
                                              University of Amsterdam

                                              Human-Computer Studies
                                                (HCS, formerly SWI)

                                                Kruislaan 419, 1098
                                                   VA  Amsterdam
                                                  The Netherlands
                                              Tel.  (+31) 20 8884671

                         SSWWII--PPrroolloogg 55..66

                        RReeffeerreennccee MMaannuuaall
            _U_p_d_a_t_e_d _f_o_r _v_e_r_s_i_o_n _5_._6_._2_7_, _J_a_n_u_a_r_y _2_0_0_7

                         _J_a_n _W_i_e_l_e_m_a_k_e_r
                    wielemak@science.uva.nl
                   http://www.swi-prolog.org

SWI-Prolog  is  a Prolog  implementation based  on a  subset of
the  WAM (Warren Abstract  Machine).   SWI-Prolog was developed
as  an _o_p_e_n  Prolog environment,  providing a powerful  and bi-
directional  interface to C in an era this was unknown to other
Prolog  implementations.  This environment  is required to deal
with  XPCE,  an object-oriented  GUI system  developed  at SWI.
XPCE  is used at SWI for the development of knowledge-intensive
graphical  applications.   As  SWI-Prolog became  more popular,
a   large  user-community  provided  requirements  that  guided
its  development.    Compatibility,  portability,  scalability,
stability  and  providing  a  powerful development  environment
have   been  the  most  important  requirements.     Edinburgh,
Quintus,  SICStus and the ISO-standard guide the development of
the  SWI-Prolog primitives.  This document gives an overview of
the features, system limits and built-in predicates.

Copyright Oc 1990--2007, University of Amsterdam


CChhaapptteerr 11..  IINNTTRROODDUUCCTTIIOONN


11..11 SSWWII--PPrroolloogg

SWI-Prolog started back in  1986 with the requirement for a  Prolog that
could handle recursive interaction with the C-language:   Prolog calling
C and  C calling  Prolog recursively.   Those  days Prolog systems  were
very aware  of its environment  and we needed such  a system to  support
interactive  applications.   Since  then,  SWI-Prolog's development  has
been guided  by requests from the  user community, especially  focussing
on (in arbitrary  order) interaction with the environment,  scalability,
(I/O)  performance,  standard  compliance,   teaching  and  the  program
development environment.

SWI-Prolog  is based  on  a very  restricted  form  of the  WAM  (Warren
Abstract Machine) described  in [Bowen & Byrd, 1983] which defines  only
7  instructions.   Prolog  can  easily be  compiled into  this  language
and the  abstract machine code  is easily  decompiled back into  Prolog.
As  it is  also  possible to  wire a  standard  4-port debugger  in  the
WAM interpreter  there is  no need  for a  distinction between  compiled
and interpreted  code.   Besides  simplifying the design  of the  Prolog
system  itself this  approach has  advantages  for program  development:
the  compiler is  simple and  fast, the  user  does not  have to  decide
in  advance whether  debugging  is required  and  the system  only  runs
slightly  slower when  in debug  mode.   The  price we  have  to pay  is
some  performance degradation  (taking  out the  debugger from  the  WAM
interpreter improves performance  by about 20%) and somewhat  additional
memory usage to help the decompiler and debugger.

SWI-Prolog  extends  the  minimal  set  of   instructions  described  in
[Bowen & Byrd, 1983]  to improve  performance.    While  extending  this
set care  has been  taken to  maintain the  advantages of  decompilation
and  tracing of  compiled  code.    The extensions  include  specialised
instructions  for unification,  predicate  invocation,  some  frequently
used built-in  predicates, arithmetic, and  control (;/2, |/2),  if-then
(->/2) and negation-by-failure (\+/1).


11..11..11 BBooookkss aabboouutt PPrroolloogg

This manual does not  describe the full syntax and semantics  of Prolog,
nor how  one should  write a  program in Prolog.    These subjects  have
been  described extensively  in  the literature.    See  [Bratko, 1986],
[Sterling & Shapiro, 1986],   and   [Clocksin & Melish, 1987].       For
more  advanced  Prolog  material  see  [O'Keefe, 1990].      Syntax  and
standard  operator declarations  confirm  to the  `Edinburgh  standard'.
Most  built  in  predicates  are  compatible  with  those  described  in
[Clocksin & Melish, 1987].      SWI-Prolog  also  offers  a  number   of
primitive  predicates compatible  with  Quintus Prolog  [Qui, 1997]  and
BIM_Prolog [BIM, 1989].

ISO  compliant  predicates  are based  on  ``Prolog:    The  Standard'',
[Deransart _e_t _a_l_., 1996], validated using [Hodgson, 1998].


11..22 SSttaattuuss

This manual describes  version 5.6 of SWI-Prolog.   SWI-Prolog has  been
used now for many  years.  The application range includes  Prolog course
material,  meta-interpreters, simulation  of parallel  Prolog,  learning
systems,  natural  language  processing,  complex  interactive  systems,
web-server and  web-server components.   Although we experienced  rather
obvious and  critical bugs can  remain unnoticed  for a remarkable  long
period, we assume  the basic Prolog system is  fairly stable.  Bugs  can
be expected in infrequently used built-in predicates.

Some bugs are known to  the author.  They are described as  footnotes in
this manual.


11..33 CCoommpplliiaannccee ttoo tthhee IISSOO ssttaannddaarrdd

SWI-Prolog 3.3.0 implements  all predicates described in ``Prolog:   The
Standard'' [Deransart _e_t _a_l_., 1996].

Exceptions  and warning  are still  weak.    Some SWI-Prolog  predicates
silently  fail on  conditions where  the ISO  specification requires  an
exception  (functor/3 for  example).    Some predicates  print  warnings
rather than raising an  exception.  All predicates where  exceptions may
be  caused due  to a  correct program  operating in  an imperfect  world
(I/O, arithmetic,  resource overflows)  should behave  according to  the
ISO standard.   In other  words:  SWI-Prolog  should be able to  execute
any program  conforming to [Deransart _e_t _a_l_., 1996]  that does not  rely
on exceptions generated by errors in the program.


11..44 SShhoouulldd yyoouu bbee uussiinngg SSWWII--PPrroolloogg??

There are a number of reasons why you better  choose a commercial Prolog
system, or another academic product:

  o _S_W_I_-_P_r_o_l_o_g _i_s _n_o_t _s_u_p_p_o_r_t_e_d
    Although  I usually fix bugs shortly  after a bug report arrives,  I
    cannot  promise anything.   Now that the  sources are provided,  you
    can always dig into them yourself.

  o _M_e_m_o_r_y _r_e_q_u_i_r_e_m_e_n_t_s _a_n_d _p_e_r_f_o_r_m_a_n_c_e _a_r_e _y_o_u_r _f_i_r_s_t _c_o_n_c_e_r_n_s
    A  number  of  commercial compilers  are  more  keen on  memory  and
    performance  than SWI-Prolog.   I do not  wish to sacrifice some  of
    the  nice features of the system, nor its portability to  compete on
    raw performance.

  o _Y_o_u _n_e_e_d _f_e_a_t_u_r_e_s _n_o_t _o_f_f_e_r_e_d _b_y _S_W_I_-_P_r_o_l_o_g
    In  this case you  may wish to  give me suggestions for  extensions.
    If  you  have great  plans, please  contact me  (you  might have  to
    implement them yourself however).

On the other hand, SWI-Prolog offers some nice facilities:

  o _N_i_c_e _e_n_v_i_r_o_n_m_e_n_t
    This includes `Do  What I Mean', automatic completion of atom names,
    history mechanism and  a tracer that operates on single key-strokes.
    Interfaces  to  some  standard  editors are  provided  (and  can  be
    extended), as well as a facility to maintain programs (see make/0).

  o _V_e_r_y _f_a_s_t _c_o_m_p_i_l_e_r
    Even  very  large applications  can  be loaded  in seconds  on  most
    machines.  If  this is not enough, there is a Quick Load Format that
    is slightly more compact and loading is almost always I/O bound.

  o _T_r_a_n_s_p_a_r_e_n_t _c_o_m_p_i_l_e_d _c_o_d_e
    SWI-Prolog  compiled code can be  treated just as interpreted  code:
    you  can list it, trace  it, etc.  This  implies you do not have  to
    decide  beforehand whether a module  should be loaded for  debugging
    or  not.  Also, performance  is much better than the  performance of
    most interpreters.

  o _P_r_o_f_i_l_i_n_g
    SWI-Prolog offers tools  for performance analysis, which can be very
    useful  to optimise  programs.   Unless you  are very familiar  with
    Prolog  and Prolog  performance  considerations this  might be  more
    helpful than a better compiler without these facilities.

  o _F_l_e_x_i_b_i_l_i_t_y
    SWI-Prolog  can  easily   be  integrated  with  C,  supporting  non-
    determinism  in Prolog calling  C as well  as C calling Prolog  (see
    section  9).  It can also be _e_m_b_e_d_d_e_d embedded in  external programs
    (see  section 9.7).  System  predicates can be redefined locally  to
    provide compatibility with other Prolog systems.

  o _I_n_t_e_g_r_a_t_i_o_n _w_i_t_h _X_P_C_E
    SWI-Prolog   offers  a   tight  integration  to   the  Object   Ori-
    ented   Package  for   User  Interface   Development,  called   XPCE
    [Anjewierden & Wielemaker, 1989].    XPCE  allows you  to  implement
    graphical  user  interfaces  that are  source-code  compatible  over
    Unix/X11,  Win32  (Windows  95/98/ME  and NT/2000/XP)  and  MacOS  X
    (darwin).


11..55 TThhee XXPPCCEE GGUUII ssyysstteemm ffoorr PPrroolloogg

The  XPCE GUI  system  for dynamically  typed  languages has  been  with
SWI-Prolog for  a long time.   It is  developed by Anjo Anjewierden  and
Jan  Wielemaker from  the department  of SWI,  University of  Amsterdam.
It  aims at  a  high-productive  development environment  for  graphical
applications based on Prolog.

Object  oriented  technology has  proven  to  be a  suitable  model  for
implementing GUIs, which  typically deal with things Prolog is  not very
good  at:   event-driven  control  and global  state.    With  XPCE,  we
designed  a system  that has  similar characteristics  that make  Prolog
such  a powerful  tool:   dynamic typing,  meta-programming and  dynamic
modification of the running system.

XPCE is  an object-system written  in the C-language.   It provides  for
the implementation of methods  in multiple languages.  New  XPCE classes
may be defined from Prolog using a simple, natural syntax.   The body of
the method is executed  by Prolog itself, providing a  natural interface
between the two systems.  Below is a very simple class definition.

:- pce_begin_class(prolog_lister, frame,
                   "List Prolog predicates").

initialise(Self) :->
        "As the C++ constructor"::
        send_super(Self, initialise, 'Prolog Lister'),
        send(Self, append, new(D, dialog)),
        send(D, append,
             text_item(predicate, message(Self, list, @arg1))),
        send(new(view), below, D).

list(Self, From:name) :->
        "List predicates from specification"::
        (   catch(term_to_atom(Term, From), _, fail)
        ->  get(Self, member, view, V),
            current_output(Old),
            pce_open(V, write, Fd),
            set_output(Fd),
            listing(Term),
            close(Fd),
            set_output(Old)
        ;   send(Self, report, error, 'Syntax error')
        ).

:- pce_end_class.

test :- send(new(prolog_lister), open).

Its  165  built-in   classes  deal  with  the  meta-environment,   data-
representation  and---of  course---graphics.     The   graphics  classes
concentrate on direct-manipulation of diagrammatic representations.

AAvvaaiillaabbiilliittyy.. XPCE  runs on  most  Unixtm platforms,  Windows  95/98/ME,
Windows NT/2000/XP and MacOS  X (using X11).  In the past,  versions for
Quintus- and SICStus Prolog as well as some Lisp  dialects have existed.
After discontinuing  active Lisp  development at SWI  the Lisp  versions
have  died.   Active  development on  the Quintus  and SICStus  versions
has  been stopped  due to  lack  of standardisation  in the  the  Prolog
community.    If adequate  standards  emerge we  are happy  to  actively
support other Prolog implementations.

IInnffoo.. further    information   is    available   from    http://www.swi-
prolog.org/packages/xpce/ or by E-mail to info@www.swi-prolog.org.


11..66 RReelleeaassee NNootteess

Collected release-notes.   This section  only contains some  highlights.
Smaller changes to especially  older releases have been removed.   For a
complete log, see the file ChangeLog from the distribution.


11..66..11 VVeerrssiioonn 11..88 RReelleeaassee NNootteess

Version  1.8 offers  a stack-shifter  to  provide dynamically  expanding
stacks  on machines  that  do  not offer  operating-system  support  for
implementing dynamic stacks.


11..66..22 VVeerrssiioonn 11..99 RReelleeaassee NNootteess

Version  1.9  offers  better portability  including  an  MS-Windows  3.1
version.  Changes to the Prolog system include:

  o _R_e_d_e_f_i_n_i_t_i_o_n _o_f _s_y_s_t_e_m _p_r_e_d_i_c_a_t_e_s
    Redefinition  of system  predicates  was allowed  silently in  older
    versions.    Version 1.9  only allows it  if the  new definition  is
    headed by a :- redefine_system_predicate/1 directive.top-level

  o _`_A_n_s_w_e_r_' _r_e_u_s_e
    The  top-level maintains a table  of bindings returned by  top-level
    goals  and  allows for  reuse  of these  bindings by  prefixing  the
    variables with the $ sign.  See section 2.8.

  o _B_e_t_t_e_r _s_o_u_r_c_e _c_o_d_e _a_d_m_i_n_i_s_t_r_a_t_i_o_n
    Allows  for proper updating of multifile predicates and  finding the
    sources of individual clauses.


11..66..33 VVeerrssiioonn 22..00 RReelleeaassee NNootteess

New features offered:

  o _3_2_-_b_i_t _V_i_r_t_u_a_l _M_a_c_h_i_n_e
    Removes various limits and improves performance.

  o _I_n_l_i_n_e _f_o_r_e_i_g_n _f_u_n_c_t_i_o_n_s
    `Simple'  foreign predicates no  longer build a Prolog  stack-frame,
    but are directly  called from the VM. Notably provides a speedup for
    the test predicates such as var/1, etc.

  o _V_a_r_i_o_u_s _c_o_m_p_a_t_i_b_i_l_i_t_y _i_m_p_r_o_v_e_m_e_n_t_s

  o _S_t_r_e_a_m _b_a_s_e_d _I_/_O _l_i_b_r_a_r_y
    All  SWI-Prolog's I/O is now  handled by the stream-package  defined
    in  the foreign include file SWI-Stream.h.   Physical I/O of  Prolog
    streams  may be  redefined through the  foreign language  interface,
    facilitating much simpler integration in window environments.


11..66..44 VVeerrssiioonn 22..55 RReelleeaassee NNootteess

Version  2.5  is  an  intermediate release  on  the  path  from  2.1  to
3.0.    All  changes are  to  the foreign-language  interface,  both  to
user- and system-predicates implemented  in the C-language.  The  aim is
twofold.   First of all  to make garbage-collection and  stack-expansion
(stack-shifts)  possible  while  foreign  code  is  active  without  the
C-programmer having  to worry  about locking  and unlocking  C-variables
pointing  to Prolog  terms.    The new  approach is  closely  compatible
to the  Quintus and  SICStus Prolog  foreign interface  using the  +term
argument specification (see their respective manuals).   This allows for
writing foreign  interfaces that  are easily portable  over these  three
Prolog platforms.

Apart from  various bug fixes  listed in the  ChangeLog file, these  are
the main changes since 2.1.0:

  o _I_S_O _c_o_m_p_a_t_i_b_i_l_i_t_y
    Many   ISO  compatibility  features   have  been  added:     open/4,
    arithmetic functions, syntax, etc.

  o _W_i_n_3_2
    Many  fixes for the Win32 (NT,  '95 and win32s) platforms.   Notably
    many  problems related  to pathnames  and a problem  in the  garbage
    collector.

  o _P_e_r_f_o_r_m_a_n_c_e
    Many  changes to  the clause  indexing system:   added  hash-tables,
    lazy computation of the index information, etc.

  o _P_o_r_t_a_b_l_e _s_a_v_e_d_-_s_t_a_t_e_s
    The   predicate  qsave_program/[1,2] allows  for   the  creating  of
    machine independent saved-states that load very quickly.


11..66..55 VVeerrssiioonn 22..66 RReelleeaassee NNootteess

Version 2.6  provides a stable implementation  of the features added  in
the 2.5.x  releases, but  at the same  time implements  a number of  new
features that may have impact on the system stability.

  o _3_2_-_b_i_t _i_n_t_e_g_e_r _a_n_d _d_o_u_b_l_e _f_l_o_a_t _a_r_i_t_h_m_e_t_i_c
    The  biggest change is the  support for full 32-bit signed  integers
    and  raw  machine-format  double precision  floats.    The  internal
    data  representation as well as  the arithmetic instruction set  and
    interface to the arithmetic functions has been changed for this.

  o _E_m_b_e_d_d_i_n_g _f_o_r _W_i_n_3_2 _a_p_p_l_i_c_a_t_i_o_n_s
    The  Win32 version has been reorganised.   The Prolog kernel is  now
    implemented  as Win32  DLL that may  be embedded in  C-applications.
    Two front ends  are provided, one for window-based operation and one
    to run as a Win32 console application.

  o _C_r_e_a_t_i_n_g _s_t_a_n_d_-_a_l_o_n_e _e_x_e_c_u_t_a_b_l_e_s
    Version  2.6.0 can create  stand-alone executables by attaching  the
    saved-state to the emulator.  See qsave_program/2.


11..66..66 VVeerrssiioonn 22..77 RReelleeaassee NNootteess

Version 2.7  reorganises the  entire data-representation  of the  Prolog
data  itself.   The  aim is  to remove  most of  the  assumption on  the
machine's memory  layout to  improve portability in  general and  enable
embedding on  systems where the memory  layout may depend on  invocation
or on how the executable is linked.  The latter  is notably a problem on
the Win32 platforms.  Porting to 64-bit architectures is feasible now.

Furthermore, 2.7 lifts the  limits on arity of predicates and  number of
variables in  a clause considerably and  allow for further expansion  at
minimal cost.


11..66..77 VVeerrssiioonn 22..88 RReelleeaassee NNootteess

With version  2.8, we declare  the data-representation changes of  2.7.x
stable.   Version  2.8 exploits  the changes  of 2.7  to support  64-bit
processors like the DEC Alpha.  As of  version 2.8.5, the representation
of  recorded  terms  has  changed,  and  terms   on  the  heap  are  now
represented  in a  compiled format.    SWI-Prolog no  longer limits  the
use of malloc()  or uses assumptions on  the addresses returned by  this
function.


11..66..88 VVeerrssiioonn 22..99 RReelleeaassee NNootteess

Version  2.9  is  the next  step  towards  version  3.0,  improving  ISO
compliance  and introducing  ISO  compliant  exception handling.     New
are catch/3, throw/1, abolish/1, write_term/[2,3], write_canonical/[1,2]
and  the  C-functions PL_exception() and  PL_throw().    The  predicates
display/[1,2] and  displayq/[1,2] have  been moved to  backcomp, so  old
code referring to them will autoload them.

The interface  to PL_open_query()  has changed.   The  _d_e_b_u_g argument  is
replaced  by a  bitwise or'ed  _f_l_a_g_s argument.    The  values FALSE  and
TRUE have their familiar meaning, making old code  using these constants
compatible.   Non-zero values  other than TRUE  (1) will be  interpreted
different.


11..66..99 VVeerrssiioonn 33..00 RReelleeaassee NNootteess

Complete  redesign   of  the   saved-state  mechanism,   providing   the
possibility of  `program resources'.   See  resource/3, open_resource/3,
and qsave_program/[1,2].


11..66..1100 VVeerrssiioonn 33..11 RReelleeaassee NNootteess

Improvements   on  exception-handling.       Allows  relating   software
interrupts (signals)  to exceptions,  handling signals in  Prolog and  C
(see on_signal/3  and PL_signal()).    Prolog stack  overflows now  raise
the resource_error  exception and thus  can be handled  in Prolog  using
catch/3.


11..66..1111 VVeerrssiioonn 33..33 RReelleeaassee NNootteess

Version 3.3  is a  major release,  changing many  things internally  and
externally.   The highlights are a  complete redesign of the  high-level
I/O system, which is  now based on explicit streams rather  then current
input/output.  The  old Edinburgh predicates (see/1, tell/1, etc.)   are
now defined on top of this layer instead of the other  way around.  This
fixes various internal problems and removes Prolog limits  on the number
of streams.

Much  progress  has been  made  to  improve ISO  compliance:    handling
strings  as  lists  of  one-character  atoms  is   now  supported  (next
to  character   codes  as  integers).      Many  more  exceptions   have
been  added and  printing  of exceptions  and messages  is  rationalised
using   Quintus   and   SICStus   Prolog   compatible   print_message/2,
message_hook/3 and print_message_lines/3.   All predicates described  in
[Deransart _e_t _a_l_., 1996] are now implemented.

As of version  3.3, SWI-Prolog adheres the  ISO _l_o_g_i_c_a_l _u_p_d_a_t_e _v_i_e_w  for
dynamic predicates.  See section 4.13.1 for details.

SWI-Prolog  3.3  includes garbage  collection  on  atoms,  removing  the
last serious memory  leak especially in text-manipulation  applications.
See  section 9.6.2.1.    In addition,  both the  user-level and  foreign
interface supports atoms holding _0_-_b_y_t_e_s.

Finally, an  alpha version of a  multi-threaded SWI-Prolog for Linux  is
added.    This version  is still  much slower  than the  single-threaded
version due  to frequent access to  `thread-local-data' as well as  some
too detailed  mutex locks.   The basic thread  API is ready for  serious
use and testing however.  See section 8.


11..66..1111..11 IInnccoommppaattiibbllee cchhaannggeess

A number  of incompatible changes  result from this upgrade.   They  are
all easily fixed however.

  o !/0_, call/1
    The  cut now behaves  according to the ISO  standard.  This  implies
    it  works in compound  goals passed  to call/1 and  is local to  the
    _c_o_n_d_i_t_i_o_n part of if-then-else as well as the argument of \+/1.

  o _a_t_o_m___c_h_a_r_s_/_2
    This  predicate  is now  ISO  compliant and  thus generates  a  list
    of  one-character atoms.    The behaviour  of the  old predicate  is
    available  in the  ---also ISO compliant---  atom_codes/2 predicate.
    Safest  repair is a  replacement of all  atom_chars  into atom_codes.
    If you do not want to change any source-code, you might want to use

    user:goal_expansion(atom_chars(A,B), atom_codes(A,B)).

  o _n_u_m_b_e_r___c_h_a_r_s_/_2
    Same applies for number_chars/2 and number_codes/2.

  o feature/2_, set_feature/2
    These  are replaced by  the ISO  compliant current_prolog_flag/2 and
    set_prolog_flag/2.   The library  backcomp provides definitions  for
    these predicates, so no source mmuusstt be updated.

  o _A_c_c_e_s_s_i_n_g _c_o_m_m_a_n_d_-_l_i_n_e _a_r_g_u_m_e_n_t_s
    This  used  to   be  provided  by  the  undocumented  '$argv'/1  and
    Quintus  compatible library unix/1.   Now  there is also  documented
    current_prolog_flag(_a_r_g_v_, _A_r_g_v).

  o _d_u_p___s_t_r_e_a_m_/_2
    Has  been deleted.   New  stream-aliases can deal  with most of  the
    problems  for which  dup_stream/2 was  designed and  dup/2 from  the
    _c_l_i_b package can with most others.

  o _o_p_/_3
    Operators  are now llooccaall ttoo mmoodduulleess.  This implies  any modification
    of  the operator-table does  not influence other  modules.  This  is
    consistent  with the proposed ISO behaviour and a necessity  to have
    any usable handling of operators in a multi-threaded environment.

  o _s_e_t___p_r_o_l_o_g___f_l_a_g_(_c_h_a_r_a_c_t_e_r___e_s_c_a_p_e_s_, _B_o_o_l_)
    This  prolog flag is now an interface to changing attributes  on the
    current source-module,  effectively making this flag module-local as
    well.   This is required for consistent handling of  sources written
    with  ISO (obligatory) character-escape sequences together  with old
    Edinburgh code.

  o _c_u_r_r_e_n_t___s_t_r_e_a_m_/_3 _a_n_d _s_t_r_e_a_m___p_o_s_i_t_i_o_n
    These predicates have been moved to quintus.


11..66..1122 VVeerrssiioonn 33..44 RReelleeaassee NNootteess

The  3.4 release  is  a consolidation  release.    It  consolidates  the
improvements  and  standard conformance  of  the  3.3 releases.     This
version  is closely  compatible  with the  3.3  version except  for  one
important change:

  o _A_r_g_u_m_e_n_t _o_r_d_e_r _i_n select/3
    The  list-processing  predicate select/3  somehow  got into  a  very
    early  version of SWI-Prolog  with the wrong  argument order.   This
    has been fixed in  3.4.0.  The correct order is select(?Elem, ?List,
    ?Rest).

    As  select/3 has  no error  conditions, runtime  checking cannot  be
    done.   To simplify  debugging, the library module checkselect  will
    print  references to  select/3  in your  source code  and install  a
    version  of select that enters the debugger if select is  called and
    the second argument is not a list.

    This   library   can    be   loaded   explicitly   or   by   calling
    check_old_select/0.


11..66..1133 VVeerrssiioonn 44..00 RReelleeaassee NNootteess

As of  version 4.0 the  standard distribution  of SWI-Prolog is  bundled
with a  number of its  popular extension packages,  among which the  now
open source XPCE GUI toolkit (see section 1.5).   No significant changes
have been made to the basic SWI-Prolog engine.

Some useful tricks in the integrated environment:

  o _R_e_g_i_s_t_e_r _t_h_e _G_U_I _t_r_a_c_e_r
    Using  a call to guitracer/0,  hooks are installed that replace  the
    normal command-line driven tracer with a graphical front-end.

  o _R_e_g_i_s_t_e_r _P_c_e_E_m_a_c_s _f_o_r _e_d_i_t_i_n_g _f_i_l_e_s
    From  your initialisation file.  you  can load emacs/swi_prolog that
    cause edit/1 to use the built-in PceEmacs editor.


11..66..1144 VVeerrssiioonn 55..00 RReelleeaassee NNootteess

Version  5.0 marks  a breakpoint  in  the philosophy,  where  SWI-Prolog
moves  from  a  dual GPL/proprietary  to  a  uniform  LGPL  (Lesser  GNU
Public Licence)  schema, providing  a widely usable  Free Source  Prolog
implementation.

On  the  technical  site  the  development  environment,  consisting  of
source-level  debugger,  integrated  editor  and  various  analysis  and
navigation tools progress steadily towards a mature set of tools.

Many portability issues have been improved, including a  port to MacOS X
(Darwin).

For details, please visit the new website at http://www.swi-prolog.org


11..66..1155 VVeerrssiioonn 55..11 RReelleeaassee NNootteess

Version 5.1 is  a beta-serie introducing portable multi-threading.   See
chapter 8.   In  addition it introduces many  new facilities to  support
server applications,  such as  the new  rlimit library  to limit  system
resources and the possibility to set timeouts on input streams.


11..66..1166 VVeerrssiioonn 55..22 RReelleeaassee NNootteess

Version  5.2  consolidates   the  5.1.x  beta  series  that   introduced
threading and many related modifications to the kernel.


11..66..1177 VVeerrssiioonn 55..33 RReelleeaassee NNootteess

Version  5.3.x   is  a  development   series  for  adding   coroutining,
constraints, global variables,  cyclic terms (infinite trees) and  other
goodies  to the  kernel.   The  package JPL,  providing a  bidirectional
Java/Prolog  interface is  added to  the common  source-tree and  common
binary packages.


11..66..1188 VVeerrssiioonn 55..44 RReelleeaassee NNootteess

Version 5.4 consolidates the 5.3.x beta series.


11..66..1199 VVeerrssiioonn 55..55 RReelleeaassee NNootteess

Version  5.5.x provides  support  for  _w_i_d_e _c_h_a_r_a_c_t_e_r_s  with  UTF-8  and
UNICODE I/O  (section 2.17.1).   On both 32  and 64-bit hardware  Prolog
integers are now at  minimum 64-bit integers.  If available,  SWI-Prolog
arithmetic  uses the  GNU  GMP  library to  provided  _u_n_b_o_u_n_d_e_d  integer
arithmetic  as well  as rational  arithmetic.    Adding GMP  support  is
sponsored  by Scientific  Software and  Systems Limited,  www.sss.co.nz.
This version also incorporates clp(r) by Christian  Holzbaur, brought to
SWI-Prolog by Tom Schrijvers and Leslie De Koninck (section 11.14).


11..66..2200 VVeerrssiioonn 55..66 RReelleeaassee NNootteess

Version 5.6 consolidates the 5.5.x beta series.


11..77 DDoonnaattee ttoo tthhee SSWWII--PPrroolloogg pprroojjeecctt

If you  are happy with  SWI-Prolog, you  care it to  be around for  much
longer while it becomes  faster, more stable and with more  features you
should consider to  donate to the SWI-Prolog  foundation.  Please  visit
the page below.

    http://www.swi-prolog.org/donate.html


11..88 AAcckknnoowwlleeddggeemmeennttss

Some small parts of the Prolog code of SWI-Prolog  are modified versions
of the corresponding Edinburgh C-Prolog code:   grammar rule compilation
and  writef/2.    Also some  of  the  C-code originates  from  C-Prolog:
finding the  path of the  currently running executable  and some of  the
code underlying  absolute_file_name/2.   Ideas  on programming style  and
techniques originate from  C-Prolog and Richard O'Keefe's _t_h_i_e_f  editor.
An  important  source  of inspiration  are  the  programming  techniques
introduced by Anjo Anjewierden in PCE version 1 and 2.

I also  would like to thank  those who had the  fade of using the  early
versions of this system,  suggested extensions or reported bugs.   Among
them are Anjo Anjewierden, Huub Knops, Bob  Wielinga, Wouter Jansweijer,
Luc Peerdeman, Eric Nombden, Frank van Harmelen, Bert Rengel.

Martin Jansche  (jansche@novell1.gs.uni-heidelberg.de) has been so  kind
to reorganise the sources for version 2.1.3 of this manual.

Horst  von Brand  has been  so  kind to  fix many  typos  in the  2.7.14
manual.  Thanks!

Bart  Demoen and  Tom  Schrijvers  have helped  me  adding  coroutining,
constraints,  global  variables and  support  for  cyclic terms  to  the
kernel.   Tom has provided the  integer interval constraint solver,  the
CHR compiler and some of the coroutining predicates.

Paul Singleton has integrated Fred Dushin's  Java-calls-Prolog side with
his Prolog-calls-Java side into the current  bidirectional JPL interface
package.

Richard O'Keefe  is gratefully acknowledged for  his efforts to  educate
beginners as well as valuable comments on proposed new developments.

Scientific  Software and  Systems Limited,  www.sss.co.nz has  sponsored
the development  if the  SSL library  as well as  unbounded integer  and
rational number arithmetic.

Leslie de Koninck has made clp(QR) available to SWI-Prolog.

Markus Triska has contributed to various libraries.

Paulo  Moura's  great   experience  in  maintaining  Logtalk  for   many
Prolog systems  including SWI-Prolog  has helped in  many places  fixing
compatibility issues.   He also worked on the MacOS port and  fixed many
typos in the 5.6.9 release of the documentation.


CChhaapptteerr 22..  OOVVEERRVVIIEEWW


22..11 GGeettttiinngg ssttaarrtteedd qquuiicckkllyy


22..11..11 SSttaarrttiinngg SSWWII--PPrroolloogg


22..11..11..11 SSttaarrttiinngg SSWWII--PPrroolloogg oonn UUnniixx

By default, SWI-Prolog is installed as `pl',  though some administrators
call  it  `swipl'  or `swi-prolog'.     The  command-line  arguments  of
SWI-Prolog  itself  and  its  utility  programs   are  documented  using
standard  Unix  man pages.     SWI-Prolog  is normally  operated  as  an
interactive application simply by starting the program:

machine% pl
Welcome to SWI-Prolog (Version 5.6.26)
Copyright (c) 1990-2007 University of Amsterdam.
SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software,
and you are welcome to redistribute it under certain conditions.
Please visit http://www.swi-prolog.org for details.

For help, use ?- help(Topic). or ?- apropos(Word).

1 ?-

After  starting Prolog,  one  normally loads  a  program into  it  using
consult/1, which  --- for historical reasons  --- may be abbreviated  by
putting the  name of  the program  file between  square brackets.    The
following  goal loads  the  file  likes.pl containing  clauses  for  the
predicates likes/2:

?- [likes].
% likes compiled, 0.00 sec, 596 bytes.

Yes
?-

After this  point, Unix  and Windows users  unite, so  if you are  using
Unix please continue at section 2.1.2.


22..11..11..22 SSttaarrttiinngg SSWWII--PPrroolloogg oonn WWiinnddoowwss

After SWI-Prolog has been  installed on a Windows system,  the following
important new things are available to the user:

  o A  folder  (called  _d_i_r_e_c_t_o_r_y in  the  remainder of  this  document)
    called  pl  containing the  executables,  libraries, etc.    of  the
    system.  No files are installed outside this directory.

  o A  program  plwin.exe,  providing  a  window  for  interaction  with
    Prolog.  The  program plcon.exe is a version of SWI-Prolog that runs
    in a DOS-box.

  o The  file-extension .pl  is associated with  the program  plwin.exe.
    Opening  a .pl file will cause plwin.exe to start,  change directory
    to  the directory in  which the file-to-open  resides and load  this
    file.

The  normal   way  to  start  with   the  likes.pl  file  mentioned   in
section 2.1.1.1  is by simply double-clicking  this file in the  Windows
explorer.


22..11..22 EExxeeccuuttiinngg aa qquueerryy

After loading a program,  one can ask Prolog queries about  the program.
The query below asks Prolog what food `sam' likes.   The system responds
with X = <_v_a_l_u_e> if it can prove the goal for a certain _X. The  user can
type the semi-colon  (;) if (s)he wants  another solution, or return  if
(s)he is satisfied, after which Prolog will say YYeess.   If Prolog answers
NNoo,  it  indicates it  cannot  find any  (more)  answers to  the  query.
Finally, Prolog can answer using an error message  to indicate the query
or program contains an error.

?- likes(sam, X).

X = dahl ;

X = tandoori ;

...

X = chips ;

No
?-


22..22 TThhee uusseerr''ss iinniittiiaalliissaattiioonn ffiillee

After  the necessary  system  initialisation  the system  consults  (see
consult/1) the user's startup file.  The base-name  of this file follows
conventions of  the operating  system.   On MS-Windows,  it is the  file
pl.ini  and on  Unix systems  .plrc.   The  file is  searched using  the
file_search_path/2 clauses for user_profile.  The table below  shows the
default value for this search-path.

     _______________________________________________________________
     |________________|UUnniixx__||WWiinnddoowwss________________________________________________________________________________||
     || llooccaall  ||.    |.                                              |

     | hhoommee   ||~    |%HOME% or %HOMEDRIVE%\%HOMEPATH%               |
     |_global_|____|SWI-Home_directory_or_%WINDIR%_or_%SYSTEMROOT%_|_

After the  first startup  file is found  it is  loaded and Prolog  stops
looking for further startup files.  The name of the  startup file can be
changed with the  `-f file' option.   If _F_i_l_e denotes an absolute  path,
this file is loaded,  otherwise the file is searched for using  the same
conventions as for the default startup file.  Finally,  if _f_i_l_e is none,
no file is loaded.


22..33 IInniittiiaalliissaattiioonn ffiilleess aanndd ggooaallss

Using  command-line  arguments (see  section  2.4),  SWI-Prolog  can  be
forced to  load files  and execute queries  for initialisation  purposes
or  non-interactive operation.    The  most  commonly used  options  are
-f file or  -s file to  make Prolog load  a file,  -g goal to define  an
initialisation  goal and  -t goal to  define the  _t_o_p_-_l_e_v_e_l _g_o_a_l.    The
following  is a  typical example  for starting  an application  directly
from the command-line.

machine% pl -s load.pl -g go -t halt

It  tells  SWI-Prolog to  load  load.pl,  start  the  application  using
the  _e_n_t_r_y_-_p_o_i_n_t  go/0  and  ---instead  of   entering  the  interactive
top-level---  exit after  completing  go/0.    The  -q  may be  used  to
suppress all informational messages.

In  MS-Windows,  the  same  can  be  achieved  using  a  short-cut  with
appropriately  defined  command-line  arguments.      A  typically  seen
alternative  is to  write  a file  run.pl  with content  as  illustrated
below.  Double-clicking run.pl will start the application.

:- [load].                      % load program
:- go.                          % run it
:- halt.                        % and exit

Section  2.10.2.1 discusses  further scripting  options  and chapter  10
discusses the  generation of runtime executables.   Runtime  executables
are  a mean  to  deliver executables  that  do  not require  the  Prolog
system.


22..44 CCoommmmaanndd--lliinnee ooppttiioonnss

The full set of command-line options is given below:

--hheellpp
    When  given as  the only  option, it summarises  the most  important
    options.

--vv
    When  given as the  only option, it  summarises the version and  the
    architecture identifier.

--aarrcchh
    When   given  as  the  only  option,  it  prints   the  architecture
    identifier  (see  current_prolog_flag(arch,  Arch)) and  exits.    See
    also -dump-runtime-variables.

--dduummpp--rruunnttiimmee--vvaarriiaabblleess
    When  given as  the only option,  it prints  a sequence of  variable
    settings  that can  be  used in  shell-scripts to  deal with  Prolog
    parameters.   This feature is  also used by plld (see section  9.7).
    Below is a typical example of using this feature.

    eval `pl -dump-runtime-variables`
    cc -I$PLBASE/include -L$PLBASE/runtime/$PLARCH ...

--qq
    Set  the prolog-flag  verbose to  silent, suppressing  informational
    and banner messages.

--LL_s_i_z_e_[_k_m_]
    Give  local stack limit (2 Mbytes default).   Note that there  is no
    space  between the size option  and its argument.   By default,  the
    argument  is interpreted in  Kbytes.   Postfixing the argument  with
    m  causes the argument to be  interpreted in Mbytes.  The  following
    example specifies 32 Mbytes local stack.

    % pl -L32m

    A maximum is  useful to stop buggy programs from claiming all memory
    resources.   -L0 sets the limit to the highest possible value.   See
    section 2.18.

--GG_s_i_z_e_[_k_m_]
    Give  global  stack limit  (4 Mbytes  default).    See -L  for  more
    details.

--TT_s_i_z_e_[_k_m_]
    Give  trail  stack  limit  (4  Mbytes  default).     This  limit  is
    relatively  high because trail-stack overflows are not  often caused
    by program bugs.  See -L for more details.

--AA_s_i_z_e_[_k_m_]
    Give  argument stack limit (1 Mbytes  default).  The argument  stack
    limits  the  maximum  nesting of  terms  that  can be  compiled  and
    executed.    SWI-Prolog does `last-argument  optimisation' to  avoid
    many  deeply nested  structure  using this  stack.   Enlarging  this
    limit is only necessary in extreme cases.  See -L for more details.

--cc _f_i_l_e _._._.
    Compile files into an `intermediate code file'.  See section 2.10.

--oo _o_u_t_p_u_t
    Used  in combination  with -c  or -b  to determine  output file  for
    compilation.

--OO
    Optimised  compilation.  See current_prolog_flag/2flag  optimise for
    details.

--nnooddeebbuugg
    Disable   debugging.        See   the   current_prolog_flag/2   flag
    generate_debug_info for details.

--ss _f_i_l_e
    Use  _f_i_l_e as a  script-file.   The script file  is loaded after  the
    initialisation  file  specified with  the -f file  option.    Unlike
    -f file,  using -s does  not stop Prolog  from loading the  personal
    initialisation file.

--ff _f_i_l_e
    Use _f_i_l_e as  initialisation file instead of the default .plrc (Unix)
    or pl.ini (Windows).   `-f none' stops SWI-Prolog from searching for
    a  startup file.    This option  can be  used as  an alternative  to
    -s file  that stops Prolog from loading the  personal initialisation
    file.  See also section 2.2.

--FF _s_c_r_i_p_t
    Selects  a startup-script from the  SWI-Prolog home directory.   The
    script-file  is  named <_s_c_r_i_p_t>.rc.    The  default _s_c_r_i_p_t  name  is
    deduced  from  the  executable,  taking the  leading  alphanumerical
    characters  (letters, digits and underscore) from  the program-name.
    -F none stops looking  for a script.  Intended for simple management
    of  slightly  different  versions.    One  could for  example  write
    a  script  iso.rc  and  then select  ISO  compatibility  mode  using
    pl -F iso or make a link from iso-pl to pl.

--gg _g_o_a_l
    _G_o_a_l  is executed just before entering the top level.  Default  is a
    predicate  which prints the  welcome message.   The welcome  message
    can  thus be suppressed by  giving -g true.   _g_o_a_l can be a  complex
    term.   In this case quotes  are normally needed to protect  it from
    being expanded by the Unix shell.

--tt _g_o_a_l
    Use  _g_o_a_l  as  interactive top-level  instead  of the  default  goal
    prolog/0.    _g_o_a_l can  be a  complex term.   If  the top-level  goal
    succeeds  SWI-Prolog exits  with status  0.   If it  fails the  exit
    status  is  1.    This  flag also  determines  the goal  started  by
    break/0  and abort/0.   If you want to  stop the user from  entering
    interactive  mode  start the  application  with `-g goal'  and  give
    `halt' as top-level.

--ttttyy
    Unix  only.      Switches  controlling  the  terminal  for  allowing
    single-character  commands to the tracer and  get_single_char/1.   By
    default  manipulating  the terminal  is  enabled unless  the  system
    detects  it is not  connected to a  terminal or it  is running as  a
    GNU-Emacs  inferior process.   This flag  is sometimes required  for
    smooth interaction with other applications.

--nnoossiiggnnaallss
    Inhibit any signal  handling by Prolog, a property that is sometimes
    desirable  for embedded  applications.   This option  sets the  flag
    signals to false.  See section 9.6.20.1 for details.

--xx _b_o_o_t_f_i_l_e
    Boot  from _b_o_o_t_f_i_l_e instead  of the system's default  boot file.   A
    bootfile is a  file resulting from a Prolog compilation using the -b
    or -c option or a program saved using qsave_program/[1,2].

--pp _a_l_i_a_s_=_p_a_t_h_1_[_:_p_a_t_h_2 _._._._]
    Define  a path alias for file_search_path.  _a_l_i_a_s is the name  of the
    alias,  _p_a_t_h_1 _._._.  is  a list of values for  the alias.  On  Windows
    the  list-separator is ;.   On other  systems it is :.   A value  is
    either  a term of the form  alias(value) or pathname.  The  computed
    aliases  are added  to file_search_path/2 using  asserta/1, so  they
    precede  predefined values  for the  alias.   See file_search_path/2
    for details on using this file-location mechanism.

--
    Stops  scanning for more  arguments, so you  can pass arguments  for
    your  application after this  one.   See current_prolog_flag/2 using
    the flag argv for obtaining the command-line arguments.

The following options  are for system maintenance.   They are given  for
reference only.

--bb _i_n_i_t_f_i_l_e _._._.-c _f_i_l_e _._._.
    Boot  compilation.   _i_n_i_t_f_i_l_e  _._._.   are compiled  by the  C-written
    bootstrap  compiler,  _f_i_l_e  _._._.    by  the normal  Prolog  compiler.
    System maintenance only.

--dd _l_e_v_e_l
    Set  debug  level to  _l_e_v_e_l.    Only  has effect  if the  system  is
    compiled with the -DO_DEBUG flag.  System maintenance only.


22..55 GGNNUU EEmmaaccss IInntteerrffaaccee

The default  Prolog mode for  GNU-Emacs can be  activated by adding  the
following rules to your Emacs initialisation file:

(setq auto-mode-alist
      (append
       '(("\\.pl" . prolog-mode))
       auto-mode-alist))
(setq prolog-program-name "pl")
(setq prolog-consult-string "[user].\n")
;If you want this.  Indentation is either poor or I don't use
;it as intended.
;(setq prolog-indent-width 8)

Unfortunately the  default Prolog mode  of GNU-Emacs  is not very  good.
An  alternative  prolog.el  file for  GNU-Emacs  20  is  available  from
http://www.freesoft.cz/ pdm/software/emacs/prolog-mode/  and  for   GNU-
Emacs 19 from http://w1.858.telia.com/ u85810764/Prolog-mode/index.html


22..66 OOnnlliinnee HHeellpp

Online  help  provides a  fast  lookup  and browsing  facility  to  this
manual.   The online  manual can show predicate  definitions as well  as
entire sections of the manual.

The online help  is displayed from the  file 'MANUAL'. The file  helpidx
provides  an index  into  this  file.    'MANUAL'  is created  from  the
LaTeX sources  with a modified version  of dvitty, using overstrike  for
printing bold  text and  underlining for  rendering italic text.    XPCE
is shipped  with swi_help,  presenting the information  from the  online
help in a  hypertext window.  The prolog-flag  write_help_with_overstrike
controls whether  or not help/1  writes its  output using overstrike  to
realise bold  and underlined  output or  not.   If  this prolog-flag  is
not  set it  is initialised  by the  help library  to true  if the  TERM
variable equals  xterm and false  otherwise.  If  this default does  not
satisfy you, add the  following line to your personal startup  file (see
section 2.2):

:- set_prolog_flag(write_help_with_overstrike, true).


hheellpp
    Equivalent to help(help/1).


hheellpp((_+_W_h_a_t))
    Show specified part of the manual.  _W_h_a_t is one of:

          <_N_a_m_e>/<_A_r_i_t_y> Give help on specified predicate
          <_N_a_m_e>         Give  help on  named  predicate with  any
                         arity or  C interface function  with that
                         name
          <_S_e_c_t_i_o_n>      Display  specified  section.      Section
                         numbers are dash-separated  numbers:  2-3
                         refers  to  section 2.3  of  the  manual.

                         Section   numbers  are   obtained   using
                         apropos/1.

    Examples:

       ?- help(assert).     Give help on predicate assert
       ?- help(3-4).        Display section 3.4 of the manual
       ?- help('PL_retry'). Give    help   on    interface   function
                            PL_retry()

    See also apropos/1,  and the SWI-Prolog home page at http://www.swi-
    prolog.org,  which provides  a FAQ,  an HTML version  of manual  for
    online browsing and HTML and PDF versions for downloading.


aapprrooppooss((_+_P_a_t_t_e_r_n))
    Display all predicates,  functions and sections that have _P_a_t_t_e_r_n in
    their  name or summary  description.   Lowercase letters in  _P_a_t_t_e_r_n
    also match a corresponding uppercase letter.  Example:

        ?- apropos(file).  Display  predicates,  functions and  sec-
                           tions that have  `file' (or `File', etc.)
                           in their summary description.


eexxppllaaiinn((_+_T_o_E_x_p_l_a_i_n))
    Give an explanation on  the given `object'.  The argument may be any
    Prolog data object.   If the argument is an atom, a term of the form
    _N_a_m_e_/_A_r_i_t_y  or a term  of the  form _M_o_d_u_l_e_:_N_a_m_e_/_A_r_i_t_y, explain  will
    try to explain the predicate as well as possible references to it.


eexxppllaaiinn((_+_T_o_E_x_p_l_a_i_n_, _-_E_x_p_l_a_n_a_t_i_o_n))
    Unify  _E_x_p_l_a_n_a_t_i_o_n with an explanation for _T_o_E_x_p_l_a_i_n.   Backtracking
    yields further explanations.


22..77 CCoommmmaanndd--lliinnee hhiissttoorryy

SWI-Prolog offers a query substitution mechanism called `history'.   The
availability of  this feature is controlled  by set_prolog_flag/2,  using
the  history prolog-flag.    By  default, history  is available  if  the
prolog-flag readline  is false.    To enable  this feature,  remembering
the last  50 commands,  put the  following into your  startup file  (see
section 2.2):

:- set_prolog_flag(history, 50).

The history  system allows the  user to compose  new queries from  those
typed  before and  remembered by  the  system.   The  available  history
commands are  shown in  table 2.1.   History  expansion is  not done  if
these sequences appear in quoted atoms or strings.
             ______________________________________________
             | !!.   |Repeat last query                     |

             | !nr.  |Repeat query numbered <_n_r>            |
             | !str. |Repeat last query starting with <_s_t_r> |
             | h.    |Show history of commands              |
             |_!h.___|Show_this_list_______________________ |

                      Table 2.1:  History commands


22..88 RReeuussee ooff ttoopp--lleevveell bbiinnddiinnggss

Bindings resulting  from the  successful execution of  a top-level  goal
are asserted  in a  database.   These values  may be  reused in  further
top-level  queries as  $Var.    Only the  latest binding  is  available.
Example:
1 ?- maplist(plus(1), "hello", X).

X = [105,102,109,109,112]

Yes
2 ?- format('~s~n', [$X]).
ifmmp

Yes

3 ?-

                Figure 2.1:  Reusing top-level bindings

Note that variables may be set by executing =/2:

6 ?- X = statistics.

X = statistics

Yes
7 ?- $X.
28.00 seconds cpu time for 183,128 inferences
4,016 atoms, 1,904 functors, 2,042 predicates, 52 modules
55,915 byte codes; 11,239 external references

                      Limit    Allocated       In use
Heap         :                                624,820 Bytes
Local  stack :    2,048,000        8,192          404 Bytes
Global stack :    4,096,000       16,384          968 Bytes
Trail  stack :    4,096,000        8,192          432 Bytes

Yes
8 ?-


22..99 OOvveerrvviieeww ooff tthhee DDeebbuuggggeerr

SWI-Prolog has  a 6-port  tracer, extending the  standard 4-port  tracer
[Clocksin & Melish, 1987]  with two  additional  ports.    The  optional
_u_n_i_f_y port allows  the user to inspect  the result after unification  of
the head.  The _e_x_c_e_p_t_i_o_n port shows exceptions raised  by throw/1 or one
of the built-in predicates.  See section 4.9.

The standard  ports are called call,  exit, redo, fail  and unify.   The
tracer is started  by the trace/0 command,  when a spy point is  reached
and the system is  in debugging mode (see spy/1 and debug/0) or  when an
exception is raised.

The interactive top-level  goal trace/0 means ``trace the next  query''.
The  tracer shows  the  port,  displaying  the port  name,  the  current
depth of  the recursion and  the goal.   The goal  is printed using  the
Prolog predicate write_term/2.  The style is defined  by the prolog-flag
debugger_print_options  and can be modified using this flag or  using the
w, p and d commands of the tracer.
1 ?- visible(+all), leash(-exit).

Yes
2 ?- trace, min([3, 2], X).
  Call:  ( 3) min([3, 2], G235) ? creep
  Unify: ( 3) min([3, 2], G235)
  Call:  ( 4) min([2], G244) ? creep
  Unify: ( 4) min([2], 2)
  Exit:  ( 4) min([2], 2)

  Call:  ( 4) min(3, 2, G235) ? creep
  Unify: ( 4) min(3, 2, G235)
  Call:  ( 5) 3 < 2 ? creep
  Fail:  ( 5) 3 < 2 ? creep
  Redo:  ( 4) min(3, 2, G235) ? creep
  Exit:  ( 4) min(3, 2, 2)
  Exit:  ( 3) min([3, 2], 2)

Yes
[trace] 3 ?-

                       Figure 2.2:  Example trace

On _l_e_a_s_h_e_d  _p_o_r_t_s (set  with the  predicate leash/1,  default are  call,
exit, redo and  fail) the user is prompted  for an action.  All  actions
are single character  commands which are executed wwiitthhoouutt waiting  for a
return, unless the command-line option -tty is active.  Tracer options:

+ ((SSppyy))
    Set a spy point (see spy/1) on the current predicate.

- ((NNoo ssppyy))
    Remove the spy point (see nospy/1) from the current predicate.

/ ((FFiinndd))
    Search  for a port.   After the  `/', the user  can enter a line  to
    specify  the port to  search for.   This line consists  of a set  of
    letters  indicating the  port type,  followed by  an optional  term,
    that  should unify with  the goal run by  the port.   If no term  is
    specified  it is taken as a variable, searching for any port  of the
    specified  type.  If an atom is given, any goal whose  functor has a
    name equal to that atom matches.  Examples:

            /f               Search for any fail port

            /fe solve        Search for a  fail or exit  port of
                             any goal with name solve
            /c solve(a, _)   Search for a call to  solve/2 whose
                             first argument is a variable or the
                             atom a
            /a member(_, _)  Search for  any  port on  member/2.
                             This is equivalent to setting a spy
                             point on member/2.

. ((RReeppeeaatt ffiinndd))
    Repeat the last find command (see `/').

A ((AAlltteerrnnaattiivveess))
    Show all goals that have alternatives.

C ((CCoonntteexxtt))
    Toggle  `Show Context'.   If on  the context module  of the goal  is
    displayed between square brackets (see section 5).  Default is off.

L ((LLiissttiinngg))
    List the current predicate with listing/1.

a ((AAbboorrtt))
    Abort Prolog execution (see abort/0).

b ((BBrreeaakk))
    Enter a Prolog break environment (see break/0).

c ((CCrreeeepp))
    Continue execution, stop at next port.  (Also return, space).

d ((DDiissppllaayy))
    Set  the max_depth(_D_e_p_t_h) option of debugger_print_options,  limiting
    the  depth  to which  terms are  printed.    See also  the  w and  p
    options.

e ((EExxiitt))
    Terminate Prolog (see halt/0).

f ((FFaaiill))
    Force failure of the current goal.

g ((GGooaallss))
    Show the list of  parent goals (the execution stack).  Note that due
    to  tail recursion optimization a  number of parent goals might  not
    exist any more.

h ((HHeellpp))
    Show available options (also `?').

i ((IIggnnoorree))
    Ignore the current goal, pretending it succeeded.

l ((LLeeaapp))
    Continue execution, stop at next spy point.

n ((NNoo ddeebbuugg))
    Continue execution in `no debug' mode.

p ((PPrriinntt))
    Set   the   prolog-flag  debugger_print_options  to   [quoted(true),
    portray(true), max_depth(10), priority(699)].  This is the default.

r ((RReettrryy))
    Undo  all actions (except for database and i/o actions) back  to the
    call  port of  the current  goal and  resume execution  at the  call
    port.

s ((SSkkiipp))
    Continue  execution,  stop  at the  next  port  of tthhiiss  goal  (thus
    skipping all calls to children of this goal).

u ((UUpp))
    Continue  execution, stop at the next port of tthhee ppaarreenntt  goal (thus
    skipping  this goal and all calls to  children of this goal).   This
    option is useful to stop tracing a failure driven loop.

w ((WWrriittee))
    Set       the       prolog-flag       debugger_print_options       to
    [quoted(true), attributes(write), priority(699)],          bypassing
    portray/1, etc.

The   ideal  4   port  model   as  described   in   many  Prolog   books
[Clocksin & Melish, 1987] is not visible in many  Prolog implementations
because code optimisation  removes part of the choice- and  exit-points.
Backtrack  points   are  not   shown  if  either   the  goal   succeeded
deterministically or its alternatives were removed using the cut.   When
running in  debug mode (debug/0) choice  points are only destroyed  when
removed by  the cut.    In debug  mode, tail  recursion optimisation  is
switched off.

Reference information to  all predicates available for manipulating  the
debugger is in section 4.38.


22..1100 CCoommppiillaattiioonn


22..1100..11 DDuurriinngg pprrooggrraamm ddeevveellooppmmeenntt

During  program   development,  programs   are  normally  loaded   using
consult/1, or the list abbreviation.  It is  common practice to organise
a project  as a  collection of source  files and a  _l_o_a_d_-_f_i_l_e, a  Prolog
file  containing only  use_module/[1,2]  or ensure_loaded/1  directives,
possibly  with a  definition  of the  _e_n_t_r_y_-_p_o_i_n_t  of the  program,  the
predicate that  is normally used  to start  the program.   This file  is
often  called load.pl.    If the  entry-point is  called _g_o,  a  typical
session starts as:

% pl
<banner>

1 ?- [load].
<compilation messages>

Yes
2 ?- go.
<program interaction>

When  using  Windows,  the  user  may  open  load.pl  from  the  Windows
explorer,  which will cause  plwin.exe to  be started  in the  directory
holding load.pl.  Prolog loads load.pl before entering the top-level.


22..1100..22 FFoorr rruunnnniinngg tthhee rreessuulltt

There are  various options if  you want to make  your program ready  for
real usage.   The best  choice depends on whether  the program is to  be
used only  on machines  holding the SWI-Prolog  development system,  the
size of the program and the operating system (Unix vs. Windows).


22..1100..22..11 UUssiinngg PPrroollooggSSccrriipptt

New in  version 4.0.5  is the possibility  to use  a Prolog source  file
directly  as a  Unix  script-file.   the  same  mechanism is  useful  to
specify additional parameters for running a Prolog file on Windows.

If the  first letter of a  Prolog file is #,  the first line is  treated
as comment.  To  create a Prolog script, make the first line  start like
this:

    #!/path/to/pl <_o_p_t_i_o_n_s> -s

Prolog recognises this  starting sequence and causes the interpreter  to
receive the following argument-list:

    /path/to/pl <_o_p_t_i_o_n_s> -s <_s_c_r_i_p_t> -- <_S_c_r_i_p_t_A_r_g_u_m_e_n_t_s>

Instead of  -s, the user may  use -f to stop  Prolog from looking for  a
personal initialisation file.

Here is a simple script doing expression evaluation:

#!/usr/bin/pl -q -t main -f

eval :-
        current_prolog_flag(argv, Argv),
        append(_, [--|Args], Argv),
        concat_atom(Args, ' ', SingleArg),
        term_to_atom(Term, SingleArg),
        Val is Term,
        format('~w~n', [Val]).

main :-
        catch(eval, E, (print_message(error, E), fail)),
        halt.
main :-
        halt(1).

And here are two example runs:

% eval 1+2
3
% eval foo
ERROR: Arithmetic: `foo/0' is not a function
%

TThhee  WWiinnddoowwss  vveerrssiioonn supports  the  #!  construct  too,   but  here  it
serves  a rather  different role.    The  Windows shell  already  allows
the  user to  start Prolog  source files  directly  through the  Windows
file-type  association.   Windows  however makes  it rather  complicated
to provide  additional parameters, such  as the required stack-size  for
an individual  Prolog file.   The #! line  provides for this,  providing
a  more flexible  approach  then changing  the  global  defaults.    The
following starts  Prolog with unlimited stack-size  on the given  source
file:

#!/usr/bin/pl -L0 -T0 -G0 -s

....

Note the  use of  /usr/bin/pl, which  specifies the interpreter.    This
argument is ignored in the Windows version, but  required to ensure best
cross-platform compatibility.


22..1100..22..22 CCrreeaattiinngg aa sshheellll--ssccrriipptt

With  the introduction  of _P_r_o_l_o_g_S_c_r_i_p_t  (see section  2.10.2.1),  using
shell-scripts  as explained  in this  section has  become redundant  for
most applications.

Especially  on  Unix systems  and  not-too-large  applications,  writing
a  shell-script  that  simply  loads  your  application  and  calls  the
entry-point is often a good choice.  A skeleton for  the script is given
below, followed by the Prolog code to obtain the program arguments.

#!/bin/sh

base=<absolute-path-to-source>
PL=pl

exec $PL -f none -g "load_files(['$base/load'],[silent(true)])" \
         -t go -- $*

go :-
        current_prolog_flag(argv, Arguments),
        append(_SytemArgs, [--|Args], Arguments), !,
        go(Args).

go(Args) :-
        ...

On Windows  systems, similar  behaviour can  be achieved  by creating  a
shortcut to Prolog, passing the proper options or writing a .bat file.


22..1100..22..33 CCrreeaattiinngg aa ssaavveedd--ssttaattee

For  larger programs,  as  well as  for programs  that are  required  to
run  on systems  that  do not  have  the SWI-Prolog  development  system
installed,  creating a  saved  state is  the  best solution.    A  saved
state is created using qsave_program/[1,2] or using the  linker plld(1).
A  saved state  is a  file  containing machine-independent  intermediate
code  in  a  format  dedicated  for  fast  loading.    Optionally,   the
emulator may be integrated  in the saved state, creating  a single-file,
but  machine-dependent,  executable.    This  process  is  described  in
chapter 10.


22..1100..22..44 CCoommppiillaattiioonn uussiinngg tthhee --cc ccoommmmaanndd--lliinnee ooppttiioonn

This mechanism loads a series of Prolog source files  and then creates a
saved-state as qsave_program/2 does.  The command syntax is:

% pl [option ...] [-o output] -c file ...

The  _o_p_t_i_o_n_s argument  are  options to  qsave_program/2 written  in  the
format below.    The option-names  and their values  are described  with
qsave_program/2.

    --_o_p_t_i_o_n_-_n_a_m_e=_o_p_t_i_o_n_-_v_a_l_u_e

For  example,  to  create  a  stand-alone   executable  that  starts  by
executing main/0  and for which  the source  is loaded through  load.pl,
use the command

% pl --goal=main --stand_alone=true -o myprog -c load.pl

This performs exactly the same as executing

% pl
<banner>
?- [load].
?- qsave_program(myprog,
                 [ goal(main),
                   stand_alone(true)
                 ]).
?- halt.


22..1111 EEnnvviirroonnmmeenntt CCoonnttrrooll ((PPrroolloogg ffllaaggss))

The  predicates  current_prolog_flag/2 and  set_prolog_flag/2 allow  the
user  to examine  and modify  the execution  environment.   It  provides
access  to whether  optional  features are  available on  this  version,
operating  system,  foreign-code  environment,  command-line  arguments,
version,  as well  as runtime  flags to  control  the runtime  behaviour
of  certain  predicates  to  achieve  compatibility  with  other  Prolog
environments.


ccuurrrreenntt__pprroolloogg__ffllaagg((_?_K_e_y_, _-_V_a_l_u_e))

    The   predicate  current_prolog_flag/2  defines   an  interface   to
    installation   features:    options  compiled  in,  version,   home,
    etc.    With both arguments  unbound, it  will generate all  defined
    prolog-flags.   With  the `Key' instantiated  it unify the value  of
    the  prolog-flag.  Flag values are typed.  Flags marked  as bool can
    have  the values true and false.  Some prolog flags are  not defined
    in  all versions, which is  normally indicated in the  documentation
    below  as _`_`_i_f _p_r_e_s_e_n_t  _a_n_d _t_r_u_e_'_'.   A boolean prolog-flag is  true
    iff  the prolog-flag  is present  aanndd the  _V_a_l_u_e is  the atom  true.
    Tests for such flags should be written as below.

            (   current_prolog_flag(windows, true)
            ->  <Do MS-Windows things>
            ;   <Do normal things>
            )

    aabboorrtt__wwiitthh__eexxcceeppttiioonn _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         Determines how  abort/0 is realised.    See the description  of
         abort/0 for details.

    aaggcc__mmaarrggiinn _(_i_n_t_e_g_e_r_, _c_h_a_n_g_e_a_b_l_e_)
         If  this amount  of  atoms  has  been created  since  the  last
         atom-garbage collection,  perform  atom garbage  collection  at
         the  first  opportunity.    Initial  value  is  10,000.     May
         be  changed.    A  value  of 0  (zero)  disables  atom  garbage
         collection.  See also PL_register_atom().

    aallllooww__vvaarriiaabbllee__nnaammee__aass__ffuunnccttoorr _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If true  (default  is false),  Functor(arg) is  read  as if  it
         was written 'Functor'(arg).   Some applications use the  Prolog
         read/1  predicate for  reading  an application  defined  script
         language.   In these  cases, it is  often difficult to  explain
         to  non-Prolog users  of  the  application that  constants  and
         functions can only  start with a  lowercase letter.   Variables
         can be  turned into atoms  starting with  an uppercase atom  by
         calling read_term/2 using the option variable_names and binding
         the variables to their name.   Using this feature, F(x)  can be
         turned into valid syntax for such script languages.   Suggested
         by Robert van Engelen.  SWI-Prolog specific.

    aarrggvv _(_l_i_s_t_)
         List  is  a   list  of  atoms  representing  the   command-line
         arguments used  to invoke  SWI-Prolog.   Please  note that  aallll
         arguments are included in the list returned.

    aarrcchh _(_a_t_o_m_)
         Identifier for  the  hardware and  operating system  SWI-Prolog
         is running  on.   Used to  select foreign files  for the  right
         architecture.  See also section 9.4 and file_search_path/2.

    aassssoocciiaattee _(_a_t_o_m_, _c_h_a_n_g_e_a_b_l_e_)
         On  Windows systems,  this  is  set to  the  filename-extension
         (e.g. pl or pro associated with plwin.exe.

    aauuttoollooaadd _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If true (default) autoloading of library functions  is enabled.
         Note that  autoloading only works  if the  flag unknown is  _n_o_t
         set to fail.  See section 2.13.

    bbaacckkqquuootteedd__ssttrriinngg _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If  true   (default  false),   read  translates  text   between
         backquotes into a string object (see section 4.23).   This flag
         is mainly for compatibility to LPA Prolog.

    bboouunnddeedd _(_b_o_o_l_)
         ISO prolog-flag.    If true,  integer  representation is  bound
         by  min_integer  and max_integer.    If  false integers  can  be
         arbitrary large  and  the min_integer and  max_integer are  not
         present.  See section 4.26.2.1.

    cc__cccc _(_a_t_o_m_)
         Name of the  C-compiler used to  compile SWI-Prolog.   Normally
         either gcc or cc.  See section 9.7.

    cc__llddffllaaggss _(_a_t_o_m_)
         Special  linker  flags  passed   to  link  SWI-Prolog.      See
         section 9.7.

    cc__lliibbss _(_a_t_o_m_)
         Libraries passed  to the C-linker  when SWI-Prolog was  linked.
         May  be  used to  determine  the  libraries  needed  to  create
         statically linked extensions for SWI-Prolog.  See section 9.7.

    cchhaarr__ccoonnvveerrssiioonn _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         Determines  whether  character-conversion  takes   place  while
         reading terms.  See also char_conversion/2.

    cchhaarraacctteerr__eessccaappeess _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If true  (default),  read/1 interprets  \  escape sequences  in
         quoted atoms and strings.  May be changed.   This flag is local
         to the module in which it is changed.

    ccoommppiilleedd__aatt _(_a_t_o_m_)
         Describes when the  system has been  compiled.  Only  available
         if  the C-compiler  used  to  compile SWI-Prolog  provides  the
         __DATE__and __TIME__macros.

    ccoonnssoollee__mmeennuu _(_b_o_o_l_)
         Set  to true  in plwin.exe  to  indicate the  console  supports
         menus.  See also section 4.34.2.

    ddddee _(_b_o_o_l_)
         Set  to  true  if this  instance  of  Prolog  supports  DDE  as
         described in section 4.42.

    ddeebbuugg _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         Switch debugging  mode  on/off.   If  debug  mode is  activated
         the  system  traps  encountered  spy-points  (see   spy/1)  and
         trace-points  (see  trace/1).     In  addition,  tail-recursion
         optimisation is  disabled and the  system is more  conservative
         in destroying choice-points to simplify debugging.

         Disabling these optimisations can  cause the system to run  out
         of memory on  programs that behave  correctly if debug mode  is
         off.

    ddeebbuugg__oonn__eerrrroorr _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If  true,  start  the  tracer  after   an  error  is  detected.
         Otherwise just continue  execution.   The goal that raised  the
         error  will normally  fail.    See  also fileerrors/2  and  the
         prolog-flag report_error.   May be changed.   Default is  true,
         except for the runtime version.

    ddeebbuuggggeerr__pprriinntt__ooppttiioonnss _(_t_e_r_m_, _c_h_a_n_g_e_a_b_l_e_)
         This  argument is  given  as  option-list to  write_term/2  for
         printing  goals  by  the  debugger.     Modified  by  the  `w',
         `p'  and  `<_N>  d'  commands  of  the  debugger.    Default  is
         [quoted(true), portray(true), max_depth(10), attributes(portray)].

    ddeebbuuggggeerr__sshhooww__ccoonntteexxtt _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If true, show  the context module while printing a  stack-frame
         in the  tracer.   Normally controlled using  the `C' option  of
         the tracer.

    ddoouubbllee__qquuootteess _(_c_o_d_e_s_,_c_h_a_r_s_,_a_t_o_m_,_s_t_r_i_n_g_, _c_h_a_n_g_e_a_b_l_e_)
         This flag  determines  how double-quotes  strings are  read  by
         Prolog and is ---like  character_escapes--- maintained for  each
         module.    If codes  (default), a  list  of character-codes  is
         returned,  if chars  a list  of  one-character atoms,  if  atom
         double  quotes  are the  same  as  single-quotes  and  finally,
         string reads the text into a Prolog string  (see section 4.23).
         See also atom_chars/2 and atom_codes/2.

    ddyynnaammiicc__ssttaacckkss _(_b_o_o_l_)
         If true,  the system uses some  form of `sparse-memory  manage-
         ment' to realise the stacks.  If  false, malloc()/realloc() are
         used for  the stacks.   In earlier  days this had  consequences
         for foreign code.   As  of version 2.5,  this is no longer  the
         case.

         Systems  using  `sparse-memory management'  are  a  bit  faster
         as  there  is  no  stack-shifter.     On   most  systems  using
         sparse-memory management  memory  is actually  returned to  the
         system  after a  garbage collection  or  call to  trim_stacks/0
         (called by prolog/0 after finishing a user-query).

    eeddiittoorr _(_a_t_o_m_, _c_h_a_n_g_e_a_b_l_e_)
         Determines the  editor used  by edit/1.   See  section 4.4  for
         details on selecting the editor used.

    eemmaaccss__iinnffeerriioorr__pprroocceessss _(_b_o_o_l_)
         If  true,  SWI-Prolog is  running  as an  _i_n_f_e_r_i_o_r  _p_r_o_c_e_s_s  of
         (GNU/X-)Emacs.   SWI-Prolog  assumes this  is the  case if  the
         environment variable EMACS is t and INFERIOR is yes.

    eennccooddiinngg _(_a_t_o_m_, _c_h_a_n_g_e_a_b_l_e_)
         Default  encoding  used   for  opening  files  in  text   mode.
         The  initial value  is  deduced  from  the environment.     See
         section 2.17.1 for details.

    eexxeeccuuttaabbllee _(_a_t_o_m_)
         Path-name of the  running executable.  Used  by qsave_program/2
         as default emulator.

    ffiillee__nnaammee__vvaarriiaabblleess _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If   true  (default   false),   expand   $varname  and   ~   in
         arguments  of  built-in predicates  that  accept  a  file  name
         (open/3, exists_file/1,  access_file/2,  etc.).   The  predicate
         expand_file_name/2  should  be  used   to  expand   environment
         variables and wildcard patterns.  This  prolog-flag is intended
         for backward compatibility with older versions of SWI-Prolog.

    ffllooaatt__ffoorrmmaatt _(_a_t_o_m_, _c_h_a_n_g_e_a_b_l_e_)
         C-library printf()  format  specification used  by write/1  and
         friends to  determine how floating  point numbers are  printed.
         The default is %g.   The specified value is passed  to printf()
         without  further checking.    For  example,  if you  want  more
         digits printed,  %.12g will  print all floats  using 12  digits
         instead of the default 6.

         When using  quoted-write, the output  is guaranteed to  contain
         a decimal  dot or exponent,  so read/1  reads a floating  point
         number.  See also format/[1,2], write_term/[2,3].

    ggcc _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If true (default), the garbage collector is active.   If false,
         neither garbage-collection,  nor stack-shifts will take  place,
         even not on explicit request.  May be changed.

    ggeenneerraattee__ddeebbuugg__iinnffoo _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If true  (default) generate  code  that can  be debugged  using
         trace/0,  spy/1,  etc.      Can  be  set  to  false  using  the
         -nodebug.   The  predicate load_files/2 restores  the value  of
         this  flag  after loading  a  file,  causing  modifications  to
         be  local to  a  source  file.    Many of  the  libraries  have
         :- set_prolog_flag(generate_debug_info, false)  to  hide  their
         details from a normal trace.

    ggmmpp__vveerrssiioonn _(_i_n_t_e_g_e_r_)
         If  Prolog is  linked  with  GMP,  this flag  gives  the  major
         version of the GMP library used.  See also section 9.6.7.

    ggnnuu__lliibbpptthhrreeaadd__vveerrssiioonn _(_a_t_o_m_)
         Linux systems only.   Reports the  version of the Linux  thread
         library used.  See section 8.2.1 for how it may affect you.

    gguuii _(_b_o_o_l_)
         Set to true if XPCE is around and can be used for graphics.

    hhiissttoorryy _(_i_n_t_e_g_e_r_, _c_h_a_n_g_e_a_b_l_e_)
         If _i_n_t_e_g_e_r >0,  support Unix csh(1)  like history as described
         in section  2.7.    Otherwise,  only support  reusing  commands
         through the command-line  editor.  The  default is to set  this
         prolog-flag to  0 if  a  command-line editor  is provided  (see
         prolog-flag readline) and 15 otherwise.

    hhoommee _(_a_t_o_m_)
         SWI-Prolog's  notion  of   the  home-directory.      SWI-Prolog
         uses  its  home   directory  to  find   its  startup  file   as
         <_h_o_m_e>/boot32.prc(32-bit machines) or <_h_o_m_e>/boot64.prc (64-bit
         machines) and to find its library as <_h_o_m_e>/library.

    hhwwnndd _(_i_n_t_e_g_e_r_)
         In plwin.exe,  this refers to  the MS-Windows window-handle  of
         the console window.

    iinntteeggeerr__rroouunnddiinngg__ffuunnccttiioonn _(_d_o_w_n_,_t_o_w_a_r_d___z_e_r_o_)
         ISO prolog-flag  describing rounding by  // and rem  arithmetic
         functions.  Value depends on the C-compiler used.

    iissoo _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         Include some  weird ISO compatibility  that is incompatible  to
         normal SWI-Prolog behaviour.   Currently  it has the  following
         effect:

           o The  //2 (float division)  _a_l_w_a_y_s return a  float, even  if
             applied to integers that can be divided.

           o In  the standard order  of terms (see  section 4.6.1),  all
             floats are before all integers.

           o atom_length/2 yields  an instantiation  error if the  first
             argument is a number.

           o clause/[2,3]  raises  a  permission  error  when  accessing
             static predicates.

           o abolish/[1,2]  raises  a permission  error  when  accessing
             static predicates.

    llaarrggee__ffiilleess _(_b_o_o_l_)
         If present and  true, SWI-Prolog has  been compiled with  _l_a_r_g_e
         _f_i_l_e _s_u_p_p_o_r_t (LFS) and  is capable to access files  larger than
         2GB on  32-bit  hardware.   Large  file-support  is default  on
         installations built using configure that support it  and may be
         switched off using the configure option --disable-largefile.

    mmaaxx__aarriittyy _(_u_n_b_o_u_n_d_e_d_)
         ISO  prolog-flag  describing  there  is  no  maximum  arity  to
         compound terms.

    mmaaxx__iinntteeggeerr _(_i_n_t_e_g_e_r_)
         Maximum integer value  if integers are _b_o_u_n_d_e_d.   See also  the
         flag bounded and section 4.26.2.1.

    mmaaxx__ttaaggggeedd__iinntteeggeerr _(_i_n_t_e_g_e_r_)
         Maximum integer value represented as a `tagged' value.   Tagged
         integers  require  1  word  storage.      Larger  integers  are
         represented as `indirect  data' and require significantly  more
         space.

    mmiinn__iinntteeggeerr _(_i_n_t_e_g_e_r_)
         Minimum integer value  if integers are _b_o_u_n_d_e_d.   See also  the
         flag bounded and section 4.26.2.1.

    mmiinn__ttaaggggeedd__iinntteeggeerr _(_i_n_t_e_g_e_r_)
         Start of the tagged-integer value range.

    ooppeenn__sshhaarreedd__oobbjjeecctt _(_b_o_o_l_)
         If  true,  open_shared_object/2 and  friends  are  implemented,
         providing access  to shared  libraries (.so  files) or  dynamic
         link libraries (.DLL files).

    ooppttiimmiissee _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If true, compile in optimised mode.  The initial  value is true
         if Prolog was started with the -O command-line option.

         Currently   optimise   compilation   implies   compilation   of
         arithmetic, and  deletion of redundant  true/0 that may  result
         from expand_goal/2.

         Later versions might imply various other optimisations  such as
         integrating small  predicates into  their callers,  eliminating
         constant expressions and other predictable constructs.   Source
         code  optimisation is  never  applied  to predicates  that  are
         declared dynamic (see dynamic/1).

    ppiidd _(_i_n_t_)
         Process identifier of  the running Prolog  process.   Existence
         of this flag is implementation dependent.

    ppiippee _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If  true,  open(pipe(command), mode, Stream),   etc.  are  sup-
         ported.    Can  be  changed to  disable  the use  of  pipes  in
         applications testing this feature.  Not recommended.

    pprroommpptt__aalltteerrnnaattiivveess__nnoo__bbiinnddiinnggss _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If present and true,  the top-level prints for alternatives  if
         the query  succeeded with pending  choice-point, regardless  of
         whether or  not  the query  has variables.    As  there are  no
         variables to  bind it  prints the  answer _M_o_r_e_?.    Here is  an
         example

         ?- set_prolog_flag(prompt_alternatives_no_bindings, true).
         ?- (write(hello);write(world)).
         hello
         More? ;
         world

    rreeaaddlliinnee _(_b_o_o_l_)
         If  true,  SWI-Prolog  is linked  with  the  readline  library.
         This is  done by  default if  you have  this library  installed
         on your  system.    It is  also true  for  the Win32  plwin.exe
         version of SWI-Prolog, which realises a subset  of the readline
         functionality.

    rreessoouurrccee__ddaattaabbaassee _(_a_t_o_m_)
         Set to the absolute-filename of the attached state.   Typically
         this is the file boot32.prc, the file specified with  -x or the
         running executable.  See also resource/3.

    rreeppoorrtt__eerrrroorr _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If true, print  error messages, otherwise  suppress them.   May
         be changed.  See also  the debug_on_error prolog-flag.   Default
         is true, except for the runtime version.

    rruunnttiimmee _(_b_o_o_l_)
         If present and  true, SWI-Prolog is  compiled with -DO_RUNTIME,
         disabling various  useful development  features (currently  the
         tracer and profiler).

    ssaavveedd__pprrooggrraamm _(_b_o_o_l_)
         If present and true, Prolog is started from a  state saved with
         qsave_program/[1,2].

    sshhaarreedd__oobbjjeecctt__eexxtteennssiioonn _(_a_t_o_m_)
         Extension used  by  the operating  system for  shared  objects.
         .so for  most Unix  systems and  .dll for  Windows.   Used  for
         locating  files using  the  file_type  executable.    See  also
         absolute_file_name/3.

    sshhaarreedd__oobbjjeecctt__sseeaarrcchh__ppaatthh _(_a_t_o_m_)
         Name of the environment  variable used by the system  to search
         for shared objects.

    ssiiggnnaallss _(_b_o_o_l_)
         Determine  whether   Prolog  is   handling  signals   (software
         interrupts).   This flag is  false if the  hosting OS does  not
         support signal handling  or the command-line option  -nosignals
         is active.  See section 9.6.20.1 for details.

    ssyysstteemm__tthhrreeaadd__iidd _(_i_n_t_)
         On MT systems (section 8, refers to  the thread-identifier used
         by the system for the calling thread.  See also thread_self/1.

    ttaaiill__rreeccuurrssiioonn__ooppttiimmiissaattiioonn _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         Determines  whether  or  not  tail-recursion   optimisation  is
         enabled.     Normally the  value  of  this  flag  is  equal  to
         the  debug  flag.      As  programs   may  run  out  of   stack
         if  tail-recursion optimisation  is  omitted, it  is  sometimes
         necessary to enable it during debugging.

    ttiimmeezzoonnee _(_i_n_t_e_g_e_r_)
         Offset in seconds  west of GMT of  the current time-zone.   Set
         at initialization  time from the  timezone variable  associated
         with the POSIX tzset() function.  See also convert_time/2.

    ttoopplleevveell__pprriinntt__aannoonn _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If true,  top-level variables starting  with an  underscore (_)
         are printed normally.   If false they are hidden.  This  may be
         used to hide bindings in complex queries from the top-level.

    ttoopplleevveell__pprriinntt__ooppttiioonnss _(_t_e_r_m_, _c_h_a_n_g_e_a_b_l_e_)
         This  argument   is  given   as  option-list   to  write_term/2
         for   printing    results   of   queries.         Default    is
         [quoted(true), portray(true), max_depth(10), attributes(portray)].

    ttoopplleevveell__vvaarr__ssiizzee _(_i_n_t_, _c_h_a_n_g_e_a_b_l_e_)
         Maximum  size counted  in  literals of  a  term returned  as  a
         binding for a variable  in a top-level query that is  saved for
         re-use using the $ variable reference.  See section 2.8.

    ttrraaccee__ggcc _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If  true  (false  is  the  default),  garbage  collections  and
         stack-shifts  will  be  reported on  the  terminal.     May  be
         changed.

    ttttyy__ccoonnttrrooll _(_b_o_o_l_)
         Determines whether  the terminal  is switched to  raw mode  for
         get_single_char/1,  which also  reads the  user-actions for  the
         trace.  May be set.  See also the +/-tty command-line option.

    uunniixx _(_b_o_o_l_)
         If present and  true, the operating  system is some version  of
         Unix.  Defined  if the C-compiler used to compile  this version
         of  SWI-Prolog either  defines  __unix__ or  unix.    On  other
         systems this flag is not available.

    uunnkknnoowwnn _(_f_a_i_l_,_w_a_r_n_i_n_g_,_e_r_r_o_r_, _c_h_a_n_g_e_a_b_l_e_)
         Determines  the   behaviour  if  an   undefined  procedure   is
         encountered.   If  fail,  the predicates  fails silently.    If
         warn,  a warning  is printed,  and  execution continues  as  if
         the  predicate was  not  defined  and if  error  (default),  an
         existence_error exception is  raised.   This flag  is local  to
         each module.  Switching this flag to  fail disables autoloading
         and thus forces complete and consistent use of use_module/[1,2]
         to load the required libraries.

    vveerrbboossee _(_A_t_o_m_, _c_h_a_n_g_e_a_b_l_e_)
         This flags is used by print_message/2.  If its value is silent,
         messages of type informational and banner are suppressed.   The
         -q switches the value from the initial normal to silent.

    vveerrbboossee__aauuttoollooaadd _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If  true the  normal  consult  message  will be  printed  if  a
         library is autoloaded.  By default this  message is suppressed.
         Intended to be used for debugging purposes.

    vveerrbboossee__llooaadd _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If false normal consult  messages will be suppressed.   Default
         is true.  The value of this flag is normally  controlled by the
         option silent(_B_o_o_l) privided by load_files/2.

    vveerrbboossee__ffiillee__sseeaarrcchh _(_b_o_o_l_, _c_h_a_n_g_e_a_b_l_e_)
         If  true  (default   false),  print  messages  indicating   the
         progress   of  absolute_file_name/[2,3]  in   locating   files.
         Intended for  debugging  complicated file-search  paths.    See
         also file_search_path/2.

    vveerrssiioonn _(_i_n_t_e_g_e_r_)
         The version identifier is an integer with value:
                          10000_*Major+ 100_*Minor+_P_a_t_c_h

         Note that  in releases  up to 2.7.10  this prolog-flag  yielded
         an  atom   holding  the  three   numbers  separated  by   dots.
         The  current representation  is  much easier  for  implementing
         version-conditional statements.

    wwiinnddoowwss _(_b_o_o_l_)
         If present and true, the operating system  is an implementation
         of Microsoft  Windows (NT/2000/XP, etc.).    This flag is  only
         available on MS-Windows based versions.

    wwrriittee__aattttrriibbuutteess _(_a_t_o_m_, _c_h_a_n_g_e_a_b_l_e_)
         Defines  how write/1  an  friends write  attributed  variables.
         The option values are  described with the attributes option  of
         write_term/3.  Default is ignore.

    wwrriittee__hheellpp__wwiitthh__oovveerrssttrriikkee _(_b_o_o_l_)
         Internal flag used  by help/1 when writing  to a terminal.   If
         present  and true  it prints  bold  and underlined  text  using
         _o_v_e_r_s_t_r_i_k_e.

    xxppccee _(_b_o_o_l_)
         Available  and set  to  true if  the  XPCE graphics  system  is
         loaded.

    xxppccee__vveerrssiioonn _(_a_t_o_m_)
         Available and set to the version of the loaded XPCE system.


sseett__pprroolloogg__ffllaagg((_+_K_e_y_, _+_V_a_l_u_e))
    Define  a new prolog-flag or change its value.  _K_e_y is an atom.   If
    the  flag is  a system-defined  flag that is  not marked  _c_h_a_n_g_e_a_b_l_e
    above,  an  attempt to  modify the flag  yields a  permission_error.
    If  the  provided _V_a_l_u_e  does not  match  the type  of the  flag,  a
    type_error is raised.

    In  addition  to  ISO,  SWI-Prolog allows  for  user-defined  prolog
    flags.   The type of the  flag is determined from the  initial value
    and cannot be changed afterwards.


22..1122 AAnn oovveerrvviieeww ooff hhooookk pprreeddiiccaatteess

SWI-Prolog provides a large number of hooks, mainly  to control handling
messages, debugging,  startup, shut-down, macro-expansion,  etc.   Below
is  a  summary  of  all  defined  hooks  with  an  indication  of  their
portability.

  o _p_o_r_t_r_a_y_/_1
    Hook into write_term/3 to alter the way terms are printed (ISO).

  o _m_e_s_s_a_g_e___h_o_o_k_/_3
    Hook  into  print_message/2 to  alter the  way system  messages  are
    printed (Quintus/SICStus).

  o _l_i_b_r_a_r_y___d_i_r_e_c_t_o_r_y_/_1
    Hook  into absolute_file_name/3 to define  new library  directories.
    (most Prolog system).

  o _f_i_l_e___s_e_a_r_c_h___p_a_t_h_/_2
    Hook   into   absolute_file_name/3  to   define   new   search-paths
    (Quintus/SICStus).

  o _t_e_r_m___e_x_p_a_n_s_i_o_n_/_2
    Hook  into  load_files/2  to  modify  read  terms  before  they  are
    compiled (macro-processing) (most Prolog system).

  o _g_o_a_l___e_x_p_a_n_s_i_o_n_/_2
    Same as term_expansion/2 for individual goals (SICStus).

  o _p_r_o_l_o_g___l_o_a_d___f_i_l_e_/_2
    Hook  into  load_files/2  to  load  other  data-formats  for  Prolog
    sources  from `non-file' resources.   The load_files/2 predicate  is
    the ancestor of consult/1, use_module/1, etc.

  o _p_r_o_l_o_g___e_d_i_t_:_l_o_c_a_t_e_/_3
    Hook into edit/1 to locate objects (SWI).

  o _p_r_o_l_o_g___e_d_i_t_:_e_d_i_t___s_o_u_r_c_e_/_1
    Hook into edit/1 to call some internal editor (SWI).

  o _p_r_o_l_o_g___e_d_i_t_:_e_d_i_t___c_o_m_m_a_n_d_/_2
    Hook into edit/1 to define the external editor to use (SWI).

  o _p_r_o_l_o_g___l_i_s_t___g_o_a_l_/_1
    Hook  into the tracer  to list the  code associated to a  particular
    goal (SWI).

  o _p_r_o_l_o_g___t_r_a_c_e___i_n_t_e_r_c_e_p_t_i_o_n_/_4
    Hook into the tracer to handle trace-events (SWI).

  o _p_r_o_l_o_g_:_d_e_b_u_g___c_o_n_t_r_o_l___h_o_o_k_/_1
    Hook  in spy/1, nospy/1, nospyall/0 and debugging/0 to  extend these
    control-predicates to higher-level libraries.

  o _p_r_o_l_o_g_:_h_e_l_p___h_o_o_k_/_1
    Hook in help/0, help/1 and apropos/1 to extend the help-system.

  o _r_e_s_o_u_r_c_e_/_3
    Defines a new resource (not really a hook, but similar) (SWI).

  o _e_x_c_e_p_t_i_o_n_/_3
    Old  attempt  to  a  generic  hook mechanism.     Handles  undefined
    predicates (SWI).

  o _a_t_t_r___u_n_i_f_y___h_o_o_k_/_2
    Unification  hook for attributed variables.   Can be defined in  any
    module.  See section 6.1 for details.


22..1133 AAuuttoommaattiicc llooaaddiinngg ooff lliibbrraarriieess

If ---at  runtime--- an undefined predicate  is trapped the system  will
first try  to import  the predicate  from the  module's default  module.
If  this fails  the  _a_u_t_o _l_o_a_d_e_r  is activated.    On  first  activation
an  index to  all library  files in  all library  directories is  loaded
in  core  (see  library_directory/1 and  file_search_path/2).     If  the
undefined predicate  can be  located in  the one of  the libraries  that
library file  is automatically loaded  and the  call to the  (previously
undefined) predicate is restarted.  By default this  mechanism loads the
file silently.   The  current_prolog_flag/2verbose_autoload is  provided
to  get verbose  loading.    The prolog-flag  autoload  can be  used  to
enable/disable the entire auto load system.

The auto-loader only  works if the unknown  flag (see unknown/2) is  set
to  trace (default).    A more  appropriate interaction  with this  flag
should be considered.

Autoloading  only handles  (library) source  files that  use the  module
mechanism  described  in  chapter  5.     The  files   are  loaded  with
use_module/2 and only the  trapped undefined predicate will be  imported
to the module  where the undefined predicate  was called.  Each  library
directory  must hold  a file  INDEX.pl  that contains  an index  to  all
library files  in the directory.    This file consists  of lines of  the
following format:

index(Name, Arity, Module, File).

The  predicate make/0  updates the  autoload  index.   It  searches  for
all library directories (see library_directory/1 and file_search_path/2)
holding the file MKINDEX.pl or INDEX.pl.  If the  current user can write
or create  the file  INDEX.pl and  it does not  exist or  is older  than
the directory  or one  of its  files, the  index for  this directory  is
updated.  If the file MKINDEX.pl exists updating  is achieved by loading
this file, normally containing a directive calling make_library_index/2.
Otherwise make_library_index/1is  called, creating an index for all *.pl
files containing a module.

Below is an example creating a completely indexed library directory.

% mkdir ~/lib/prolog
% cd !$
% pl -g true -t 'make_library_index(.)'

If  there  are  more than  one  library  files  containing  the  desired
predicate the following search schema is followed:

 1. If  there is a  library file  that defines the  module in which  the
    undefined predicate is trapped, this file is used.

 2. Otherwise  library files  are considered  in the  order they  appear
    in  the  library_directory/1  predicate  and  within  the  directory
    alphabetically.


mmaakkee__lliibbrraarryy__iinnddeexx((_+_D_i_r_e_c_t_o_r_y))
    Create  an index for this  directory.  The  index is written to  the
    file  'INDEX.pl' in the specified directory.   Fails with a  warning
    if the directory does not exist or is write protected.


mmaakkee__lliibbrraarryy__iinnddeexx((_+_D_i_r_e_c_t_o_r_y_, _+_L_i_s_t_O_f_P_a_t_t_e_r_n_s))
    Normally  used in  MKINDEX.pl, this  predicate creates INDEX.pl  for
    _D_i_r_e_c_t_o_r_y,  indexing all files that  match one of the  file-patterns
    in _L_i_s_t_O_f_P_a_t_t_e_r_n_s.

    Sometimes  library packages consist  of one public  load file and  a
    number  of files used by  this load-file, exporting predicates  that
    should not be used  directly by the end-user.  Such a library can be
    placed  in a sub-directory of  the library and the files  containing
    public  functionality can  be  added to  the index  of the  library.
    As  an  example we  give the  XPCE library's  MKINDEX.pl,  including
    the  public  functionality of  trace/browse.pl to  the  autoloadable
    predicates for the XPCE package.

    :- make_library_index('.',
                          [ '*.pl',
                            'trace/browse.pl'
                          ]).


rreellooaadd__lliibbrraarryy__iinnddeexx
    Force  reloading  the  index  after modifying  the  set  of  library
    directories   by   changing  the   rules  for   library_directory/1,
    file_search_path/2,  adding  or  deleting  INDEX.pl  files.     This
    predicate   does   _n_o_t   update  the   INDEX.pl   files.       Check
    make_library_index/[1,2] and make/0 for updating the index files.

    Normally, the index  is reloaded automatically if a predicate cannot
    be  found  in the  index  and the  set  of library  directories  has
    changed.   Using reload_library_index/0 is necessary if  directories
    are removed or the order of the library directories is changed.


22..1144 GGaarrbbaaggee CCoolllleeccttiioonn

SWI-Prolog provides garbage-collection, last-call optimization  and atom
garbage collection.   These features  are controlled using prolog  flags
(see current_prolog_flag/2).


22..1155 SSyynnttaaxx NNootteess

SWI-Prolog uses ISO-Prolog standard syntax, which  is closely compatible
to Edinburgh Prolog syntax.   A description of this syntax can  be found
in the  Prolog books  referenced in the  introduction.   Below are  some
non-standard or non-common constructs that are accepted by SWI-Prolog:

  o 0'<_c_h_a_r>
    This  construct is not accepted by all Prolog systems that  claim to
    have  Edinburgh compatible syntax.  It describes the  character code
    of  <_c_h_a_r>.   To test  whether C is  a lower case  character one can
    use between(0'a, 0'z, C).   If character codes are enabled (default)
    <_c_h_a_r>  can use \  escape sequences.   The  sequence 0'\t represents
    the tab character using symbolic notation.

  o /* .../* ...*/ ...*/
    The  /* ...*/  comment statement  can be  nested.    This is  useful
    if  some  code with  /* ...*/  comment statements  in it  should  be
    commented out.


22..1155..11 IISSOO SSyynnttaaxx SSuuppppoorrtt

SWI-Prolog offers ISO compatible extensions to the Edinburgh syntax.


22..1155..11..11 PPrroocceessssoorr CChhaarraacctteerr SSeett

The processor character  set specifies the class of each  character used
for parsing Prolog  source text.   Character classification is fixed  to
use UCS/Unicode as  provided by the C-library  wchar_t based  primitives.
See also section 2.17.


22..1155..11..22 CChhaarraacctteerr EEssccaappee SSyynnttaaxx

Within quoted  atoms (using single quotes:   '<_a_t_o_m>'special  characters
are represented  using escape-sequences.    An escape  sequence is  lead
in  by the  backslash  (\) character.    The  list of  escape  sequences
is compatible  with the  ISO standard,  but contains  one extension  and
the interpretation of numerically specified characters  is slightly more
flexible to improve compatibility.

\a
    Alert character.  Normally the ASCII character 7 (beep).

\b
    Backspace character.

\c
    No  output.    All input  characters  up to  but not  including  the
    first  non-layout  character  are skipped.     This allows  for  the
    specification of pretty-looking  long lines.  For compatibility with
    Quintus Prolog.  Not supported by ISO. Example:

    format('This is a long line that would look better if it was \c
           split across multiple physical lines in the input')

\<RETURN>
    No  output.   Skips input till the  next non-layout character or  to
    the end of the next line.  Same intention as \c but ISO compatible.

\f
    Form-feed character.

\n
    Next-line character.

\r
    Carriage-return only (i.e. go back to the start of the line).

\t
    Horizontal tab-character.

\v
    Vertical tab-character (ASCII 11).

\xXX..\
    Hexadecimal   specification  of  a  character.      The  closing   \
    is   obligatory  according  to  the   ISO  standard,  but   optional
    in  SWI-Prolog  to  enhance  compatibility to  the  older  Edinburgh
    standard.   The code \xa\3 emits the character 10  (hexadecimal `a')
    followed  by `3'.  Characters specified this way are  interpreted as
    Unicode characters.  See also \u.

\uXXXX
    Unicode  character specification  where the  character is  specified
    using  _e_x_a_c_t_l_y 4 hexadecimal  digits.  This  is an extension to  the
    ISO  standard fixing two problems.   First of all, where  \x defines
    a  numeric character code, it  doesn't specify the character set  in
    which  the  character should  be interpreted.    Second,  it is  not
    needed to use the idiosyncratic closing \ ISO Prolog syntax.

\UXXXXXXXX
    Same as \uXXXX, but using 8 digits to cover the whole Unicode set.

\40
    Octal   character  specification.     The  rules  and   remarks  for
    hexadecimal specifications apply to octal specifications as well.

\<_c_h_a_r_a_c_t_e_r>
    Any  character immediately preceded  by a \  and not covered by  the
    above  escape sequences is copied verbatim.   Thus, '\\' is  an atom
    consisting  of a single \ and  '\'' and '''' both describe the  atom
    with a single '.

Character      escaping      is     only      available      if      the
current_prolog_flag(character_escapes, true) is active (default).    See
current_prolog_flag/2.   Character escapes conflict with writef/2  in two
ways:   \40  is interpreted  as decimal  40 by  writef/2, but  character
escapes  handling by  read has  already interpreted  as  32 (40  octal).
Also, \l is translated to  a single `l'.  It is advised to use  the more
widely supported  format/[2,3] predicate instead.    If you insist  upon
using writef/2, either switch character_escapes to false, or  use double
\\, as in writef('\\l').


22..1155..11..33 SSyynnttaaxx ffoorr nnoonn--ddeecciimmaall nnuummbbeerrss

SWI-Prolog  implements  both  Edinburgh  and  ISO   representations  for
non-decimal numbers.   According to  Edinburgh syntax, such numbers  are
written as <_r_a_d_i_x>'<_n_u_m_b_e_r>, where <_r_a_d_i_x> is  a number between 2 and 36.
ISO defines binary,  octal and hexadecimal numbers using 0[bxo]<_n_u_m_b_e_r>.
For example:  A is 0b100 \/ 0xf00  is a valid expression.   Such numbers
are always unsigned.


22..1155..11..44 UUnniiccooddee PPrroolloogg ssoouurrccee

The ISO standard  specifies the Prolog syntax  in ASCII characters.   As
SWI-Prolog supports Unicode  in source files we must extend  the syntax.
This  section describes  the implication  for  the source  files,  while
writing international source files is described in section 3.1.3.

The  SWI-Prolog Unicode  character classification  is  based on  version
4.1.0 of  the Unicode standard.    Please that char_type/2 and  friends,
intended to be used with all text except Prolog source  code is based on
the C-library locale-based classification routines.

  o _Q_u_o_t_e_d _a_t_o_m_s _a_n_d _s_t_r_i_n_g_s
    Any  character  of  any script  can  be  used in  quoted  atoms  and
    strings.      The  escape  sequences  \uXXXX  and   \UXXXXXXXX  (see
    section 2.15.1.2) were  introduced to specify Unicode code points in
    ASCII files.

  o _A_t_o_m_s _a_n_d _V_a_r_i_a_b_l_e_s
    We  handle  them in  one item  as they  are closely  related.    The
    Unicode  standard  defines  a  syntax for  identifiers  in  computer
    languages.   In this syntax identifiers start with ID_Start followed
    by  a sequence of ID_Continue codes.  Such sequences are  handled as
    a  single token  in SWI-Prolog.    The token  is a  _v_a_r_i_a_b_l_e iff  it
    starts with  an uppercase character or an underscore (_).  Otherwise
    it is an atom.   Note that many languages do not not have the notion
    of  character-case.  In such languages variables _m_u_s_t be  written as
    _name.

  o _W_h_i_t_e _s_p_a_c_e
    All  characters  marked  as separators  in  the Unicode  tables  are
    handled as layout characters.

  o _O_t_h_e_r _c_h_a_r_a_c_t_e_r_s
    The first 128 characters  follow the ISO Prolog standard.  All other
    characters  not covered  by the  rules above  are considered  `solo'
    characters:   they form  single-character atoms.   We would like  to
    have a more  appropriate distinction between what is known to Prolog
    as `solo' characters and `singleton' characters.


22..1155..11..55 SSiinngglleettoonn vvaarriiaabbllee cchheecckkiinngg

A singleton  variable is  a variable  that appears  only one  time in  a
clause.   It can always be  replaced by _, the  _a_n_o_n_y_m_o_u_s variable.   In
some  cases however  people prefer  to give  the variable  a name.    As
mistyping a variable is a common mistake, Prolog  systems generally give
a  warning (controlled  by style_check/1)  if a  variable is  used  only
once.   The system can  be informed a variable  is known to appear  once
by _s_t_a_r_t_i_n_g  it with  an underscore.    E.g. _Name.    Please note  that
any variable,  except plain _  shares with variables  of the same  name.
The term  t(_X, _X) is equivalent  to t(X, X),  which is _d_i_f_f_e_r_e_n_t  from
t(_, _).

As  Unicode requires  variables  to start  with  an underscore  in  many
languages this  schema needs to be  extended.   First we define the  two
classes of named variables.

  o _N_a_m_e_d _s_i_n_g_l_e_t_o_n _v_a_r_i_a_b_l_e_s
    Named  singletons start with  a double underscore  (__) or a  single
    underscore followed by an uppercase letter.  E.g. __var or _Var.

  o _N_o_r_m_a_l _v_a_r_i_a_b_l_e_s
    All other variables are  `normal' variables.  Note this makes _var a
    normal variable.

Any normal variable appearing  exactly ones in the clause _a_n_d  any named
singleton variables appearing  more than once are  reported.  Below  are
some examples with warnings in the right column.

___________________________________________________________________________
| test(_).      |                                                         |

| test(_a).     |Singleton variables:  [_a]                                 |
| test(A).     |Singleton variables:  [A]                                 |
| test(_A).     |                                                         |
| test(__a).    |                                                         |
| test(_, _).   |                                                         |
| test(_a, _a). |                                                         |
| test(__a, __a).S|ingleton-marked variables appearing more than once:  [__a] |

| test(_A, _A). |Singleton-marked variables appearing more than once:  [_A] |
|_test(A,_A).__|__________________________________________________________|_


22..1166 IInnffiinniittee ttrreeeess ((ccyycclliicc tteerrmmss))

SWI-Prolog  has  limited support  for  infinite  trees,  also  known  as
cyclic  terms.   Full  support  requires special  code in  all  built-in
predicates that require  recursive exploration of a  term.  The  current
version  supports cycles  terms  in  the pure  Prolog  kernel  including
the  garbage  collector  and  in  the  following   predicates:    =../2,
==/2,  =@=/2,  =/2, @</2 , @=</2,  @>=/2,  @>/2,  \==/2,  \=@=/2,  \=/2,
acyclic_term/1, bagof/3,  compare/3, copy_term/2, cyclic_term/1,  dif/2,
duplicate_term/2,  findall/3, ground/1,  hash_term/2,  numbervars/[3,4],
recorda/3,  recordz/3,   setof/3,  term_variables/2,  throw/1,   when/2,
write/1 (incomplete) .


22..1177 WWiiddee cchhaarraacctteerr ssuuppppoorrtt

SWI-Prolog  supports _w_i_d_e  _c_h_a_r_a_c_t_e_r_s, characters  with character  codes
above  255 that  cannot be  represented in  a single  _b_y_t_e.    _U_n_i_v_e_r_s_a_l
_C_h_a_r_a_c_t_e_r  _S_e_t (UCS)  is the  ISO/IEC 10646  standard  that specifies  a
unique  31-bits unsigned  integer  for any  character in  any  language.
It  is a  superset  of  16-bit Unicode,  which  in  turn is  a  superset
of ISO  8859-1 (ISO  Latin-1), a  superset of US-ASCII.  UCS can  handle
strings  holding  characters  from  multiple  languages   and  character
classification (uppercase, lowercase, digit, etc.)   and operations such
as case-conversion are unambiguously defined.

For this reason SWI-Prolog has two representations for  atoms and string
objects (see  section 4.23.   If  the text  fits in ISO  Latin-1, it  is
represented as  an array  of 8-bit characters.    Otherwise the text  is
represented as an array of 32-bit numbers.   This representational issue
is completely  transparent to  the Prolog user.    Users of the  foreign
language interface as described in section 9 sometimes need  to be aware
of these issues though.

Character coding comes into  view when characters of strings need  to be
read from  or written to file  or when they  have to be communicated  to
other software  components using  the foreign  language interface.    In
this section we only deal with I/O through streams,  which includes file
I/O as well as I/O through network sockets.


22..1177..11 WWiiddee cchhaarraacctteerr eennccooddiinnggss oonn ssttrreeaammss

Although  characters   are  uniquely  coded   using  the  UCS   standard
internally, streams  and files are byte  (8-bit) oriented and there  are
a variety of  ways to represent the larger  UCS codes in an 8-bit  octet
stream.   The most popular  one, especially in  the context of the  web,
is UTF-8.   Bytes 0 ... 127 represent simply the  corresponding US-ASCII
character, while bytes 128  ... 255 are used for multi-byte  encoding of
characters placed  higher in the  UCS space.   Especially on  MS-Windows
the  16-bit Unicode  standard, represented  by pairs  of  bytes is  also
popular.

Prolog I/O streams  have a property called _e_n_c_o_d_i_n_g which  specifies the
used encoding  that influence get_code/2 and  put_code/2 as well as  all
the other text I/O predicates.

The  default  encoding  for  files  is  derived  from  the  Prolog  flag
encoding,  which   is  initialised  from  the  environment.      If  the
environment variable  LANG ends  in "UTF-8", this  encoding is  assumed.
Otherwise  the default  is  text and  the  translation  is left  to  the
wide-character  functions  of the  C-library.         The  encoding  can
be  specified  explicitly  in load_files/2  for  loading  Prolog  source
with  an  alternative encoding,   open/4 when  opening  files  or  using
set_stream/2 on  any open  stream.    For Prolog  source  files we  also
provide the  encoding/1 directive  that can  be used  to switch  between
encodings  that are  compatible to  US-ASCII  (ascii, iso_latin_1,  utf8
and  many  locales).     See  also  section  3.1.3  for  writing  Prolog
files  with non-US-ASCII  characters  and  section 2.15.1.4  for  syntax
issues.  For additional information and Unicode  resources, please visit
http://www.unicode.org/.

SWI-Prolog currently defines and supports the following encodings:

oocctteett
    Default  encoding for binary streams.  This causes the stream  to be
    read and written fully untranslated.

aasscciiii
    7-bit  encoding  in 8-bit  bytes.   Equivalent  to  iso_latin_1,  but
    generates errors and warnings on encountering values above 127.

iissoo__llaattiinn__11
    8-bit  encoding supporting many western languages.  This  causes the
    stream to be read and written fully untranslated.

tteexxtt
    C-library  default locale encoding for text  files.  Files are  read
    and  written using the C-library functions mbrtowc()  and wcrtomb().
    This may be the  same as one of the other locales, notably it may be
    the  same as iso_latin_1 for western languages  and utf8 in a  UTF-8
    context.

uuttff88
    Multi-byte encoding of full UCS, compatible to ascii.  See above.

uunniiccooddee__bbee
    Unicode  _B_i_g  _E_n_d_i_a_n.     Reads  input  in  pairs  of  bytes,   most
    significant byte first.  Can only represent 16-bit characters.

uunniiccooddee__llee
    Unicode  _L_i_t_t_l_e  _E_n_d_i_a_n.    Reads input  in  pairs of  bytes,  least
    significant byte first.  Can only represent 16-bit characters.

Note that not all encodings can represent all characters.   This implies
that  writing text  to a  stream  may cause  errors because  the  stream
cannot represent these characters.   The behaviour of a stream  on these
errors can  be controlled  using set_stream/2.   Initially the  terminal
stream write  the characters using Prolog  escape sequences while  other
streams generate an I/O exception.


22..1177..11..11 BBOOMM:: BByyttee OOrrddeerr MMaarrkk

From  section  2.17.1,  you  may  have  got  the  impression  text-files
are  complicated.   This  section  deals with  a related  topic,  making
live  often easier  for the  user, but  providing another  worry to  the
programmer.   BBOOMM or  _B_y_t_e _O_r_d_e_r _M_a_r_k_e_r is  a technique for  identifying
Unicode text-files as well  as the encoding they use.  Such  files start
with  the Unicode  character 0xFEFF,  a non-breaking,  zero-width  space
character.   This is a pretty unique  sequence that is not likely to  be
the start of  a non-Unicode file and uniquely distinguishes  the various
Unicode file  formats.   As it is  a zero-width  blank, it even  doesn't
produce any output.  This solves all problems, or ...

Some formats start of as US-ASCII and may contain  some encoding mark to
switch to UTF-8,  such as the encoding="UTF-8" in  an XML header.   Such
formats often  explicitly forbid the  the use of a  UTF-8 BOM. In  other
cases there  is additional information telling  the encoding making  the
use of a BOM redundant or even illegal.

The  BOM  is handled  by  SWI-Prolog  open/4  predicate.    By  default,
text-files are  probed for the BOM  when opened for reading.   If a  BOM
is found, the encoding is set accordingly and  the property bom(_t_r_u_e) is
available through stream_property/2.   When opening a file for  writing,
writing a BOM can be requested using the option bom(_t_r_u_e) with open/4.


22..1188 SSyysstteemm lliimmiittss


22..1188..11 LLiimmiittss oonn mmeemmoorryy aarreeaass

SWI-Prolog has  a number of  memory areas which are  only enlarged to  a
certain limit.   The  default sizes for these  areas should suffice  for
most applications, but big  applications may require larger ones.   They
are  modified by  command-line options.    The table  below shows  these
areas.   The first column  gives the option name  to modify the size  of
the area.  The option character is immediately followed  by a number and
optionally by  a k or  m.   With k or  no unit  indicator, the value  is
interpreted in Kbytes (1024 bytes), with m, the  value is interpreted in
Mbytes (10241* 024 bytes).

The local-,  global- and  trail-stack are  limited to 128  Mbytes on  32
bit processors,  or more  generally to  2 to the power bits-per-long - 5
bytes.

The  PrologScript facility  described  in  section 2.10.2.1  provides  a
mechanism for  specifying options with  the load-file.   On Windows  the
default stack-sizes  are controlled  using the Windows  registry on  the
key  HKEY_CURRENT_USER\Software\SWI\Prolog using  the  names  localSize,
globalSize and trailSize.   The value is a DWORD expressing  the default
stack size  in Kbytes.   A  GUI for modifying  these values is  provided
using the XPCE package.  To use this, start the  XPCE manual tools using
manpce/0, after which you find _P_r_e_f_e_r_e_n_c_e_s in the _F_i_l_e menu.
       ___________________________________________________________
       |_Option_|Default_|Area_name______|Description____________|_||-L||2Mllooccaa||llTssttaacckkhe|l||||ocal|stack is used

       |        |                        to  store   the  execu- |             |            ||
       |        |                        tion  environments   of |             |            ||
       |        |                        procedure  invocations. |             |            ||
       |        |                        The  space for  an  en- |             |            ||
       |        |                        vironment is  reclaimed |             |            ||
       |        |                        when  it  fails,  exits |             |            ||
       |        |                        without leaving  choice |             |            ||
       |        |                        points,   the  alterna- |             |            ||
       |        |                        tives are  cut of  with |             |            ||
       |        |                                                |             |            ||

       |        |                        the  !/0  predicate  or |             |            ||
       |        |                        no  choice points  have |             |            ||
       |        |                        been created  since the |             |            ||
       |        |                        invocation and the last |             |            ||
       |        |                        subclause  is   started |             |            ||
       |        |                        (tail recursion optimi- |             |            ||
       ||       ||      |               ||sation).                ||            |            ||

       |   -G   | 4M    |gglloobbaall ssttaacckk   ||Theusglobaled stacktois store| terms
       |        |       |               ||created during Prolog's |
       |        |       |               ||execution.    Terms  on |
       |        |       |               ||                        |
       |        |       |               ||this stack will  be re- |
       |        |       |               ||claimed by backtracking |
       |        |       |               ||to a  point before  the |
       |        |       |               ||term  was   created  or |
       |        |       |               ||by  garbage  collection |

       |        |       |               ||(provided  the term  is |
       ||       ||      ||              ||no||longer referenced).  ||
       |   -T   | 4M    |ttrraaiill ssttaacckk    ||Theusetraild stackto isstore|  as-
       |        |       |               ||signments during execu- |
       |        |       |               ||                        |
       |        |       |               ||tion.   Entries on this |
       |        |       |               ||stack remain  alive un- |

       |        |       |               ||til backtracking before |
       |        |       |               ||the  point of  creation |
       |        |       |               ||or the  garbage collec- |
       ||       ||      ||              ||tor determines||they are ||
       ||| -A   |||1M   |||aarrgguummeenntt ssttaacc||norkkneeded||any||longer.Th|e||argument stack  is

       |        |        |               used || to   store   one |
       |        |        |               of   ||the   intermediate|
       |        |        |               code ||interpreter's reg-|
       |        |        |               ister||s.     The  amount|
       |        |        |               of sp||ace needed on this|
       |        |        |               stack||is determined en- |
       |        |        |               tirel||y by the  depth in|

       |        |        |               which||terms  are nested |
       |        |        |               in  t||he   clauses  that|
       |        |        |               const||itute the program.|
       |        |        |               Overf||low is most likely|
       |        |        |               when ||using long strings|
       |        |        |               in a ||clause.           |
       |        |        |               In ad||dition, this stack|
       |        |        |               is us||ed by  some built-|
       |        |        |                    ||                  |
       |        |        |               in pr||edicates to handle|

       |        |        |               cycli||c terms.   Its de-|
       |        |        |               fault|| size   limit  is |
       |        |        |               propo||rtional   to   the|
       |        |        |               globa||l stack limit such|
       |        |        |               that || it  will   never |
       |________|________|_______________overf||low.______________|_

                        Table 2.2:  Memory areas


22..1188..11..11 TThhee hheeaapp

With  the heap,  we  refer  to the  memory  area  used by  malloc()  and
friends.  SWI-Prolog uses the area to store  atoms, functors, predicates
and their  clauses, records and  other dynamic data.   As of  SWI-Prolog
2.8.5, no limits are  imposed on the addresses returned by  malloc() and
friends.

On  some machines,  the  runtime stacks  described above  are  allocated
using `sparse  allocation'.   Virtual space up to  the limit is  claimed
at startup and committed and released while the area  grows and shrinks.
On Win32  platform this  is realised using  VirtualAlloc() and  friends.
On Unix systems this is realised using mmap().


22..1188..22 OOtthheerr LLiimmiittss

CCllaauusseess  The only  limit  on  clauses  is their  arity  (the  number  of
    arguments  to the head),  which is  limited to 1024.   Raising  this
    limit is easy and relatively cheap, removing it is harder.

AAttoommss aanndd SSttrriinnggss  SWI-Prolog  has no  limits  on  the  sizes  of  atoms
    and  strings.   read/1  and its derivatives  however normally  limit
    the  number  of newlines  in  an  atom or  string  to 5  to  improve
    error  detection  and recovery.    This  can  be switched  off  with
    style_check/1.

    The  number  of  atoms  is  limited  to  16777216  (16M)  on  32-bit
    machines.   On  64-bit machines  this is virtually  unlimited.   See
    also section 9.6.2.1.

MMeemmoorryy aarreeaass  On 32-bit hardware, SWI-Prolog data is packed in  a 32-bit
    word,  which contains  both type and  value information.   The  size
    of  the various memory areas  is limited to 128  Mb for each of  the
    areas,  except  for the  program heap,  which is  not limited.    On
    64-bit hardware there are no meaningful limits.

IInntteeggeerrss  Integers  are  64-bit  on  32  as  well  as  64-bit  machines.
    Integers  up to the value of  the max_tagged_integer prolog-flag  are
    represented more efficiently  on the stack.  For clauses and records
    the difference is much smaller.

FFllooaattss  Floating  point  numbers  are  represented  as  C-native  double
    precision floats, 64 bit IEEE on most machines.


22..1188..33 RReesseerrvveedd NNaammeess

The boot compiler  (see -b option) does  not support the module  system.
As large parts of  the system are written in Prolog itself we  need some
way to  avoid name clashes  with the  user's predicates, database  keys,
etc.   Like Edinburgh C-Prolog [Pereira, 1986] all predicates,  database
keys, etc. that should  be hidden from the user start with a  dollar ($)
sign (see style_check/1).


22..1199 SSWWII--PPrroolloogg aanndd 6644--bbiitt mmaacchhiinneess

SWI-Prolog support for  64-bit machines started with version 2.8  on DEC
Alpha  CPUs running  Linux.   Initially  64-bit  hardware was  developed
to  deal  with  the  addressing  demands  of  large  databases,  running
primarily on  expensive server hardware.   Recently  (2007) we see  CPUs
that support  64-bit addressing become  commonplace, even in  low-budget
desktop hardware.   Most todays 64-bit platforms are capable  of running
both 32-bit and 64-bit applications.  This asks  for some clarifications
on the advantages and drawbacks of 64-bit addressing for (SWI-)Prolog.


22..1199..11 SSuuppppoorrtteedd ppllaattffoorrmmss

On  Unix  systems,  64-bit addressing  is  configured  using  configure.
Traditionally,  both  long and  void*  are  64-bits on  these  machines.
Version  5.6.26 introduces  support for  64-bit  MS-Windows (Windows  XP
and Vista  64-bit editions) on  amd64 (x64) hardware.   Win64 uses  long
integers of only  32-bits.  Version  5.6.26 introduces support for  such
platforms.


22..1199..22 CCoommppaarriinngg 3322-- aanndd 6644--bbiittss PPrroolloogg

Most of Prolog's memory-usage consists of pointers.   This indicates the
primary drawback:   Prolog memory  usage almost  doubles when using  the
64 bit  addressing model.   Using  more memory  means copying more  data
between CPU and main memory, slowing down the system.

What than  are the advantages?   First  of all, SWI-Prolog's  addressing
of the  Prolog stacks  does not  cover the  whole address  space due  to
the  use of  _t_y_p_e _t_a_g  _b_i_t_s and  _g_a_r_b_a_g_e _c_o_l_l_e_c_t_i_o_n  _f_l_a_g_s.   On  32-bit
hardware the  stacks are limited to  128MB each.   This tends to be  too
low for demanding applications  on modern hardware.  On  64-bit hardware
the limit is 232 times higher, exceeding the  addressing capabilities of
todays CPUs and operating  systems.  This implies Prolog can  be started
with stacks sizes that use the full capabilities of your hardware.

Multi-threaded applications profit much more.   SWI-Prolog threads claim
the full stacksize limit in _v_i_r_t_u_a_l _a_d_d_r_e_s_s _s_p_a_c_e and  each thread comes
with its  own set  of stacks.    This approach  quickly exhaust  virtual
memory  on  32-bit systems  but  poses  no problems  when  using  64-bit
addresses.

The implications  theoretical performance loss  due to increased  memory
bandwidth implied by  exchanging wider pointers depend on the  design of
the  hardware.   We  only have  data for  the popular  IA32 vs.    AMD64
architectures.   Here is appears that the  loss is compensated for by  a
an instruction set that  has been optimized for modern programming.   In
particular, the  AMD64 has  more registers and  the relative  addressing
capabilities  have been  improved.    Where  we  see a  10%  performance
degradation when placing the SWI-Prolog kernel in a  Unix shared object,
we cannot  find a measurable  difference on AMD64.   Current  SWI-Prolog
(5.6.26) runs at practically the same speed on IA32 and AMD64.


22..1199..33 CChhoooossiinngg bbeettwweeeenn 3322-- aanndd 6644--bbiittss PPrroolloogg

For those  cases where  we can choose  between 32-  and 64-bits,  either
because the hardware and OS support both or because  we can still choose
the hardware and OS, we give guidelines for this decision.

First of  all, if SWI-Prolog  needs to be linked  against 32- or  64-bit
native libraries,  there  is no choice  as it  is not  possible to  link
32- and  64-bit code into  a single  executable.   Only if all  required
libraries are available  in both sizes and  there is no clear reason  to
use either the different characteristics of Prolog become important.

Prolog  applications  that  require more  than  the  128MB  stack  limit
provided in 32-bit  addressing mode must use  the 64-bit edition.   Note
however that the limits  must be doubled to accommodate the  same Prolog
application.

If the system is  tight on physical memory, 32-bit Prolog has  the clear
advantage to use  only slightly more than  half of the memory of  64-bit
Prolog.    This argument  applies as  long as  the  application fits  in
the _v_i_r_t_u_a_l  _a_d_d_r_e_s_s _s_p_a_c_e of  the machine.   The virtual address  space
of  32-bit hardware  is 4GB,  but  in many  cases the  operating  system
provides less to user applications.  Virtual memory  usage of SWI-Prolog
is roughly  the program size  (_h_e_a_p) plus the  sum of the  stack-limits.
If there are  multiple threads, each thread  has its own stacks and  the
stack-limits must be summed over the running threads.

The  only  standard  SWI-Prolog library  adding  significantly  to  this
calculation is  the RDF  database provided by  the _s_e_m_w_e_b  package.   It
uses approximately 80 bytes per triple on 32-bit hardware  and 150 bytes
on 64-bit hardware.  Details depend on how  many different resources and
literals appear  in the dataset  as well  as desired additional  literal
indexes.

Summarizing,  if applications  are small  enough to  fit comfortably  in
virtual  and physical  memory simply  take  the model  used by  most  of
the applications  on the  OS. If  applications require  more than  128MB
per  stack,  use the  64-bit  edition.    If applications  approach  the
size  of physical  memory,  fit in  the  128MB stack  limit and  fit  in
virtual memory, the 32-bit version has clear advantages.   For demanding
applications  on 64-bit  hardware  with  more than  about  6GB  physical
memory the 64-bit model is the model of choice.


CChhaapptteerr 33..  IINNIITTIIAALLIISSIINNGG AANNDD MMAANNAAGGIINNGG AA PPRROOLLOOGG PPRROOJJEECCTT

Prolog text-books  give you  an overview of  the Prolog  language.   The
manual tells  you what predicates  are provided in  the system and  what
they do.  This chapter wants to explain how to run a  project.  There is
no ultimate `right'  way to do this.   Over the years we developed  some
practice in  this area and  SWI-Prolog's commands  are there to  support
this practice.   This chapter  describes the conventions and  supporting
commands.

The first two sections  (section 3.1 and section 3.2 only  require plain
Prolog.    The remainder  discusses the  use of  the built-in  graphical
tools that require the XPCE graphical library installed on your system.


33..11 TThhee pprroojjeecctt ssoouurrccee--ffiilleess

Organisation  of  source-files  depends largely  on  the  size  of  your
project.    If  you  are doing  exercises  for  a Prolog  course  you'll
normally use one  file for each exercise.   If you have a small  project
you'll work work with  one directory holding a couple of files  and some
files to link it  all together.  Even bigger projects will  be organised
in sub-projects each using their own directory.


33..11..11 FFiillee NNaammeess aanndd LLooccaattiioonnss


33..11..11..11 FFiillee NNaammee EExxtteennssiioonnss

The first consideration  is what extension to use for  the source-files.
Tradition  calls for  .pl,  but conflicts  with Perl  force the  use  of
another extension on systems where extensions have  global meaning, such
as MS-Windows.  On such systems .pro is the common alternative.

All versions of SWI-Prolog load files with the extension  .pl as well as
with the registered alternative extension without  explicitly specifying
the  extension.    For  portability  reasons we  propose  the  following
convention:

IIff tthheerree iiss nnoo ccoonnfflliicctt  because  you   do   not   use   a   conflicting
    application  or the system does not force a unique  relation between
    extension and application, use .pl.

WWiitthh aa ccoonnfflliicctt  choose .pro and  use this extension  for the files  you
    want  to load  through your  file-manager.   Use .pl  for all  other
    files for maximal portability.


33..11..11..22 PPrroojjeecctt DDiirreeccttoorriieess

Large projects are generally composed of sub-projects,  each using their
own directory or  directory-structure.  If  nobody else will ever  touch
your  files and  you use  only one  computer there  is  little to  worry
about, but this is rarely the case with a large project.

To  improve  portability,   SWI-Prolog  uses  the  POSIX   notation  for
filenames, which  uses the  forward slash (/)  to separate  directories.
Just before  hitting the file-system  it uses prolog_to_os_filename/2 to
convert the  filename to the conventions  used by the hosting  operating
system.  It  is _s_t_r_o_n_g_l_y advised to write paths using the  /, especially
on systems using the \ for this purpose (MS-Windows).   Using \ violates
the  portability rules  and requires  you to  _d_o_u_b_l_e the  \  due to  the
Prolog quoted-atom escape rules.

Portable  code should  use  prolog_to_os_filename/2to  convert  computed
paths  into system-paths  when  constructing  commands for  shell/1  and
friends.


33..11..11..33 SSuubb--pprroojjeeccttss uussiinngg sseeaarrcchh--ppaatthhss

Thanks to Quintus, Prolog adapted an extensible  mechanism for searching
files using file_search_path/2.   This mechanism allows for  comfortable
and readable specifications.

Suppose  you  have  extensive  library  packages   on  graph-algorithms,
set-operations  and  GUI-primitives.    These  sub-projects  are  likely
candidates for re-use in future projects.  A good choice  is to create a
directory with sub-directories for each of these sub-projects.

Next,  there are  three options.    One is  to add  the sub-projects  to
the  directory-hierarchy of  the current  project.   Another  is to  use
a completely  dislocated directory  and finally the  sub-project can  be
added to the SWI-Prolog hierarchy.  Using local  installation, a typical
file_search_path/2is:

:- prolog_load_context(directory, Dir),
   asserta(user:file_search_path(myapp, Dir)).

user:file_search_path(graph, myapp(graph)).
user:file_search_path(ui,    myapp(ui)).

For  using sub-projects  in  the  SWI-Prolog hierarchy  one  should  use
the path-alias  swi as  basis.   For a  system-wide installation use  an
absolute-path.

Extensive  sub-projects with  a  small  well-defined API  should  define
a   load-file  using   use_module/1   calls  to   import   the   various
library-components and export the API.


33..11..22 PPrroojjeecctt SSppeecciiaall FFiilleess

There are  a number of  tasks you typically carry  out on your  project,
such as  loading it, creating  a saved-state, debugging it,  etc.   Good
practice  on large  projects is  to  define small  files that  hold  the
commands to execute such a task, name this file after  the task and give
it  a file-extension  that makes  starting easy  (see section  3.1.1.1).
The task _l_o_a_d is generally central to these tasks.   Here is a tentative
list.

  o load.pl
    Use  this file  to set  up the  environment (prolog  flags and  file
    search  paths) and load the sources.  Quite commonly this  file also
    provides  convenient predicates  to parse  command-line options  and
    start the application.

  o run.pl
    Use  this file to start the application.  Normally it  loads load.pl
    in  silent-mode,  and  calls one  of  the starting  predicates  from
    load.pl.

  o save.pl
    Use this file  to create a saved-state of the application by loading
    load.pl and  call qsave_program/2to generate  a saved-state with the
    proper options.

  o debug.pl
    Loads  the program for  debugging.   In addition to loading  load.pl
    this  file defines rules for portray/1 to modify printing  rules for
    complex  terms and customisation rules for the debugger  and editing
    environment.  It may start some of these tools.


33..11..33 IInntteerrnnaattiioonnaall ssoouurrccee ffiilleess

As  discussed  in   section  2.17,  SWI-Prolog  supports   international
character  handling.   Its  internal encoding  is  UNICODE. I/O  streams
convert  to/from this  internal format.    This  sections discusses  the
options for source-files not in US-ASCII.

SWI-Prolog  can  read  files  in  any  of  the  encodings  described  in
section 2.17.    Two encodings  are of particular  interest.   The  text
encoding  deals with  the  current  _l_o_c_a_l_e,  the default  used  by  this
computer for  representing text files.   The encodings  utf8, unicode_le
and unicode_be are _U_N_I_C_O_D_E encodings:  they can represent---in  the same
file---characters of  virtually any known language.   In addition,  they
do so unambiguously.

If  one wants  to  represent non  US-ASCII text  as  Prolog terms  in  a
source-file there are several options:

  o _U_s_e _e_s_c_a_p_e _s_e_q_u_e_n_c_e_s
    This approach describes  NON-ASCII as sequences of the form \_o_c_t_a_l\.
    The  numerical argument is interpreted as a UNICODE character.   The
    resulting  Prolog file is  strict 7-bit US-ASCII,  but if there  are
    many NON-ASCII characters it becomes very unreadable.

  o _U_s_e _l_o_c_a_l _c_o_n_v_e_n_t_i_o_n_s
    Alternatively  the file  may be specified  using local  conventions,
    such  as the EUC  encoding for Japanese text.   The disadvantage  is
    portability.   If the file is moved to another machine  this machine
    must  be using the  same _l_o_c_a_l_e or  the file is  unreadable.   There
    is  no elegant if files from multiple locales must be united  in one
    application  using this technique.   In other words, it is  fine for
    local projects in countries with uniform locale conventions.

  o _U_s_i_n_g _U_T_F_-_8 _f_i_l_e_s
    The best way  to specify source files with many NON-ASCII characters
    is  definitely the use  of UTF-8 encoding.   Prolog can be  notified
    two ways of  this encoding, using a UTF-8 _B_O_M (see section 2.17.1.1)
    or  using  the  directive  :- encoding(utf8)..    Many  todays  text
    editors,  including PceEmacs,  are capable  of editing UTF-8  files.
    Projects  that started  using local conventions  can be be  re-coded
    using  the Unix iconv tool or often using a commands offered  by the
    editor.


33..22 UUssiinngg mmoodduulleess

Modules have  been debated fiercely  in the Prolog world.   Despite  all
counter-arguments we feel they are extremely useful because

  o _T_h_e_y _h_i_d_e _l_o_c_a_l _p_r_e_d_i_c_a_t_e_s
    This  is the  reason they  have been  invented in  the first  place.
    Hiding  provides  two features.    They  allow for  short  predicate
    names  without worrying about conflicts.  Given the  flat name-space
    introduced  by modules, they  still require meaningful module  names
    as well as meaningful names for exported predicates.

  o _T_h_e_y _d_o_c_u_m_e_n_t _t_h_e _i_n_t_e_r_f_a_c_e
    Possibly  more important then avoiding name-conflicts is  their role
    in  documenting  which part  of the  file is  for  public usage  and
    which  is private.   When editing  a module you  may assume you  can
    reorganise  anything  but the  name and  semantics  of the  exported
    predicates without worrying.

  o _T_h_e_y _h_e_l_p _t_h_e _e_d_i_t_o_r
    The  PceEmacs built-in editor  does on-the-fly cross-referencing  of
    the  current module, colouring predicates based on their  origin and
    usage.    Using modules,  the editor  can quickly find  out what  is
    provided  by the imported  modules by reading  just the first  term.
    This  allows it to indicate real-time which predicates are  not used
    or not defined.

Using modules  is generally  easy.   Only if  you write  meta-predicates
(predicates reasoning about  other predicates) that are exported from  a
module good understanding of resolution of terms to  predicates inside a
module is required.  Here is a typical example from readutil.

:- module(read_util,
          [ read_line_to_codes/2,       % +Fd, -Codes
            read_line_to_codes/3,       % +Fd, -Codes, ?Tail
            read_stream_to_codes/2,     % +Fd, -Codes
            read_stream_to_codes/3,     % +Fd, -Codes, ?Tail
            read_file_to_codes/3,       % +File, -Codes, +Options
            read_file_to_terms/3        % +File, -Terms, +Options
          ]).


33..33 TThhee tteesstt--eeddiitt--rreellooaadd ccyyccllee

SWI-Prolog does not enforce  the use of a particular editor  for writing
down  Prolog  source  code.    Editors  are  complicated  programs  that
must  be mastered  in  detail for  real  productive programming  and  if
you  are familiar  with  a specific  editor  you  should not  be  forced
to change.    You may  specify your  favourite editor  using the  prolog
flag editor,  the environment variable EDITOR  or by defining rules  for
prolog_edit:edit_source/1 (see section 4.4).

The  use  of  a built-in  editor,  which  is  selected  by  setting  the
prolog-flag  editor to  pce_emacs,  has advantages.    The  XPCE  _e_d_i_t_o_r
object around  which the  built-in PceEmacs  is built can  be opened  as
a Prolog  stream allowing  analysis of  your source by  the real  Prolog
system.


33..33..11 LLooccaattiinngg tthhiinnggss ttoo eeddiitt

The central  predicate for  editing something is  edit/1, an  extensible
front-end that searches for objects (files, predicates,  modules as well
as  XPCE classes  and methods)  in the  Prolog database.    If  multiple
matches are  found it  provides a choice.    Together with the  built-in
completion on atoms  bound to the TAB key  this provides a quick way  to
edit objects:

?- edit(country).
Please select item to edit:

  1 chat:country/10      '/staff/jan/lib/prolog/chat/countr.pl':16
  2 chat:country/1       '/staff/jan/lib/prolog/chat/world0.pl':72

Your choice?


33..33..22 EEddiittiinngg aanndd iinnccrreemmeennttaall ccoommppiillaattiioonn

One of  the nice features  of Prolog  is that the  code can be  modified
while  the program  is running.    Using  pure Prolog  you  can trace  a
program,  find it  is misbehaving,  enter  a _b_r_e_a_k  _e_n_v_i_r_o_n_m_e_n_t,  modify
the  source code,  reload it  and finally  do _r_e_t_r_y  on the  misbehaving
predicate  and  try  again.      This  sequence  is  not   uncommon  for
long-running programs.   For faster  programs one normally aborts  after
understanding  the misbehaviour,  edit  the source,  reload it  and  try
again.

One of the nice features of SWI-Prolog is the  availability of make/0, a
simple predicate that checks  all loaded source files to see  which ones
you have modified.  It then reloads these  files, considering the module
from which the file was loaded originally.   This greatly simplifies the
trace-edit-verify development cycle.  After the tracer  reveals there is
something wrong with prove/3, you do:

?- edit(prove).

Now  edit the  source,  possibly switching  to  other files  and  making
multiple changes.   After  finishing invoke  make/0, either through  the
editor UI  () Compile/Make  (Control-C Control-M)) or  on the  top-level
and watch the files being reloaded.

?- make.
% show compiled into photo_gallery 0.03 sec, 3,360 bytes


33..44 UUssiinngg tthhee PPcceeEEmmaaccss bbuuiilltt--iinn eeddiittoorr


33..44..11 AAccttiivvaattiinngg PPcceeEEmmaaccss

Initially edit/1  uses the  editor specified in  the EDITOR  environment
variable.   There are two ways to  force it to use the built-in  editor.
One is to  set the prolog-flag editor  to pce_emacs and the other is  by
starting the editor explicitly using the emacs/[0,1] predicates.


33..44..22 BBlluuffffiinngg tthhrroouugghh PPcceeEEmmaaccss

PceEmacs closely  mimics Richard Stallman's  GNU-Emacs commands,  adding
features from  modern window-based  editors to make  it more  acceptable
for beginners.

At the  basis, PceEmacs  maps keyboard sequences  to methods defined  on
the extended  _e_d_i_t_o_r object.   Some frequently  used commands are,  with
their key-binding, presented  in the menu-bar above each  editor window.
A complete  overview of the  bindings for the  current _m_o_d_e is  provided
through ) Help/Show key bindings (Control-h Control-b).


33..44..22..11 EEddiitt mmooddeess

Modes  are the  heart of  (Pce)Emacs.   Modes  define dedicated  editing
support for  a particular  kind of (source-)text.    For our purpose  we
want _P_r_o_l_o_g _m_o_d_e.   Their are various  ways to make PceEmacs use  Prolog
mode for a file.

  o _U_s_i_n_g _t_h_e _p_r_o_p_e_r _e_x_t_e_n_s_i_o_n
    If  the file  ends in .pl  or the  selected alternative (e.g.  .pro)
    extension, Prolog mode is selected.

  o _U_s_i_n_g #!/path/to/pl
    If  the  file  is a  _P_r_o_l_o_g  _S_c_r_i_p_t  file, starting  with  the  line
    #!/path/to/pl options -s, Prolog  mode is selected regardless of the
    extension

  o _U_s_i_n_g -*- Prolog -*-
    If the above sequence  appears in the first line of the file (inside
    a Prolog comment) Prolog mode is selected.

  o _E_x_p_l_i_c_i_t _s_e_l_e_c_t_i_o_n
    Finally,  using ) File/Mode/Prolog (y)ou  can switch to Prolog  mode
    explicitly.


33..44..22..22 FFrreeqquueennttllyy uusseedd eeddiittoorr ccoommmmaannddss

Below we list a few important commands and how to activate them.

  o _C_u_t_/_C_o_p_y_/_P_a_s_t_e
    These  commands  follow Unix/X11  traditions.    You're best  suited
    with  a three-button mouse.   After  selecting using the  left-mouse
    (double-click  uses word-mode  and triple  line-mode), the  selected
    text   is  _a_u_t_o_m_a_t_i_c_a_l_l_y  copied  to  the  clipboard   (X11  primary
    selection  on  Unix).   _C_u_t  is achieved  using the  DEL  key or  by
    typing  something else  at the location.    _P_a_s_t_e is achieved  using
    the  middle-mouse (or wheel)  button.   If you  don't have a  middle
    mouse-button,  pressing the left- and right-button at the  same time
    is interpreted as a  middle-button click.  If nothing helps there is
    the ) Edit/Paste menu-entry.  Text is pasted at the caret-location.

  o _U_n_d_o
    Undo  is bound to the GNU-Emacs Control-_ as well as  the MS-Windows
    Control-Z sequence.

  o _A_b_o_r_t
    Multi-key sequences can be aborted at any stage using Control-G.

  o _F_i_n_d
    Find  (Search) is  started  using Control-S  (forward) or  Control-R
    (backward).      PceEmacs  implements  _i_n_c_r_e_m_e_n_t_a_l  _s_e_a_r_c_h.     This
    is  difficult  to  use  for novices,  but  very  powerful  once  you
    get  the  clue.    After  one of  the  above start-keys  the  system
    indicates  search mode in the  status line.   As you are typing  the
    search-string,  the  system searches  for it,  extending the  search
    with  every character you  type.   It illustrates the current  match
    using a green background.

    If  the target  cannot be found,  PceEmacs warns  you and no  longer
    extends  the  search-string.   During  search  some characters  have
    special  meaning.  Typing anything but these characters  commits the
    search, re-starting normal edit mode.  Special commands are:

    Control-S
         Search for next forwards.

    Control-R
         Search for next backwards.

    Control-W
         Extend search to next word-boundary.

    Control-G
         Cancel search, go back to where it started.

    ESC
         Commit search, leaving caret at found location.

    Backspace
         Remove a character from the search string.

  o _D_y_n_a_m_i_c _A_b_b_r_e_v_i_a_t_i_o_n
    Also  called _d_a_b_b_r_e_v  is  an important  feature of  Emacs clones  to
    support  programming.   After  typing the  first few  letters of  an
    identifier  you may hit Alt-/, causing PceEmacs to  search backwards
    for  identifiers that start  the same and  using it to complete  the
    text  you typed.   A second  Alt-/ searches further  backwards.   If
    there  are no hits  before the caret  it starts searching  forwards.
    With  some practice, this system allows for very fast  entering code
    with nice and readable identifiers (or other difficult long words).

  o _O_p_e_n _(_a _f_i_l_e_)
    Is  called ) File/Find file  (Control-x Control-f).  By default  the
    file  is loaded into the current window.   If you want to  keep this
    window,  Hit Alt-s or  click the little  icon at the bottom-left  to
    make the window _s_t_i_c_k_y.

  o _S_p_l_i_t _v_i_e_w
    Sometimes  you want to look at two places  of the same file.   To do
    this,  use Control-x 2 to create  a new window pointing to  the same
    file.   Do not worry, you  can edit as well as move around  in both.
    Control-x 1 kills all other windows running on the same file.

These were the  most commonly used commands.   In section section  3.4.3
we discuss specific support for dealing with Prolog source code.


33..44..33 PPrroolloogg MMooddee

In  the previous  section (section  3.4.2) we  explained  the basics  of
PceEmacs.     Here  we  continue  with  Prolog  specific  functionality.
Possibly  the most  interesting is  _S_y_n_t_a_x _h_i_g_h_l_i_g_h_t_i_n_g.    Unlike  most
editors  where  this  is  based  on  simple  patterns,  PceEmacs  syntax
highlighting  is  achieved   by  Prolog  itself  actually  reading   and
interpreting the  source as you  type it.   There  are three moments  at
which PceEmacs checks (part of) the syntax.

  o _A_f_t_e_r _t_y_p_i_n_g _a .
    After  typing a .  that is  not preceded by  a _s_y_m_b_o_l character  the
    system  assumes you completed a clause,  tries to find the start  of
    this  clause and verifies the syntax.   If this process succeeds  it
    colours  the elements  of the  clause according to  the rules  given
    below.    Colouring is  done using  information from  the last  full
    check  on this file.  If it fails, the syntax error  is displayed in
    the status line and the clause is not coloured.

  o _A_f_t_e_r _t_h_e _c_o_m_m_a_n_d Control-c Control-s
    Acronym  for CCcheck SSyntax it performs the same checks as  above for
    the  clause surrounding the caret.   On a syntax error however,  the
    caret is moved to the expected location of the error.

  o _A_f_t_e_r _p_a_u_s_i_n_g _f_o_r _t_w_o _s_e_c_o_n_d_s
    After  a short  pause (2  seconds), PceEmacs  opens the  edit-buffer
    and  reads it  as a  whole, creating  an index  of defined,  called,
    dynamic,  imported and exported predicates.  After  completing this,
    it  re-reads the file and colours  all clauses and calls with  valid
    syntax.

  o _A_f_t_e_r _t_y_p_i_n_g Control-l Control-l
    The Control-l commands  re-centers the window (scrolls the window to
    make  the caret the  center of  the window).   Hitting this  command
    twice starts the same process as above.

TThhee ccoolloouurr sscchheemmaa

itself  is  defined  in  emacs/prolog_colour.     The  colouring  can  be
extended and  modified using multifile  predicates.   Please check  this
source-file for details.   In general,  underlined objects have a  popup
(right-mouse button) associated for common commands such  as viewing the
documentation or source.   BBoolldd text is used to indicate  the definition
of  objects (typically  predicates  when using  plain  Prolog).    Other
colours follow intuitive conventions.  See table 3.4.3.
          _____________________________________________________
          |______________________Clauses_______________________|
          | Blue bold  |Head of an exported predicate          |
          | Red bold   |Head of a predicate that is not called |

          |_Black_Bold_|Head_of_remaining_predicates___________|
          |______________Calls_in_the_clause-body______________|
          | Blue       |Call to built-in or imported predicate |
          | Red        |Call to not-defined predicate          |
          |_Purple_____|Call_to_dynamic_predicate______________|
          |___________________Other_entities___________________|

          | Dark green |Comment                                |
          | Dark blue  |Quoted atom or string                  |
          |_Brown______|Variable_______________________________|

                     Table 3.1:  Colour conventions

LLaayyoouutt ssuuppppoorrtt Layout  is not `just nice',  it is _e_s_s_e_n_t_i_a_l for  writing
readable code.   There is  much debate on  the proper layout of  Prolog.
PceEmacs,  being a  rather small  project supports  only one  particular
style for layout.  Below are examples of typical constructs.

head(arg1, arg2).

head(arg1, arg2) :- !.

head(Arg1, arg2) :- !,
        call1(Arg1).

head(Arg1, arg2) :-
        (   if(Arg1)
        ->  then
        ;   else
        ).

head(Arg1) :-
        (   a
        ;   b
        ).

head :-
        a(many,
          long,
          arguments(with,
                    many,
                    more),
          and([ a,
                long,
                list,
                with,
                a,
              | tail
              ])).

PceEmacs uses the  same conventions as GNU-Emacs.   The TAB key  indents
the current  line according  to the  syntax rules.    Alt-q indents  all
lines  of the  current clause.    It provides  support for  head,  calls
(indented 1  tab), if-then-else,  disjunction and argument-lists  broken
across multiple lines as illustrated above.


33..44..33..11 FFiinnddiinngg yyoouurr wwaayy aarroouunndd

The command Alt-.   extracts name and arity from the caret  location and
jumps (after conformation  or edit) to the definition of  the predicate.
It does  so based on the  source-location database of loaded  predicates
also used  by edit/1.   This makes locating  predicates reliable if  all
sources are loaded and up-to-date (see make/0).

In addition,  references to files  in use_module/[1,2], consult/1,  etc.
are red if the file cannot be found and underlined blue  if the file can
be loaded.  A popup allows for opening the referenced file.


33..55 TThhee GGrraapphhiiccaall DDeebbuuggggeerr

SWI-Prolog offers  two debuggers.   One is the traditional  text-console
based 4-port Prolog tracer and the other is  a window-based source-level
debugger.    The window-based  debugger  requires XPCE  installed.    It
operates  based  on   the  prolog_trace_interception/4 hook   and  other
low-level functionality described in chapter 12.

Window-based tracing  provides much better overview  due to the  eminent
relation to your source-code, a clear list of  named variables and their
bindings as well  as a graphical overview  of the call and  choice-point
stack.  There are  some drawbacks though.  Using a textual trace  on the
console one can  scroll back and examine  the past, while the  graphical
debugger just presents a (much better) overview of the current state.


33..55..11 IInnvvookkiinngg tthhee wwiinnddooww--bbaasseedd ddeebbuuggggeerr

Whether the  text-based or window-based debugger  is used is  controlled
using  the predicates  guitracer/0 and  noguitracer/0.   Entering  debug
mode is controlled  using the normal predicates  for this:  trace/0  and
spy/1.    In  addition,  PceEmacs prolog  mode  provides the  command  )
Prolog/Break  at (Control-c b)  to insert  a break-point  at a  specific
location in the source-code.

The graphical tracer is  particulary useful for debugging threads.   The
tracer must be loaded from the main thread before it can  be used from a
background thread.


gguuiittrraacceerr
    This  predicate  installs the  above-mentioned hooks  that  redirect
    tracing  to  the  window-based  environment.    No  window  appears.
    The  debugger window  appears as actual  tracing is started  through
    trace/0,  by hitting a spy-point  defined by spy/1 or a  break-point
    defined using PceEmacs command ) Prolog/Break at (Control-c b).


nnoogguuiittrraacceerr
    Disable  the hooks  installed by  guitracer/0,  reverting to  normal
    text-console based tracing.


ggttrraaccee
    Utility defined as guitracer,trace.


ggddeebbuugg
    Utility defined as guitracer,debug.


ggssppyy((_+_P_r_e_d_i_c_a_t_e))
    Utility defined as guitracer,spy(Predicate).


33..66 TThhee PPrroolloogg NNaavviiggaattoorr

Another  tool is  the  _P_r_o_l_o_g  _N_a_v_i_g_a_t_o_r.    This  tool can  be  started
from  PceEmacs using  the command  ) Browse/Prolog  navigator, from  the
GUI  debugger or  using  the  programmatic IDE  interface  described  in
section 3.8.


33..77 CCrroossss rreeffeerreenncceerr

A  cross-referencers is  a  tool  examining the  caller-callee  relation
between predicates  and using this  information to explicate  dependency
relations between  source files, find  calls to non-existing  predicates
and predicates  for which no  callers can be  found.   Cross-referencing
is useful during  program development, reorganisation, cleanup,  porting
and  other program  maintenance tasks.    The dynamic  nature of  Prolog
makes the  task non-trivial.   Goals can  be created dynamically  call/1
after construction  of a  goal term.   Abtract  interpretation can  find
some  of such  calls, but  the ultimately  they can  come from  external
communication, making  it completely impossible  to predict the  callee.
In other words,  the cross-referencer has only partial  understanding of
the  program and  its results  are necessarily  incomplete.   Still,  it
provides valuable information to the developer.

SWI-Prolog's cross-referencer  is split into  two parts.   The  standard
Prolog  library prolog_xref  is an  extensible library  for  information
gathering described in section 11.17 and the XPCE

library pce_xref provides a graphical frontend for  the cross-referencer
described here.   We demonstrate the tool on CHAT80, a  natural language
question  and answer  system by  Fernando C.N.  Pereira  and David  H.D.
Warren.


ggxxrreeff
    Run  cross-referencer on  all currently loaded  files and present  a
    graphical  overview of  the result.   As  the predicate operates  on
    the  currently loaded application it  must be run after loading  the
    application.

The lleefftt wwiinnddooww  (see figure ???? provides  browsers for loaded files  and
predicates.  To avoid long file paths the file  hierarchy has three main
branches.  The first is the current directory holding the  sources.  The
second is  marked alias and  below it  are the file-search-path  aliases
(see file_search_path/2 and absolute_file_name/3).   Here you find  files
loaded from  the system as  well as modules of  the program loaded  from
other locations  using file  search path.   All  loaded files that  fall
outside these  categories are  below the  last branch  called /.    File
where  the system  found  suspicious  dependencies are  marked  with  an
exclamation mark.   This also holds for directories holding  such files.
Clicking on a file opens a _F_i_l_e _i_n_f_o window in the right pane.

The FFiillee iinnffoo  window shows a file,  its main properties, its  undefined
and not-called predicates and its import- and export  relations to other
files  in the  project.   Both  predicates and  files can  be opened  by
clicking  on them.    The number  of callers  in a  file  for a  certain
predicate is  indicated with  a blue  underlined number.   A  left-click
will open a list and allows to edit the calling predicate.

The DDeeppeennddeenncciieess  (see figure ????) window  displays a graphical  overview
of dependencies  between files.   Using the  background menu a  complete
graph of the project can be created.  It is  also possible to drag files
onto the  graph window and  use the menu on  the nodes to  incrementally
expand the  graph.   The underlined  blue text  indicates the number  of
predicates used in the destination file.  Left-clicking  opens a menu to
open the definition or select one of the callers.

MMoodduullee  aanndd nnoonn--mmoodduullee  ffiilleess The  cross-referencer threads  module  and
non-module  project files  differently.    Module  files  have  explicit
import  and  export   relations  and  the  tool  shows  the   usage  and
consistency of the  relations.  Using  the menu-command Header the  tool
creates a  consistent import list  for the module  that can be  included
in the  file.   The tool computes the  dependency relations between  the
non-module  files.   If  the user  wishes to  convert  the project  into
a module-based  one the Header command  generates an appropriate  module
header and import list.  Note that the  cross-referencer may have missed
dependencies  and does  not  deal with  meta-predicates defined  in  one
module and called in another.  Such problems must be resolved manually.

SSeettttiinnggss The  following settings  can be  controlled  from the  settings
menu:

WWaarrnn aauuttoollooaadd
    By  default disabled.   If enabled, modules that require  predicates
    to  be  autoloaded are  flagged with  a warning  and  the file  info
    window of a module shows the required autoload predicates.

WWaarrnn nnoott ccaalllleedd
    If  enabled (default),  the file-overview  shows an  alert icon  for
    files that have predicates that are not called.


33..88 AAcccceessssiinngg tthhee IIDDEE ffrroomm yyoouurr pprrooggrraamm

Over  the years  a collection  of IDE  components  have been  developed,
each with their  own interface.   In addition, some of these  components
require  each other  and loading  IDE components  must be  on demand  to
avoid the IDE  being part of a  saved-state (see qsave_program/2).   For
this  reason,  access  to the  IDE  will  be concentrated  on  a  single
interface called prolog_ide/1:


pprroolloogg__iiddee((_+_A_c_t_i_o_n))
    This  predicate ensures the IDE  enabling XPCE component is  loaded,
    creates  the XPCE class _p_r_o_l_o_g___i_d_e and sends  _A_c_t_i_o_n to its one  and
    only  instance \index{@prolog_ide}\objectname{prolog_ide}.    _A_c_t_i_o_n
    is one of the following:

    ooppeenn__nnaavviiggaattoorr((_+_D_i_r_e_c_t_o_r_y))
         Open  the Prolog  Navigator  (see  section 3.6)  in  the  given
         _D_i_r_e_c_t_o_r_y.

    ooppeenn__ddeebbuugg__ssttaattuuss
         Open a window to edit spy- and trace-points.

    ooppeenn__qquueerryy__wwiinnddooww
         Opens  a  little  window to  run  Prolog  queries  from  a  GUI
         component.

    tthhrreeaadd__mmoonniittoorr
         Open a graphical  window indicating existing threads and  their
         status.

    ddeebbuugg__mmoonniittoorr
         Open a graphical front-end for the debug  library that provides
         an overview of the topics and catches messages.

    xxrreeff
         Open  a  graphical  front-end  for  the  cross-referencer  that
         provides an overview of predicates and their callers.


33..99 SSuummmmaarryy ooff tthhee iiDDEE

The  SWI-Prolog  development   environment  consists  of  a  number   of
interrelated but  not (yet) integrated  tools.   Here is  a list of  the
most important features and tips.

  o _A_t_o_m _c_o_m_p_l_e_t_i_o_n
    The  console  completes a  partial atom  on the  TAB  key and  shows
    alternatives on the command Alt-?.

  o _U_s_e edit/1 _t_o _f_i_n_d_i_n_g _l_o_c_a_t_i_o_n_s
    The  command edit/1 takes the name  of a file, module, predicate  or
    other  entity registered  through  extensions and  starts the  users
    preferred editor at the right location.

  o _S_e_l_e_c_t _e_d_i_t_o_r
    External   editors  are  selected   using  the  EDITOR   environment
    variable, by setting  the prolog flag editor or by defining the hook
    prolog_edit:edit_source/1.

  o _U_p_d_a_t_e _P_r_o_l_o_g _a_f_t_e_r _e_d_i_t_i_n_g
    Using make/0, all files you have edited are re-loaded.

  o _P_c_e_E_m_a_c_s
    Offers  syntax-highlighting and checking based on  real-time parsing
    of the editor's buffer, layout-support and navigation support.

  o _U_s_i_n_g _t_h_e _g_r_a_p_h_i_c_a_l _d_e_b_u_g_g_e_r
    The   predicates  guitracer/0   and  noguitracer/0  switch   between
    traditional  text-based and window-based debugging.   The tracer  is
    activated  using the trace/0, spy/1  or menu-items from PceEmacs  or
    the PrologNavigator.

  o _T_h_e _P_r_o_l_o_g _N_a_v_i_g_a_t_o_r
    Shows  the file-structure and structure inside the file.   It allows
    for loading files, editing, setting spy-points, etc.


CChhaapptteerr 44..  BBUUIILLTT--IINN PPRREEDDIICCAATTEESS


44..11 NNoottaattiioonn ooff PPrreeddiiccaattee DDeessccrriippttiioonnss

We have  tried to  keep the  predicate descriptions  clear and  concise.
First  the predicate  name is  printed  in bold  face, followed  by  the
arguments in  italics.   Arguments  are preceded by  a `+',  `-' or  `?'
sign.    `+'  indicates the  argument is  input  to the  predicate,  `-'
denotes output and  `?'  denotes `either  input or output'.   Constructs
like  `op/3' refer  to the  predicate `op'  with arity  `3'.    Finally,
arguments may  have the `:'   specifier, which  implies the argument  is
module-sensitive.   Normally the argument  is a _c_a_l_l_a_b_l_e term  referring
to  a  predicate  in  a specific  module.     See  section  5  for  more
information on module-handing.


44..22 CChhaarraacctteerr rreepprreesseennttaattiioonn

In  traditional (Edinburgh-)  Prolog, characters  are represented  using
_c_h_a_r_a_c_t_e_r_-_c_o_d_e_s.   Character codes are  integer indices into a  specific
character set.   Traditionally  the character  set was 7-bits  US-ASCII.
8-bit  character sets  have  been allowed  for  a long  time,  providing
support for national  character sets, of which iso-latin-1  (ISO 8859-1)
is applicable  to many western  languages.   Text-files are supposed  to
represent a sequence of character-codes.

ISO Prolog introduces three types, two of which  are used for characters
and one for accessing binary streams (see open/4).  These types are:

  o _c_o_d_e
    A  _c_h_a_r_a_c_t_e_r_-_c_o_d_e is  an  integer representing  a single  character.
    As  files  may  use  multi-byte encoding  for  supporting  different
    character  sets (utf-8 encoding for example), reading a code  from a
    text-file is in general not the same as reading a byte.

  o _c_h_a_r
    Alternatively,  characters  may  be  represented  as  _o_n_e_-_c_h_a_r_a_c_t_e_r_-
    _a_t_o_m_s.    This is  a  very natural  representation, hiding  encoding
    problems  from  the  programmer as  well  as providing  much  easier
    debugging.

  o _b_y_t_e
    Bytes are used for accessing binary-streams.

The  current  version  of  SWI-Prolog  does  not   provide  support  for
multi-byte character encoding.  This implies for example  that it is not
capable of  breaking a  multi-byte encoded  atom into characters.    For
SWI-Prolog, bytes  and codes  are the same  and one-character-atoms  are
simple atoms containing one byte.

To  ease  the  pain  of  these  multiple  representations,  SWI-Prolog's
built-in  predicates dealing  with character-data  work  as flexible  as
possible:   they accept  data in  any of these  formats as  long as  the
interpretation  is unambiguous.     In addition,  for  output  arguments
that are  instantiated, the character  is extracted before  unification.
This implies  that the following two  calls are identical, both  testing
whether the next input characters is an a.

peek_code(Stream, a).
peek_code(Stream, 97).

These  multiple-representations  are  handled  by  a   large  number  of
built-in  predicates,   all   of  which   are  ISO-compatible.       For
converting  between  code and  character  there  is  char_code/2.     For
breaking  atoms  and  numbers  into  characters  are  are  atom_chars/2,
atom_codes/2,  number_codes/2 and  number_chars/2.    For  character  I/O
on  streams  there  is get_char/[1,2],  get_code/[1,2],  get_byte/[1,2],
peek_char/[1,2],   peek_code/[1,2],   peek_byte/[1,2],   put_code/[1,2],
put_char/[1,2]  and  put_byte/[1,2].     The  prolog-flag  double_quotes
(see current_prolog_flag/2) controls how  text between double-quotes  is
interpreted.


44..33 LLooaaddiinngg PPrroolloogg ssoouurrccee ffiilleess

This section deals  with loading Prolog source-files.   A Prolog  source
file is a plain  text file containing a Prolog program or  part thereof.
Prolog source files come in three flavours:

 AA ttrraaddiittiioonnaall   Prolog  source  file  contains  a  Prolog  clauses  and
    directives,  but no  _m_o_d_u_l_e_-_d_e_c_l_a_r_a_t_i_o_n.   They are normally  loaded
    using consult/1 or ensure_loaded/1.

 AA mmoodduullee   Prolog source file  starts with a module  declaration.   The
    subsequent Prolog code  is loaded into the specified module and only
    the _p_u_b_l_i_c predicates  are made available to the context loading the
    module.   Module  files are normally loaded  using use_module/[1,2].
    See chapter 5 for details.

 AAnn iinncclluuddee   Prolog source file is loaded using the include/1 directive
    and normally contains only directives.

Prolog  source-files are  located  using  absolute_file_name/3 with  the
following options:

locate_prolog_file(Spec, Path) :-
        absolute_file_name(Spec,
                           [ file_type(prolog),
                             access(read)
                           ],
                           Path).

The file_type(_p_r_o_l_o_g) option is  used to determine the extension of  the
file using  prolog_file_type/2.    The default extension  is .pl.    _S_p_e_c
allows  for the  _p_a_t_h_-_a_l_i_a_s construct  defined  by absolute_file_name/3.
The most commonly used path-alias is library(_L_i_b_r_a_r_y_F_i_l_e).   The example
below  loads the  library  file  ordsets.pl (containing  predicates  for
manipulating ordered sets).

:- use_module(library(ordsets)).

SWI-Prolog   recognises    grammar   rules    (DCG)   as   defined    in
[Clocksin & Melish, 1987].   The user may define additional  compilation
of the source  file by defining the  dynamic predicates term_expansion/2
and  goal_expansion/2.     Transformations by  term_expansion/2  overrule
the systems  grammar rule  transformations.   It is  not allowed to  use
assert/1, retract/1 or any other  database predicate in term_expansion/2
other than for local computational purposes.

Directives  may be  placed  anywhere  in a  source  file,  invoking  any
predicate.    They are  executed when  encountered.    If the  directive
fails, a warning is printed.  Directives are specified  by :-/1 or ?-/1.
There is no difference between the two.

SWI-Prolog   does   not   have   a   separate   reconsult/1   predicate.
Reconsulting  is  implied automatically  by  the  fact that  a  file  is
consulted which is already loaded.


llooaadd__ffiilleess((_+_F_i_l_e_s_, _+_O_p_t_i_o_n_s))
    The  predicate load_files/2 is the parent  of all the other  loading
    predicates  except for include/1.   It  currently supports a  subset
    of  the options of Quintus load_files/2.   _F_i_l_e_s is either a  single
    source-file,  or a list  of source-files.   The specification for  a
    source-file  is handed to absolute_file_name/2.  See this  predicate
    for  the supported expansions.   _O_p_t_i_o_n_s is a list of options  using
    the format

         _O_p_t_i_o_n_N_a_m_e(_O_p_t_i_o_n_V_a_l_u_e)

    The following options are currently supported:

    aauuttoollooaadd((_B_o_o_l))
         If  true  (default false),  indicate  this  load  is  a  _d_e_m_a_n_d
         load.   This  implies that,  depending  on the  setting of  the
         prolog-flag  verbose_autoload  the  load-action  is  printed  at
         level informational  or silent.   See also  print_message/2 and
         current_prolog_flag/2.

    ddeerriivveedd__ffrroomm((_F_i_l_e))
         Indicate that the  loaded file is derived  from _F_i_l_e.  Used  by
         make/0 to  time-check and  load the original  file rather  than
         the derived file.

    eennccooddiinngg((_E_n_c_o_d_i_n_g))
         Specify the way  characters are encoded in  the file.   Default
         is taken  from the prolog  flag encoding.   See section  2.17.1
         for details.

    eexxppaanndd((_B_o_o_l))
         If true, run the  filenames through expand_file_name/2 and load
         the returned  files.   Default is false,  except for  consult/1
         which is intended  for interactive use.   Flexible location  of
         files is defined by file_search_path/2.

    iiff((_C_o_n_d_i_t_i_o_n))
         Load the  file only  if the specified  condition is  satisfied.
         The value  true loads the  file unconditionally, changed  loads
         the file  if it  was not loaded  before, or  has been  modified
         since it was loaded the last time, not_loaded loads the file if
         it was not loaded before.

    iimmppoorrttss((_L_i_s_t_O_r_A_l_l))
         If  all and  the  file is  a  module file,  import  all  public
         predicates.  Otherwise import only the named predicates.   Each
         predicate is referred to as <_n_a_m_e>/<_a_r_i_t_y>.   This option has no
         effect if the file is not a module file.

    mmuusstt__bbee__mmoodduullee((_B_o_o_l))
         If true,  raise an  error if  the file  is not  a module  file.
         Used by use_module/[1,2].

    qqccoommppiillee((_B_o_o_l))
         If this call appears in a directive of a file  that is compiled
         into Quick Load Format using qcompile/1 and this  flag is true,
         the contents  of the argument  files are  included in the  .qlf
         file instead of the loading directive.

    ssiilleenntt((_B_o_o_l))
         If  true, load  the  file  without printing  a  message.    The
         specified  value is  the  default for  all  files loaded  as  a
         result of loading the specified files.  This  option writes the
         prolog flag verbose_load with the negation of _B_o_o_l.

    ssttrreeaamm((_I_n_p_u_t))
         This SWI-Prolog  extension compiles  the data  from the  stream
         _I_n_p_u_t.    If  this option  is  used,  _F_i_l_e_s  must be  a  single
         atom  which is  used to  identify  the source-location  of  the
         loaded clauses  as well as  remove all clauses  if the data  is
         re-consulted.

         This  option  is   added  to  allow  compiling  from   non-file
         locations such as databases, the web, the  _u_s_e_r (see consult/1)
         or other servers.

    The  load_files/2 predicate  can be  hooked to  load  other data  or
    data  from other objects than  files.  See  prolog_load_file/2for  a
    description and http_load for an example.


ccoonnssuulltt((_+_F_i_l_e))
    Read _F_i_l_e as a Prolog  source file.  _F_i_l_e may be a list of files, in
    which  case all members are consulted in turn.  _F_i_l_e may  start with
    the  Unix shell special  sequences ~,  <_u_s_e_r> and $<_v_a_r>.  _F_i_l_e  may
    also be library(Name),  in which case the libraries are searched for
    a  file with the specified  name.  See  also library_directory/1 and
    file_search_path/2.  consult/1  may be abbreviated by just  typing a
    number of file names in a list.  Examples:

        ?- consult(load).       % consult load or load.pl
        ?- [library(quintus)].  % load Quintus compatibility library
        ?- [user].

    The  predicate  consult/1 is  equivalent  to load_files(Files,  []),
    except for handling  the special file user, which reads clauses from
    the terminal.  See also the stream(_I_n_p_u_t) option of load_files/2.


eennssuurree__llooaaddeedd((_+_F_i_l_e))
    If the file  is not already loaded, this is equivalent to consult/1.
    Otherwise,   if  the  file  defines  a  module,  import  all  public
    predicates.    Finally,  if the  file is  already loaded,  is not  a
    module  file and the context module  is not the global user  module,
    ensure_loaded/1 will call consult/1.

    With the semantics, we  hope to get as closely possible to the clear
    semantics  without the presence  of a module  system.   Applications
    using modules should consider using use_module/[1,2].

    Equivalent to load_files(Files, [if(not_loaded)]).


iinncclluuddee((_+_F_i_l_e))
    Pretend  the  terms   in  _F_i_l_e  are  in  the  source-file  in  which
    :- include(File)  appears.  The  include construct is only  honoured
    if  it appears  as  a directive  in a  source-file.   Normally  _F_i_l_e
    contains a sequence of directives.


rreeqquuiirree((_+_L_i_s_t_O_f_N_a_m_e_A_n_d_A_r_i_t_y))
    Declare  that  this file/module  requires the  specified  predicates
    to  be defined ``with  their commonly accepted  definition''.   This
    predicate originates from  the Prolog portability layer for XPCE. It
    is  intended to  provide a  portable mechanism  for specifying  that
    this module requires the specified predicates.

    The implementation normally  first verifies whether the predicate is
    already defined.   If not, it will search the libraries and load the
    required library.

    SWI-Prolog, having autoloading,  does nnoott load the library.  Instead
    it  creates a  procedure header  for the  predicate if  it does  not
    exist.    This will flag  the predicate  as `undefined'.   See  also
    check/0 and autoload/0.


eennccooddiinngg((_+_E_n_c_o_d_i_n_g))
    This  directive can appear anywhere in  a source file to define  how
    characters  are  encoded in  the remainder  of  the file.    It  can
    be  used in  files that  are encoded  with a  superset of  US-ASCII,
    currently UTF-8 and ISO Latin-1.  See also section 2.17.1.


mmaakkee
    Consult  all source  files that  have been changed  since they  were
    consulted.   It checks _a_l_l  loaded source files:  files loaded  into
    a  compiled state  using pl -c ...  and files  loaded using  consult
    or  one  of  its  derivatives.    The  predicate  make/0  is  called
    after  edit/1, automatically reloading all  modified files.  It  the
    user  uses an  external editor  (in  a separate  window), make/0  is
    normally  used to update  the program after  editing.  In  addition,
    make/0  updates the  autoload indices  (see section  2.13) and  runs
    list_undefined/0  from  the check  library  to report  on  undefined
    predicates.


lliibbrraarryy__ddiirreeccttoorryy((_?_A_t_o_m))
    Dynamic  predicate used  to specify  library directories.    Default
    ./lib,  ~/lib/prolog and  the system's library  (in this order)  are
    defined.    The  user may  add library  directories using  assert/1,
    asserta/1 or remove system defaults using retract/1.


ffiillee__sseeaarrcchh__ppaatthh((_+_A_l_i_a_s_, _?_P_a_t_h))
    Dynamic  predicate used to specify `path-aliases'.  This  feature is
    best described using an example.  Given the definition

    file_search_path(demo, '/usr/lib/prolog/demo').

    the  file specification demo(myfile)  will be expanded to  /usr/lib/
    prolog/demo/myfile.   The second  argument of file_search_path/2 may
    be another alias.

    Below  is the  initial definition  of the file  search path.    This
    path  implies  swi(<_P_a_t_h>)  refers  to  a  file  in  the  SWI-Prolog
    home   directory.     The  alias  foreign(<_P_a_t_h>) is   intended  for
    storing   shared  libraries  (.so  or   .DLL  files).     See   also
    load_foreign_library/[1,2].

    user:file_search_path(library, X) :-
            library_directory(X).
    user:file_search_path(swi, Home) :-
            current_prolog_flag(home, Home).
    user:file_search_path(foreign, swi(ArchLib)) :-
            current_prolog_flag(arch, Arch),
            atom_concat('lib/', Arch, ArchLib).
    user:file_search_path(foreign, swi(lib)).

    The  file_search_path/2expansion  is used by all loading  predicates
    as well as by absolute_file_name/[2,3].

    The  prolog-flag  verbose_file_search  can be  set  to true  to  help
    debugging Prolog's search for files.


eexxppaanndd__ffiillee__sseeaarrcchh__ppaatthh((_+_S_p_e_c_, _-_P_a_t_h))
    Unifies  _P_a_t_h  with   all  possible  expansions  of  the  file  name
    specification _S_p_e_c.  See also absolute_file_name/3.


pprroolloogg__ffiillee__ttyyppee((_?_E_x_t_e_n_s_i_o_n_, _?_T_y_p_e))
    This  dynamic multifile predicate defined in module  user determines
    the  extensions considered by file_search_path/2.  _E_x_t_e_n_s_i_o_n is  the
    filename  extension without the leading  dot, _T_y_p_e denotes the  type
    as  used by the file_type(_T_y_p_e) option of  file_search_path/2.   Here
    is the initial definition of prolog_file_type/2:

    user:prolog_file_type(pl,       prolog).
    user:prolog_file_type(Ext,      prolog) :-
            current_prolog_flag(associate, Ext),
            Ext \== pl.
    user:prolog_file_type(qlf,      qlf).
    user:prolog_file_type(Ext,      executable) :-
            current_prolog_flag(shared_object_extension, Ext).

    Users  may  wish to  change  the extension  used for  Prolog  source
    files  to avoid  conflicts (for  example with  perl) as  well as  to
    be  compatible with  some specific  implementation.   The  preferred
    alternative extension is .pro.


ssoouurrccee__ffiillee((_?_F_i_l_e))
    True  if _F_i_l_e is a loaded Prolog source file.  _F_i_l_e  is the absolute
    and canonical path to the source-file.


ssoouurrccee__ffiillee((_?_P_r_e_d_, _?_F_i_l_e))
    Is   true   if  the   predicate  specified   by   _P_r_e_d  was   loaded
    from   file  _F_i_l_e,  where  _F_i_l_e  is  an  absolute  path   name  (see
    absolute_file_name/2).  Can be used with  any instantiation pattern,
    but the database  only maintains the source file for each predicate.
    See also clause_property/2.


pprroolloogg__llooaadd__ccoonntteexxtt((_?_K_e_y_, _?_V_a_l_u_e))
    Determine loading context.  The following keys are defined:

      ________________________________________________________________
      |__KKeeyy______________________||DDeessccrriippttiioonn________________________________________________________________________||
      || module        |Module into which file is loaded               |
      | source        |File  loaded.    Returns  the  original  Prolog|
      |               |file when loading  a .qlf file.   Compatible to|

      |               |SICStus Prolog.                                |
      | file          |Currently  equivalent  to  file.     In  future|
      |               |versions it  may report a different  values for|
      |               |files being loaded using include/1.            |
      | stream        |Stream identifier (see current_input/1)        |
      | directory     |Directory in which _F_i_l_e lives.                 |
      | term_position |Position of last  term read.  Term of  the form|

      |               |'$stream_position'(0,<_L_i_n_e>,0,0,0).    See  also|
      |_______________|stream_position_data/3.________________________|

    Quintus compatibility predicate.  See also source_location/2.


ssoouurrccee__llooccaattiioonn((_-_F_i_l_e_, _-_L_i_n_e))
    If the last term  has been read from a physical file (i.e., not from
    the file user or  a string), unify _F_i_l_e with an absolute path to the
    file  and _L_i_n_e with the  line-number in the file.   New code  should
    use prolog_load_context/2.


tteerrmm__eexxppaannssiioonn((_+_T_e_r_m_1_, _-_T_e_r_m_2))
    Dynamic  and  multifile  predicate,  normally  not defined.     When
    defined  by the user all terms  read during consulting are given  to
    this predicate.   If the predicate succeeds Prolog will assert _T_e_r_m_2
    in  the database rather then the read term (_T_e_r_m_1).  _T_e_r_m_2  may be a
    term of a the form  `?- _G_o_a_l' or `:- _G_o_a_l'.  _G_o_a_l is then treated as
    a  directive.  If _T_e_r_m_2 is a  list all terms of the list  are stored
    in  the database or  called (for directives).   If  _T_e_r_m_2 is of  the
    form  below, the system will assert _C_l_a_u_s_e and record  the indicated
    source-location with it.

         '$source_location'(<_F_i_l_e>, <_L_i_n_e>):<_C_l_a_u_s_e>

    When compiling a  module (see chapter 5 and the directive module/2),
    expand_term/2  will first try  term_expansion/2 in the module  being
    compiled  to  allow  for  term-expansion rules  that  are  local  to
    a  module.     If  there  is  no  local  definition,  or  the  local
    definition  fails  to translate  the  term,  expand_term/2 will  try
    term_expansion/2  in module user.    For compatibility with  SICStus
    and  Quintus Prolog,  this feature  should not  be used.   See  also
    expand_term/2, goal_expansion/2 and expand_goal/2.


eexxppaanndd__tteerrmm((_+_T_e_r_m_1_, _-_T_e_r_m_2))
    This  predicate  is  normally  called by  the  compiler  to  perform
    preprocessing.   First it calls term_expansion/2.   If this predicate
    fails  it performs  a grammar-rule translation.    If this fails  it
    returns the first argument.


ggooaall__eexxppaannssiioonn((_+_G_o_a_l_1_, _-_G_o_a_l_2))
    Like   term_expansion/2,   goal_expansion/2  provides   for   macro-
    expansion  of Prolog  source-code.   Between  expand_term/2 and  the
    actual  compilation, the body of clauses analysed and the  goals are
    handed  to expand_goal/2, which  uses the goal_expansion/2 hook to do
    user-defined expansion.

    The  predicate goal_expansion/2 is first  called in the module  that
    is  being compiled, and then on the user module.  If _G_o_a_l  is of the
    form  _M_o_d_u_l_e:_G_o_a_l where _M_o_d_u_l_e is instantiated,  goal_expansion/2 is
    called on _G_o_a_l using rules from module _M_o_d_u_l_e followed by user.

    Only  goals  appearing  in  the  body  of  clauses  when  reading  a
    source-file  are expanded using mechanism,  and only if they  appear
    literally  in the clause, or  as an argument to the  meta-predicates
    not/1,  call/1,  once/1, ignore/1,  findall/3,  bagof/3, setof/3  or
    forall/2.    A real predicate  definition is  required to deal  with
    dynamically constructed calls.


eexxppaanndd__ggooaall((_+_G_o_a_l_1_, _-_G_o_a_l_2))
    This  predicate  is  normally  called by  the  compiler  to  perform
    preprocessing.   First it calls goal_expansion/2.  If this  fails it
    returns the first argument.


aatt__iinniittiiaalliizzaattiioonn((_+_G_o_a_l))
    Register   _G_o_a_l   to   be   run   when   the   system   initialises.
    Initialisation  takes place after  reloading a .qlf (formerly  .wic)
    file  as well as after reloading a  saved-state.  The hooks  are run
    in  the order they were registered.  A warning message is  issued if
    _G_o_a_l fails, but execution continues.  See also at_halt/1


aatt__hhaalltt((_+_G_o_a_l))
    Register  _G_o_a_l to  be run  from PL_cleanup(), which  is called  when
    the  system halts.   The  hooks are  run in the  reverse order  they
    were  registered  (FIFO). Success  or failure  executing  a hook  is
    ignored.   If  the hook  raises an exception  this is printed  using
    print_message/2.    An attempt  to call  halt/[0,1] from  a hook  is
    ignored.


iinniittiiaalliizzaattiioonn((_+_G_o_a_l))
    Call  _G_o_a_l and  register it using  at_initialization/1.   Directives
    that  do  other things  than  creating clauses,  records,  flags  or
    setting  predicate attributes should normally be written  using this
    tag  to ensure the  initialisation is executed  when a saved  system
    starts.  See also qsave_program/[1,2].


ccoommppiilliinngg
    True  if the system is compiling source files with the -c  option or
    qcompile/1  into an intermediate code file.  Can be used  to perform
    conditional code  optimisations in term_expansion/2(see  also the -O
    option) or to omit execution of directives during compilation.


pprreepprroocceessssoorr((_-_O_l_d_, _+_N_e_w))
    Read  the input file via a  Unix process that acts as  preprocessor.
    A  preprocessor is specified  as an atom.   The first occurrence  of
    the  string `%f' is replaced by the  name of the file to  be loaded.
    The  resulting atom  is called as  a Unix  command and the  standard
    output  of this command is loaded.   To use the Unix  C preprocessor
    one should define:

    ?- preprocessor(Old, '/lib/cpp -C -P %f'), consult(...).

    Old = none


44..33..11 LLooaaddiinngg ffiilleess,, aaccttiivvee ccooddee aanndd tthhrreeaaddss

Traditionally,  Prolog environments  allow for  reloading files  holding
currently active code.   In particular, the following sequence  is valid
use of the development environment:

  o Trace a goal

  o Find unexpected behaviour of a predicate

  o Enter a _b_r_e_a_k using the bb command

  o Fix the sources and reload them using make/0

  o Exit the break, _r_e_t_r_y using the rr command

Goals running  during the  reload keep  running on  the old  definition,
while new  goals use  the reloaded definition,  which is  why the  _r_e_t_r_y
must be used _a_f_t_e_r the reload.  This implies  that clauses of predicates
that are  active during  the reload  cannot be  reclaimed.   Normally  a
small amount of dead clauses should not be an  issue during development.
Such clauses can be reclaimed with garbage_collect_clauses/0.


ggaarrbbaaggee__ccoolllleecctt__ccllaauusseess
    leanup  all _d_i_r_t_y  predicates,  where dirty  predicates are  defined
    to  be predicates  that have  both old  and new  definitions due  to
    reloading  a  source  file while  the  predicate  was active.     Of
    course,  predicates that are  active using garbage_collect_clauses/0
    cannot  be reclaimed and remain _d_i_r_t_y.   Predicate are -like  atoms-
    shared resources and  therefore all threads are suspended during the
    execution of this predicate.


44..33..11..11 TThhrreeaaddss aanndd rreellooaaddiinngg rruunnnniinngg ccooddee

As of version 5.5.30, there is basic thread-safety  for reloading source
files while  other threads are  executing code  defined in these  source
files.   Reloading a file freezes  all threads after marking the  active
predicates originating from  the file being reloaded.   The threads  are
resumed after the file  has been loaded.  In addition,  after completing
loading the outermost file the system runs garbage_collect_clauses/0.

What does that mean?   Unfortunately it does _n_o_t mean we  can `hot-swap'
modules.   Consider the case where thread  A is executing the recursive
predicate P.   We `fix'  P and reload.   The already  running goals for
P  continue to  run the  old definition,  but new  recursive calls  will
use the  new definition!   Many  similar cases  can be constructed  with
dependent predicates.

It provides  some basic security for  reloading files in  multi-threaded
applications during  development.   In  the above  scenarios the  system
does not crash  uncontrolled, but behaves like  any broken program:   it
may return the wrong bindings, wrong truth value or raise an exception.

Future versions  may have an  `update now' facility.   Such as  facility
can be implemented  on top of the _l_o_g_i_c_a_l  _u_p_d_a_t_e _v_i_e_w.  It would  allow
threads to do a controlled update between processing independent jobs.


44..33..22 QQuuiicckk llooaadd ffiilleess

SWI-Prolog supports compilation of individual or  multiple Prolog source
files into `Quick Load Files'.  A `Quick Load  Files' (.qlf file) stores
the contents of the file in a precompiled format.

These files load considerably faster than source files  and are normally
more compact.    They are  machine independent  and may  thus be  loaded
on any  implementation of  SWI-Prolog.   Note however  that clauses  are
stored as virtual  machine instructions.   Changes to the compiler  will
generally make old compiled files unusable.

Quick Load Files  are created using qcompile/1.   They are loaded  using
consult/1  or one  of  the other  file-loading predicates  described  in
section 4.3.   If consult is given  the explicit .pl file, it will  load
the Prolog  source.  When  given the .qlf file,  it will load the  file.
When no extension is specified, it will load the  .qlf file when present
and the .pl file otherwise.


qqccoommppiillee((_+_F_i_l_e))
    Takes  a single  file specification  like  consult/1 (i.e.,  accepts
    constructs  like library(LibFile)  and,  in addition  to the  normal
    compilation,   creates  a   _Q_u_i_c_k  _L_o_a_d  _F_i_l_e   from  _F_i_l_e.      The
    file-extension  of this file is  .qlf.  The  base name of the  Quick
    Load File is the same as the input file.

    If   the   file  contains   `:- consult(+File)',   `:- [+File]'   or
    :- load_files(+File, [qcompile(true), ...])   statements,  the   re-
    ferred  files  are  compiled  into  the  same  .qlf  file.     Other
    directives will be stored  in the .qlf file and executed in the same
    fashion as when loading the .pl file.

    For  term_expansion/2,  the same rules  as described in section  2.10
    apply.

    Conditional  execution   or  optimisation  may  test  the  predicate
    compiling/0.

    Source  references (source_file/2) in the  Quick Load File refer  to
    the Prolog source file from which the compiled code originates.


44..44 LLiissttiinngg aanndd EEddiittoorr IInntteerrffaaccee

SWI-Prolog  offers an  extensible  interface which  allows the  user  to
edit objects  of the program:   predicates,  modules, files,  etc.   The
editor interface is  implemented by edit/1 and consists of  three parts:
_l_o_c_a_t_i_n_g, _s_e_l_e_c_t_i_n_g and _s_t_a_r_t_i_n_g _t_h_e _e_d_i_t_o_r.

Any of  these parts may  be extended or redefined  by adding clauses  to
various multi-file  (see multifile/1) predicates  defined in the  module
prolog_edit.

The built-in  edit specifications for  edit/1 (see prolog_edit:locate/3)
are described below.

   ___________________________________________________________________
   |__________________________________________FFuullllyy__ssppeecciiffiieedd__oobbjjeeccttss____________________________________________||
   || <_M_o_d_u_l_e>:<_N_a_m_e>/<_A_r_i_t_y>R|efers a predicate                      |
   | module(<_M_o_d_u_l_e>)      |Refers to a module                       |
   | file(<_P_a_t_h>)          |Refers to a file                         |

   |_source_file(<_P_a_t_h>)___R|efers_to_a_loaded_source-file___________|_
   |__________________________________________AAmmbbiigguuoouuss__ssppeecciiffiiccaattiioonnss__________________________________________||
   || <_N_a_m_e>/<_A_r_i_t_y>        R|efers this predicate in any module      |
   | <_N_a_m_e>                |Refers  to  (1) named  predicate  in  any|
   |                       |module  with any  arity,  (2) a  (source)|
   |_______________________|file_or_(3)_a_module.____________________|_


eeddiitt((_+_S_p_e_c_i_f_i_c_a_t_i_o_n))
    First  exploits prolog_edit:locate/3 to translate  _S_p_e_c_i_f_i_c_a_t_i_o_n into
    a  list  of  _L_o_c_a_t_i_o_n_s.    If there  is  more than  one  `hit',  the
    user  is  asked  to select  from  the  locations found.     Finally,
    prolog_edit:edit_source/1  is used  to  invoke the  user's  preferred
    editor.   Typically, edit/1 can  be handed the name of  a predicate,
    module, basename of a file, XPCE class, XPCE method, etc.


eeddiitt
    Edit the `default' file  using edit/1.  The default file is the file
    loaded  with the  command-line option -s  or, in  windows, the  file
    loaded by double-clicking from the Windows shell.


pprroolloogg__eeddiitt::llooccaattee((_+_S_p_e_c_, _-_F_u_l_l_S_p_e_c_, _-_L_o_c_a_t_i_o_n))
    Where  _S_p_e_c is  the  specification provided  through edit/1.    This
    multifile  predicate  is  used to  enumerate  locations at  with  an
    object satisfying the given  _S_p_e_c can be found.  _F_u_l_l_S_p_e_c is unified
    with  the complete specification for  the object.  This  distinction
    is  used to  allow for ambiguous  specifications.   For example,  if
    _S_p_e_c  is  an  atom,  which appears  as  the base-name  of  a  loaded
    file  and as  the name  of a predicate,  _F_u_l_l_S_p_e_c will  be bound  to
    file(_P_a_t_h) or _N_a_m_e/_A_r_i_t_y.

    _L_o_c_a_t_i_o_n  is a list of attributes  of the location.  Normally,  this
    list  will contain  the term file(_F_i_l_e)  and ---if available---  the
    term line(_L_i_n_e).


pprroolloogg__eeddiitt::llooccaattee((_+_S_p_e_c_, _-_L_o_c_a_t_i_o_n))
    Same  as prolog_edit:locate/3, but only  deals with  fully-specified
    objects.


pprroolloogg__eeddiitt::eeddiitt__ssoouurrccee((_+_L_o_c_a_t_i_o_n))
    Start  editor on _L_o_c_a_t_i_o_n.   See prolog_edit:locate/3 for the  format
    of  a location  term.   This  multi-file predicate  is normally  not
    defined.  If it succeeds, edit/1 assumes the editor is started.

    If  it fails, edit/1 uses  its internal defaults, which are  defined
    by  the prolog-flag editor  and/or the environment variable  EDITOR.
    The  following  rules  apply.    If  the prolog-flag  editor  is  of
    the  format $<_n_a_m_e>,  the  editor is  determined by  the environment
    variable  <_n_a_m_e>.   Else, if this flag  is pce_emacs or built_in _a_n_d
    XPCE  is  loaded or  can  be loaded,  the  built-in Emacs  clone  is
    used.  Else,  if the environment EDITOR is set, this editor is used.
    Finally,  vi  is used  as default  on Unix  systems  and notepad  on
    Windows.

    See   the  default  user   preferences  file  dotfiles/dotplrc   for
    examples.


pprroolloogg__eeddiitt::eeddiitt__ccoommmmaanndd((_+_E_d_i_t_o_r_, _-_C_o_m_m_a_n_d))
    Determines  how _E_d_i_t_o_r is  to be invoked using  shell/1.  _E_d_i_t_o_r  is
    the  determined editor  (see edit_source/1), without  the full  path
    specification,  and without  possible (exe) extension.   _C_o_m_m_a_n_d  is
    an  atom describing  the command.   The  pattern %f  is replaced  by
    the  full file-name  of the  location, and  %d by  the line  number.
    If  the  editor can  deal with  starting at  a specified  line,  two
    clauses  should be provided,  one holding only  the %f pattern,  and
    one holding both patterns.

    The  default contains  definitions for vi,  emacs, emacsclient,  vim
    and notepad (latter without line-number version).

    Please contribute your specifications to jan@swi.psy.uva.nl.


pprroolloogg__eeddiitt::llooaadd
    Normally  not-defined  multifile  predicate.    This  predicate  may
    be  defined  to provide  loading hooks  for  user-extensions to  the
    edit  module.   For example,  XPCE provides the  code below to  load
    swi_edit,  containing definitions to locate  classes and methods  as
    well as to bind this package to the PceEmacs built-in editor.

    :- multifile prolog_edit:load/0.

    prolog_edit:load :-
            ensure_loaded(library(swi_edit)).


lliissttiinngg((_+_P_r_e_d))
    List  specified predicates  (when an  atom is  given all  predicates
    with  this name will  be listed).   The listing  is produced on  the
    basis of the  internal representation, thus losing user's layout and
    variable name information.  See also portray_clause/1.


lliissttiinngg
    List all predicates of the database using listing/1.


ppoorrttrraayy__ccllaauussee((_+_C_l_a_u_s_e))
    Pretty  print a  clause.   A clause  should be specified  as a  term
    `<_H_e_a_d> :- <_B_o_d_y>'.    Facts are  represented as  `<_H_e_a_d> :- true'  or
    simply  <_H_e_a_d>.  Variables  in the clause are written as  A, B, ....
    Singleton variables are written as _.  See also portray_clause/2.


ppoorrttrraayy__ccllaauussee((_+_S_t_r_e_a_m_, _+_C_l_a_u_s_e))
    Pretty print a clause to _S_t_r_e_a_m.  See portray_clause/1 for details.


44..55 VVeerriiffyy TTyyppee ooff aa TTeerrmm


vvaarr((_+_T_e_r_m))
    True if _T_e_r_m currently is a free variable.


nnoonnvvaarr((_+_T_e_r_m))
    True if _T_e_r_m currently is not a free variable.


iinntteeggeerr((_+_T_e_r_m))
    True if _T_e_r_m is bound to an integer.


ffllooaatt((_+_T_e_r_m))
    True if _T_e_r_m is bound to a floating point number.


rraattiioonnaall((_+_T_e_r_m))
    True  if _T_e_r_m  is  bound to  a rational  number.   Rational  numbers
    include integers.


rraattiioonnaall((_+_T_e_r_m_, _-_N_u_m_e_r_a_t_o_r_, _-_D_e_n_o_m_i_n_a_t_o_r))
    True  if  _T_e_r_m  is  a  rational  number  with  given  _N_u_m_e_r_a_t_o_r  and
    _D_e_n_o_m_i_n_a_t_o_r.   The _N_u_m_e_r_a_t_o_r and _D_e_n_o_m_i_n_a_t_o_r are in  canonical form,
    which  means _D_e_n_o_m_i_n_a_t_o_r  is  a positive  integer and  there are  no
    common divisors between _N_u_m_e_r_a_t_o_r and _D_e_n_o_m_i_n_a_t_o_r.


nnuummbbeerr((_+_T_e_r_m))
    True if _T_e_r_m is bound to an integer or floating point number.


aattoomm((_+_T_e_r_m))
    True if _T_e_r_m is bound to an atom.


ssttrriinngg((_+_T_e_r_m))
    True if _T_e_r_m is  bound to a string.  Note that string here refers to
    the  built-in atomic type string as described in section  4.23, Text
    in double quotes  such as "hello" creates a _l_i_s_t of _c_h_a_r_a_c_t_e_r _c_o_d_e_s.
    We illustrate the issues in the example queries below.

    ?- write("hello").
    [104, 101, 108, 108, 111]
    ?- string("hello").
    No
    ?- is_list("hello").
    Yes


aattoommiicc((_+_T_e_r_m))
    True  if  _T_e_r_m is  bound to  an atom,  string,  integer or  floating
    point  number.  Note that string  refers to the built-in type.   See
    string/1.    Strings in  the classical  Prolog sense  are lists  and
    therefore compound.


ccoommppoouunndd((_+_T_e_r_m))
    True  if _T_e_r_m is bound to a  compound term.  See also  functor/3 and
    =../2.


ccaallllaabbllee((_+_T_e_r_m))
    True  if _T_e_r_m is bound to an atom  or a compound term, so it  can be
    handed without type-error to call/1, functor/3 and =../2.


ggrroouunndd((_+_T_e_r_m))
    True if _T_e_r_m holds no free variables.


ccyycclliicc__tteerrmm((_+_T_e_r_m))
    True  if _T_e_r_m contains cycles, i.e. is  an infinite term.   See also
    acyclic_term/1 and section 2.16.


aaccyycclliicc__tteerrmm((_+_T_e_r_m))
    True  if  _T_e_r_m  does  not contain  cycles,  i.e.  can  be  processed
    recursively   in  finite   time.      See  also   cyclic_term/1  and
    section 2.16.


44..66 CCoommppaarriissoonn aanndd UUnniiffiiccaattiioonn oorr TTeerrmmss


44..66..11 SSttaannddaarrdd OOrrddeerr ooff TTeerrmmss

Comparison and  unification of arbitrary  terms.   Terms are ordered  in
the so called ``standard order''.  This order is defined as follows:

 1. _V_a_r_i_a_b_l_e_s <_N_u_m_b_e_r_s <_A_t_o_m_s <_S_t_r_i_n_g_s <_C_o_m_o_u_n_d _T_e_r_m_s

 2. Variables  are  sorted  by  address.     Attaching  attributes  (see
    section 6.1) does not affect the ordering.

 3. _A_t_o_m_s are compared alphabetically.

 4. _S_t_r_i_n_g_s are compared alphabetically.

 5. _N_u_m_b_e_r_s  are compared  by value.   Integers  and floats are  treated
    identically.   If the prolog_flag (see current_prolog_flag/2)  iso is
    defined, all floating point numbers precede all integers.

 6. _C_o_m_p_o_u_n_d  terms are  first checked  on  their arity,  then on  their
    functor-name  (alphabetically)  and  finally  recursively  on  their
    arguments, leftmost argument first.


_+_T_e_r_m_1 == _+_T_e_r_m_2
    True if _T_e_r_m_1 is  equivalent to _T_e_r_m_2.  A variable is only identical
    to a sharing variable.


_+_T_e_r_m_1 \== _+_T_e_r_m_2
    Equivalent to \+Term1 == Term2.


_+_T_e_r_m_1 = _+_T_e_r_m_2
    Unify _T_e_r_m_1 with _T_e_r_m_2.  True if the unification succeeds.


uunniiffyy__wwiitthh__ooccccuurrss__cchheecckk((_+_T_e_r_m_1_, _+_T_e_r_m_2))
    As  =/2, but  using _s_o_u_n_d_-_u_n_i_f_i_c_a_t_i_o_n.    That is,  a variable  only
    unifies  to  a term  if  this term  does  not contain  the  variable
    itself.  To illustrate this, consider the two goals below:

    1 ?- A = f(A).

    A = f(f(f(f(f(f(f(f(f(f(...))))))))))
    2 ?- unify_with_occurs_check(A, f(A)).

    No

    I.e.   the  first  creates  a  _c_y_c_l_i_c_-_t_e_r_m,  which  is   printed  as
    an  infinitely   nested  f/1  term  (see  the  max_depth  option  of
    write_term/2).  The second executes  logically sound unification and
    thus fails.


_+_T_e_r_m_1 \= _+_T_e_r_m_2
    Equivalent to \+Term1 = Term2.


_+_T_e_r_m_1 =@= _+_T_e_r_m_2
    True  if  _T_e_r_m_1  is  `structurally  equal' to  _T_e_r_m_2.     Structural
    equivalence  is weaker  than equivalence (==/2),  but stronger  than
    unification  (=/2).  Two terms are structurally equal if  their tree
    representation  is identical  and they  have the  same `pattern'  of
    variables.  Examples:

               a  =@=  A       false
               A  =@=  B       true
          x(A,A)  =@=  x(B,C)  false
          x(A,A)  =@=  x(B,B)  true
          x(A,B)  =@=  x(C,D)  true

    The  predicates  =@=/2  and  \=@=/2  are  cycle-safe.     Attributed
    variables  are considered  structurally equal  iff their  attributes
    are structurally equal.


_+_T_e_r_m_1 \=@= _+_T_e_r_m_2
    Equivalent to `\+Term1 =@= Term2'.


_+_T_e_r_m_1 @< _+_T_e_r_m_2
    True if _T_e_r_m_1 is before _T_e_r_m_2 in the standard order of terms.


_+_T_e_r_m_1 @=< _+_T_e_r_m_2
    True if both terms  are equal (==/2) or _T_e_r_m_1 is before _T_e_r_m_2 in the
    standard order of terms.


_+_T_e_r_m_1 @> _+_T_e_r_m_2
    True if _T_e_r_m_1 is after _T_e_r_m_2 in the standard order of terms.


_+_T_e_r_m_1 @>= _+_T_e_r_m_2
    True  if both terms are equal (==/2) or _T_e_r_m_1 is after _T_e_r_m_2  in the
    standard order of terms.


ccoommppaarree((_?_O_r_d_e_r_, _+_T_e_r_m_1_, _+_T_e_r_m_2))
    Determine or test  the _O_r_d_e_r between two terms in the standard order
    of terms.  _O_r_d_e_r is one of <, >  or =, with the obvious meaning.


??==((_@_T_e_r_m_1_, _@_T_e_r_m_2))
    Decide  whether the  equality  of _T_e_r_m_1  and _T_e_r_m_2  can be  compared
    safely,  i.e. whether  the result of  Term1 == Term2 can change  due
    to  further instantiation  of  either term.    It is  defined as  by
    ?=(A,B) :- (A==B ; A \= B), !. See also dif/2.


uunniiffiiaabbllee((_@_X_, _@_Y_, _-_U_n_i_f_i_e_r))
    If  _X and _Y  can unify,  unify _U_n_i_f_i_e_r with  a list of  _V_a_r = _V_a_l_u_e,
    representing  the  bindings required  to make  _X  and _Y  equivalent.
    This  predicate can handle cyclic  terms.  Attributed variables  are
    handles as normal variables.  Associated hooks are _n_o_t executed.


44..77 CCoonnttrrooll PPrreeddiiccaatteess

The predicates of  this section implement control structures.   Normally
the constructs in this  section, except for repeat/0, are  translated by
the compiler.   Please  note that complex goals  passed as arguments  to
meta-predicates such  as findall/3 below cause  the goal to be  compiled
to a  temporary location before  execution.   It is  faster to define  a
sub-predicate (i.e. one_character_atom/1 in the example below)  and make
a call to this simple predicate.

one_character_atoms(As) :-
        findall(A, (current_atom(A), atom_length(A, 1)), As).


ffaaiill
    Always  fail.   The  predicate fail/0  is translated  into a  single
    virtual machine instruction.


ttrruuee
    Always  succeed.  The predicate  true/0 is translated into a  single
    virtual machine instruction.


rreeppeeaatt
    Always succeed, provide an infinite number of choice points.


!
    Cut.    Discard choice  points of  parent frame  and frames  created
    after the parent frame.   As of SWI-Prolog 3.3, the semantics of the
    cut are compliant with  the ISO standard.  This implies that the cut
    is  transparent to  ;/2, ->/2  and  *->/2.   Cuts  appearing in  the
    _c_o_n_d_i_t_i_o_n  part of ->/2 and  *->/2 as well  as in \+/1 are local  to
    the condition.

             t1 :- (a, !, fail ; b).        % cuts a/0 and t1/0
             t2 :- (a -> b, !  ; c).        % cuts b/0 and t2/0
             t3 :- call((a, !, fail ; b)).  % cuts a/0
             t4 :- \+(a, !, fail ; b).      % cuts a/0


_+_G_o_a_l_1 , _+_G_o_a_l_2
    Conjunction.   True if both `Goal1'  and `Goal2' can be proved.   It
    is  defined as  (this  definition does  not lead  to a  loop as  the
    second comma is handled by the compiler):

    Goal1, Goal2 :- Goal1, Goal2.


_+_G_o_a_l_1 ; _+_G_o_a_l_2
    The `or' predicate is defined as:

    Goal1 ; _Goal2 :- Goal1.
    _Goal1 ; Goal2 :- Goal2.


_+_G_o_a_l_1 | _+_G_o_a_l_2
    Equivalent  to ;/2.    Retained for compatibility  only.   New  code
    should use ;/2.


_+_C_o_n_d_i_t_i_o_n -> _+_A_c_t_i_o_n
    If-then  and  If-Then-Else.    The  ->/2  construct commits  to  the
    choices  made  at  its  left-hand  side,   destroying  choice-points
    created  inside the  clause (by  ;/2), or  by goals  called by  this
    clause.   Unlike !/0, the  choice-point of the predicate as  a whole
    (due  to multiple clauses)  is nnoott destroyed.   The combination  ;/2
    and ->/2  acts as if defines by:

    If -> Then; _Else :- If, !, Then.
    If -> _Then; Else :- !, Else.
    If -> Then :- If, !, Then.

    Please  note that (If -> Then) acts  as (If -> Then ; ffaaiill),  making
    the  construct _f_a_i_l if the condition fails.  This  unusual semantics
    is part of the ISO and all de-facto Prolog standards.


_+_C_o_n_d_i_t_i_o_n *-> _+_A_c_t_i_o_n _; _+_E_l_s_e
    This  construct implements  the so-called `soft-cut'.   The  control
    is  defined as follows:   If _C_o_n_d_i_t_i_o_n succeeds  at least once,  the
    semantics  is the same  as (_C_o_n_d_i_t_i_o_n, _A_c_t_i_o_n).   If _C_o_n_d_i_t_i_o_n  does
    not  succeed, the  semantics is that  of (\+ _C_o_n_d_i_t_i_o_n,  _E_l_s_e).   In
    other  words, If _C_o_n_d_i_t_i_o_n succeeds at least once, simply  behave as
    the conjunction of _C_o_n_d_i_t_i_o_n and _A_c_t_i_o_n, otherwise execute _E_l_s_e.

    The  construct _A *-> _B, i.e.  without an _E_l_s_e branch, is  translated
    as the normal conjunction _A, _B.


\+ _+_G_o_a_l
    True  if `Goal' cannot  be proven (mnemonic:   + refers to  _p_r_o_v_a_b_l_e
    and  the backslash  (\)  is normally  used to  indicate negation  in
    Prolog).


44..88 MMeettaa--CCaallll PPrreeddiiccaatteess

Meta-call predicates  are used to  call terms  constructed at run  time.
The basic meta-call mechanism offered by SWI-Prolog is  to use variables
as a  subclause (which  should of  course be bound  to a  valid goal  at
runtime).   A  meta-call is  slower than a  normal call  as it  involves
actually searching the database at runtime for the  predicate, while for
normal calls this search is done at compile time.


ccaallll((_+_G_o_a_l))
    Invoke  _G_o_a_l as a  goal.   Note that clauses  may have variables  as
    subclauses, which is identical to call/1.


ccaallll((_+_G_o_a_l_, _+_E_x_t_r_a_A_r_g_1_, _._._.))
    Append  _E_x_t_r_a_A_r_g_1_, _E_x_t_r_a_A_r_g_2_,  _._._.   to  the argument  list of  _G_o_a_l
    and  call the result.   For  example, call(plus(1), 2, X) will  call
    plus/3, binding _X to 3.

    The call/[2..]   construct is handled by the compiler, which implies
    that  redefinition as  a predicate has  no effect.   The  predicates
    call/[2-6]  are defined as true  predicates, so they can be  handled
    by interpreted code.


aappppllyy((_+_T_e_r_m_, _+_L_i_s_t))
    Append  the members of  _L_i_s_t to the arguments  of _T_e_r_m and call  the
    resulting  term.   For  example:   apply(plus(1), [2, X]) will  call
    plus(1, 2, X).   apply/2 is  incorporated in the virtual machine  of
    SWI-Prolog.   This implies that the overhead can be compared  to the
    overhead  of call/1.  New code should use call/[2..]   if the length
    of _L_i_s_t is  fixed, which is more widely supported and faster because
    there is no need to build and examine the argument list.


nnoott((_+_G_o_a_l))
    True  if _G_o_a_l cannot  be proven.   Retained for compatibility  only.
    New code should use \+/1.


oonnccee((_+_G_o_a_l))
    Defined as:

    once(Goal) :-
            Goal, !.

    once/1  can  in  many  cases  be replaced  with  ->/2.     The  only
    difference  is how the  cut behaves  (see !/0).   The following  two
    clauses are identical:

    1) a :- once((b, c)), d.
    2) a :- b, c -> d.


iiggnnoorree((_+_G_o_a_l))
    Calls  _G_o_a_l  as once/1,  but succeeds,  regardless  of whether  _G_o_a_l
    succeeded or not.  Defined as:

    ignore(Goal) :-
            Goal, !.
    ignore(_).


ccaallll__wwiitthh__ddeepptthh__lliimmiitt((_+_G_o_a_l_, _+_L_i_m_i_t_, _-_R_e_s_u_l_t))
    If  _G_o_a_l can be proven  without recursion deeper than _L_i_m_i_t  levels,
    call_with_depth_limit/3 succeeds,  binding  _R_e_s_u_l_t  to  the  deepest
    recursion  level  used  during the  proof.    Otherwise,  _R_e_s_u_l_t  is
    unified  with depth_limit_exceeded  if the limit was exceeded  during
    the  proof,  or the  entire predicate  fails if  _G_o_a_l fails  without
    exceeding _L_i_m_i_t.

    The  depth-limit is  guarded by the  internal machinery.   This  may
    differ  from the depth computed based  on a theoretical model.   For
    example,  true/0  is  translated  into an  inlined  virtual  machine
    instruction.   Also, repeat/0 is not implemented as below, but  as a
    non-deterministic foreign predicate.

    repeat.
    repeat :-
            repeat.

    As  a result,  call_with_depth_limit/3may  still loop infinitely  on
    programs  that should  theoretically finish  in finite time.    This
    problem  can be cured by  using Prolog equivalents to such  built-in
    predicates.

    This   predicate  may  be   used  for  theorem-provers  to   realise
    techniques  like  _i_t_e_r_a_t_i_v_e _d_e_e_p_e_n_i_n_g.    It  was implemented  after
    discussion with Steve Moyle smoyle@ermine.ox.ac.uk.


ccaallll__cclleeaannuupp((_:_G_o_a_l_, _+_C_a_t_c_h_e_r_, _:_C_l_e_a_n_u_p))
    Calls   _G_o_a_l.      If  _G_o_a_l  is  completely   finished,  either   by
    deterministic  success,  failure,  its  choice-point  being  cut  or
    raising  an exception  and _C_a_t_c_h_e_r unifies  to the termination  code
    (see  below), _C_l_e_a_n_u_p  is called.    Success or  failure of  _C_l_e_a_n_u_p
    is  ignored and possible choice-points it created are  destroyed (as
    once/1).    If  cleanup  throws an  exception  this is  executed  as
    normal.

    _C_a_t_c_h_e_r  is  unified  with  a  term  describing  how  the  call  has
    finished.  If this unification fails, _C_l_e_a_n_u_p is _n_o_t called.

    eexxiitt
         _G_o_a_l succeeded without leaving any choice-points.

    ffaaiill
         _G_o_a_l failed.

    !
         _G_o_a_l succeeded with  choice-points and these are now  discarded
         by the execution of a cut (or other pruning of  the search tree
         such as if-then-else).

    eexxcceeppttiioonn((_E_x_c_e_p_t_i_o_n))
         _G_o_a_l raised the given _E_x_c_e_p_t_i_o_n.

    Typical  use of this predicate is cleanup of permanent  data storage
    required to execute  _G_o_a_l, close file-descriptors, etc.  The example
    below  provides a  non-deterministic search  for a term  in a  file,
    closing the stream as needed.

    term_in_file(Term, File) :-
            open(File, read, In),
            call_cleanup(term_in_stream(Term, In), _, close(In)).

    term_in_stream(Term, In) :-
            repeat,
            read(In, T),
            (   T == end_of_file
            ->  !, fail
            ;   T = Term
            ).

    Note  that  this  predicate is  impossible  to implement  in  Prolog
    other  then reading all terms into  a list, close the file  and call
    member/2  because without  call_cleanup/3 there  is no  way to  gain
    control if the choice-point left by repeat is killed by a cut.

    This  predicate is a SWI-Prolog extension.   See also call_cleanup/2
    for compatibility to other Prolog implementations.


ccaallll__cclleeaannuupp((_:_G_o_a_l_, _:_C_l_e_a_n_u_p))
    This  predicate  is  equivalent to  call_cleanup(_G_o_a_l_,  ___,  _C_l_e_a_n_u_p),
    calling   _C_l_e_a_n_u_p   regardless  of   the  reason   for   termination
    and  without  providing   information.     This  predicate  provides
    compatibility to a number of other Prolog implementations.


44..99 IISSOO ccoommpplliiaanntt EExxcceeppttiioonn hhaannddlliinngg

SWI-Prolog defines the predicates catch/3 and throw/1  for ISO compliant
raising  and catching  of exceptions.    In  the current  implementation
(4.0.6), most of  the built-in predicates generate exceptions,  but some
obscure predicates merely print a message, start the  debugger and fail,
which was the normal behaviour before the introduction of exceptions.


ccaattcchh((_:_G_o_a_l_, _+_C_a_t_c_h_e_r_, _:_R_e_c_o_v_e_r))
    Behaves  as call/1 if  no exception is  raised when executing  _G_o_a_l.
    If  a exception  is raised  using throw/1 while  _G_o_a_l executes,  and
    the  _G_o_a_l is the innermost goal  for which _C_a_t_c_h_e_r unifies with  the
    argument  of throw/1, all choice-points  generated by _G_o_a_l are  cut,
    the  system backtracks to the start of catch/3 while  preserving the
    thrown exception term and _R_e_c_o_v_e_r is called as in call/1.

    The  overhead of calling a  goal through catch/3 is very  comparable
    to  call/1.  Recovery from  an exception is much slower,  especially
    if the exception-term is large due to the copying thereof.


tthhrrooww((_+_E_x_c_e_p_t_i_o_n))
    Raise  an exception.   The  system looks  for the innermost  catch/3
    ancestor  for which _E_x_c_e_p_t_i_o_n unifies  with the _C_a_t_c_h_e_r argument  of
    the catch/3 call.  See catch/3 for details.

    ISO  demands throw/1 to make a copy of _E_x_c_e_p_t_i_o_n, walk up  the stack
    to a catch/3 call,  backtrack and try to unify the copy of _E_x_c_e_p_t_i_o_n
    with  _C_a_t_c_h_e_r.   SWI-Prolog delays  making a  copy of _E_x_c_e_p_t_i_o_n  and
    backtracking  until it actually found a matching catch/3 goal.   The
    advantage  is that we can start  the debugger at the first  possible
    location  while preserving the entire exception context if  there is
    no  matching catch/3  goal.   This  approach can  lead to  different
    behaviour  if _G_o_a_l and _C_a_t_c_h_e_r of catch/3 call share variables.   We
    assume this to be  highly unlikely and could not think of a scenario
    where this is useful.

    If  an exception is raised in a callback from C (see chapter  9) and
    not  caught in the  same call-back, PL_next_solution() fails and  the
    exception context can be retrieved using PL_exception().


44..99..11 DDeebbuuggggiinngg aanndd eexxcceeppttiioonnss

Before  the introduction  of exceptions  in SWI-Prolog  a runtime  error
was handled  by printing  an error  message, after  which the  predicate
failed.   If  the prolog_flag (see current_prolog_flag/2) debug_on_error
was in effect  (default), the tracer was  switched on.  The  combination
of the  error message and trace  information is generally sufficient  to
locate the error.

With exception handling,  things are different.   A programmer may  wish
to trap an  exception using catch/3 to avoid  it reaching the user.   If
the exception  is not  handled by user-code,  the interactive  top-level
will trap it to prevent termination.

If  we  do  not  take  special  precautions,   the  context  information
associated with  an unexpected exception (i.e,  a programming error)  is
lost.  Therefore,  if an exception is raised, which is not  caught using
catch/3 and  the top-level is  running, the error  will be printed,  and
the system will enter trace mode.

If the system  is in an non-interactive  callback from foreign code  and
there is no catch/3  active in the current context, it  cannot determine
whether or  not the  exception will  be caught by  the external  routine
calling Prolog.    It will  then base  its behaviour on  the prolog_flag
debug_on_error:

  o _c_u_r_r_e_n_t___p_r_o_l_o_g___f_l_a_g_(_d_e_b_u_g___o_n___e_r_r_o_r_, _f_a_l_s_e_)
    The  exception does  not trap the  debugger and  is returned to  the
    foreign  routine  calling Prolog,  where it  can  be accessed  using
    PL_exception().  This is the default.

  o _c_u_r_r_e_n_t___p_r_o_l_o_g___f_l_a_g_(_d_e_b_u_g___o_n___e_r_r_o_r_, _t_r_u_e_)
    If the exception is  not caught by Prolog in the current context, it
    will trap the tracer to help analysing the context of the error.

While looking for the  context in which an exception takes place,  it is
advised to switch on debug mode using the predicate debug/0.


44..99..22 TThhee eexxcceeppttiioonn tteerrmm

Built-in  predicates generates  exceptions  using a  term  error(_F_o_r_m_a_l_,
_C_o_n_t_e_x_t).    The  first argument  is  the  `formal' description  of  the
error,  specifying the class  and generic  defined context  information.
When applicable,  the ISO  error-term definition  is used.   The  second
part  describes some  additional context  to help  the programmer  while
debugging.    In its  most  generic form  this  is a  term of  the  form
context(_N_a_m_e_/_A_r_i_t_y_,  _M_e_s_s_a_g_e), where _N_a_m_e/_A_r_i_t_y  describes the  built-in
predicate  that raised  the error,  and _M_e_s_s_a_g_e  provides an  additional
description of the error.  Any part of this structure  may be a variable
if no information was present.


44..99..33 PPrriinnttiinngg mmeessssaaggeess

The predicate print_message/2 may be  used to print a message term  in a
human readable  format.   The other predicates  from this section  allow
the user  to refine  and extend  the message system.    The most  common
usage of  print_message/2 is to  print error  messages from  exceptions.
The code below  prints errors encountered during the execution  of _G_o_a_l,
without  further propagating  the  exception  and without  starting  the
debugger.

        ...,
        catch(Goal, E,
              ( print_message(error, E),
                fail
              )),
        ...

Another common  use is to  defined message_hook/3 for printing  messages
that are normally _s_i_l_e_n_t, suppressing messages,  redirecting messages or
make something happen in addition to printing the message.


pprriinntt__mmeessssaaggee((_+_K_i_n_d_, _+_T_e_r_m))
    The  predicate print_message/2 is  used to  print messages,  notably
    from  exceptions  in  a human-readable  format.    _K_i_n_d  is  one  of
    informational,   banner,  warning,  error,   help  or  silent.     A
    human-readable message is printed to the stream user_error.

    If  the prolog flag  (see current_prolog_flag/2) verbose is  silent,
    messages  with _K_i_n_d informational, or banner are treated  as silent.
    See -q.

    This  predicate first translates  the _T_e_r_m into  a list of  `message
    lines'  (see print_message_lines/3for  details).  Next it  will call
    the hook  message_hook/3to allow the  user intercepting the message.
    If  message_hook/3 fails it  will print the  message unless _K_i_n_d  is
    silent.

    The  print_message/2  predicate  and  its  rules  are  in  the  file
    <_p_l_h_o_m_e>/boot/messages.pl,   which  may   be   inspected  for   more
    information  on the error messages and related error terms.   If you
    need  to report errors  from your own predicates,  we advise you  to
    stick  to the existing error terms  if you can; but should  you need
    to  invent new ones, you can define corresponding error  messages by
    asserting clauses for  prolog:message.  You will need to declare the
    predicate as multifile.

    See also message_to_string/2.


pprriinntt__mmeessssaaggee__lliinneess((_+_S_t_r_e_a_m_, _+_P_r_e_f_i_x_, _+_L_i_n_e_s))
    Print a  message (see print_message/2) that has  been translated to a
    list of message elements.  The elements of this list are:

    <_F_o_r_m_a_t>--<_A_r_g_s>
         Where _F_o_r_m_a_t is an atom and _A_r_g_s is a list  of format argument.
         Handed to format/3.

    flush
         If this  appears as the  last element,  _S_t_r_e_a_m is flushed  (see
         flush_output/1) and no final newline is generated.

    at_same_line
         If this  appears as  first element,  no prefix  is printed  for
         the first line  and the line-position is  not forced to 0  (see
         format/1, ~N).

    <_F_o_r_m_a_t>
         Handed to format/3 as format(Stream, Format, []).

    nnll
         A new line  is started and if  the message is not complete  the
         _P_r_e_f_i_x is printed too.

    See also print_message/2 and message_hook/3.


mmeessssaaggee__hhooookk((_+_T_e_r_m_, _+_K_i_n_d_, _+_L_i_n_e_s))
    Hook  predicate that may be define  in the module user to  intercept
    messages  from  print_message/2.    _T_e_r_m  and _K_i_n_d  are the  same  as
    passed to  print_message/2.  _L_i_n_e_s is a  list of format statements as
    described with print_message_lines/3.  See also message_to_string/2.

    This  predicate should  be defined  dynamic and  multifile to  allow
    other modules defining clauses for it too.


mmeessssaaggee__ttoo__ssttrriinngg((_+_T_e_r_m_, _-_S_t_r_i_n_g))
    Translates  a message-term into a  string object (see section  4.23.
    Primarily  intended  to  write  messages to  Windows  in  XPCE  (see
    section 1.5) or other GUI environments.


44..1100 HHaannddlliinngg ssiiggnnaallss

As  of  version  3.1.0,   SWI-Prolog  is  capable  to   handle  software
interrupts  (signals) in  Prolog as  well as  in foreign  (C) code  (see
section 9.6.12).

Signals are used to handle internal errors (execution  of a non-existing
CPU  instruction,  arithmetic  domain  errors,  illegal  memory  access,
resource  overflow,   etc.),  as   well  as  for  dealing   asynchronous
inter-process communication.

Signals  are  defined  by  the POSIX  standard  and  part  of  all  Unix
machines.    The  MS-Windows  Win32  provides  a subset  of  the  signal
handling  routines,   lacking  the   vital  functionality  to  raise   a
signal in  another thread for  achieving asynchronous inter-process  (or
inter-thread) communication (Unix kill() function).


oonn__ssiiggnnaall((_+_S_i_g_n_a_l_, _-_O_l_d_, _:_N_e_w))
    Determines  the reaction on  _S_i_g_n_a_l.   _O_l_d is  unified with the  old
    behaviour, while the behaviour  is switched to _N_e_w.  As with similar
    environment-control  predicates,  the  current  value  is  retrieved
    using on_signal(Signal, Current, Current).

    The  action  description  is  an  atom  denoting  the  name  of  the
    predicate  that  will be  called  if _S_i_g_n_a_l  arrives.    on_signal/3
    is  a meta-predicate, which  implies that <_M_o_d_u_l_e>:<_N_a_m_e>  refers the
    <_N_a_m_e>/1 in the module <_M_o_d_u_l_e>.

    Two  predicate-names have  special meaning.    throw implies  Prolog
    will  map  the  signal  onto  a Prolog  exception  as  described  in
    section  4.9.   default resets  the handler  to the settings  active
    before SWI-Prolog manipulated the handler.

    Signals   bound  to  a  foreign  function  through  PL_signal()  are
    reported using the term $foreign_function(_A_d_d_r_e_s_s).

    After  receiving a signal mapped to throw, the exception  raised has
    the structure

         error(signal(<_S_i_g_N_a_m_e>, <_S_i_g_N_u_m>), <_C_o_n_t_e_x_t>)

    One possible usage of  this is, for example, to limit the time spent
    on  proving a goal.  This  requires a little C-code for  setting the
    alarm timer (see chapter 9):

    #include <SWI-Prolog.h>
    #include <unistd.h>

    foreign_t
    pl_alarm(term_t time)
    { double t;

      if ( PL_get_float(time, &t) )
      { alarm((long)(t+0.5));

        PL_succeed;
      }

      PL_fail;
    }

    install_t
    install()
    { PL_register_foreign("alarm", 1, pl_alarm, 0);
    }

    Next,  we can  define the  Prolog below.   This will  run _G_o_a_l  just
    as  once/1, throwing  the exception  error(_s_i_g_n_a_l_(_a_l_r_m_,  ___)_, __) if  a
    timeout occurs.

    :- load_foreign_library(alarm).

    :- on_signal(alrm, _, throw).

    :- module_transparent
            call_with_time_limit/2.

    call_with_time_limit(MaxTime, Goal) :-
            alarm(MaxTime),
            call_cleanup(Goal, _, alarm(0)), !.

    The  signal  names are  defined  by the  POSIX standard  as  symbols
    of  the form  SIG_<_S_I_G_N_A_M_E>.    The Prolog name  for a  signal is the
    lowercase  version of <_S_I_G_N_A_M_E>.  The predicate current_signal/3 may
    be used to map between names and signals.

    Initially,  some  signals  are  mapped to  throw,  while  all  other
    signals  are default.    The following signals  throw an  exception:
    ill, fpe, segv, pipe, alrm, bus, xcpu, xfsz and vtalrm.


ccuurrrreenntt__ssiiggnnaall((_?_N_a_m_e_, _?_I_d_, _?_H_a_n_d_l_e_r))
    Enumerate  the  currently defined  signal  handling.   _N_a_m_e  is  the
    signal  name,  _I_d is  the numerical  identifier and  _H_a_n_d_l_e_r is  the
    currently defined handler (see on_signal/3).


44..1100..11 NNootteess oonn ssiiggnnaall hhaannddlliinngg

Before  deciding  to deal  with  signals  in  your  application,  please
consider the following:

  o _P_o_r_t_a_b_i_l_i_t_y
    On MS-Windows, the  signal interface is severely limited.  Different
    Unix  brands support  different sets  of signals,  and the  relation
    between signal name and number may vary.

  o _S_a_f_e_t_y
    Signal   handling   is   not   completely  safe   in   the   current
    implementation,  especially  if throw  is used  in combination  with
    external  foreign  code.    The  system  will use  the  C  longjmp()
    construct  to  direct control  to  the innermost  PL_next_solution(),
    thus  forcing an external procedure to be abandoned at  an arbitrary
    moment.   Most likely not all SWI-Prologs own foreign code  is (yet)
    safe  too.   For  the multi-threaded  versions this  is even  worse:
    signals can easily violate thread synchronisation consistency.

    The  C-interface described  in  section 9.6.12  provides the  option
    PL_SIGSYNC for registering a signal handler  that delays delivery of
    signals  to a safe point.   Unfortunately this may cause signals  to
    be delayed for a long time if Prolog is executing foreign code.

  o _G_a_r_b_a_g_e _C_o_l_l_e_c_t_i_o_n
    The  garbage  collector  will block  all  signals that  are  handled
    by  Prolog.    While handling  a  signal, the  garbage-collector  is
    disabled.

  o _T_i_m_e _o_f _d_e_l_i_v_e_r_y
    Normally  delivery  is immediate  (or as  defined  by the  operating
    system  used).   Signals are blocked  when the garbage collector  is
    active,  and internally delayed if they occur within in  a `critical
    section'.  The critical sections are generally very short.


44..1111 TThhee ``bblloocckk'' ccoonnttrrooll--ssttrruuccttuurree

The  block/3 predicate  and  friends  have been  introduced  before  ISO
compatible  catch/3  exception  handling  for  compatibility  with  some
Prolog implementation.    The only  feature not covered  by catch/3  and
throw/1 is the possibility to execute global cuts.   New code should use
catch/3 and throw/1 to deal with exceptions.


bblloocckk((_+_L_a_b_e_l_, _+_G_o_a_l_, _-_E_x_i_t_V_a_l_u_e))
    Execute  _G_o_a_l in a _b_l_o_c_k.   _L_a_b_e_l is the name  of the block.   _L_a_b_e_l
    is  normally an  atom, but  the system imposes  no type  constraints
    and  may even be a variable.   _E_x_i_t_V_a_l_u_e is normally unified  to the
    second argument of an exit/2 call invoked by _G_o_a_l.


eexxiitt((_+_L_a_b_e_l_, _+_V_a_l_u_e))
    Calling  exit/2 makes the innermost _b_l_o_c_k which _L_a_b_e_l  unifies exit.
    The  block's _E_x_i_t_V_a_l_u_e is unified with  _V_a_l_u_e.  If this  unification
    fails the block fails.


ffaaiill((_+_L_a_b_e_l))
    Calling  fail/1 makes the innermost  _b_l_o_c_k which _L_a_b_e_l unifies  fail
    immediately.  Implemented as

    fail(Label) :- !(Label), fail.


!((_+_L_a_b_e_l))
    Cut  all  choice-points created  since the  entry  of the  innermost
    _b_l_o_c_k which _L_a_b_e_l unifies.


44..1122 DDCCGG GGrraammmmaarr rruulleess

Grammar rules  form a comfortable interface  to _d_i_f_f_e_r_e_n_c_e_-_l_i_s_t_s.   They
are designed  both to support  writing parsers  that build a  parse-tree
from a list as for  generating a flat list from a term.   Unfortunately,
Definite  Clause  Grammar (DCG)  handling  is  not part  of  the  Prolog
standard.   Most Prolog  engines implement DCG,  but the details  differ
slightly.

Grammar rules  look like  ordinary clauses  using -->/2  for  separating
the head  and body rather then  :-/2.   Expanding grammar rules is  done
by expand_term/2, which  adds two additional  argument to each term  for
representing the difference list.   We will illustrate the  behaviour by
defining a rule-set for parsing an integer.

integer(I) -->
        digit(D0),
        digits(D),
        { number_chars(I, [D0|D])
        }.

digits([D|T]) -->
        digit(D), !,
        digits(T).
digits([]) -->
        [].

digit(D) -->
        [D],
        { code_type(D, digit)
        }.

The  body of  a grammar  rule  can contain  three  types of  terms.    A
compound  term interpreted  as a  reference  to a  grammar-rule.    Code
between {...}  is interpreted  as a  reference to  ordinary Prolog  code
and finally,  a list  is interpreted  as a  sequence of literals.    The
Prolog control-constructs (\+/1,  ->/2, ;//2, ,/2  and !/0) can be  used
in grammar rules.

Grammar rule-sets are called using the built-in  predicates phrase/2 and
phrase/3:


pphhrraassee((_+_R_u_l_e_S_e_t_, _+_I_n_p_u_t_L_i_s_t))
    Equivalent to phrase(RuleSet, InputList, []).


pphhrraassee((_+_R_u_l_e_S_e_t_, _+_I_n_p_u_t_L_i_s_t_, _-_R_e_s_t))
    Activate  the rule-set with given name.  `InputList' is the  list of
    tokens to parse,  `Rest' is unified with the remaining tokens if the
    sentence is parsed  correctly.  The example below calls the rule-set
    `integer' defined above.

    ?- phrase(integer(X), "42 times", Rest).

    X = 42
    Rest = [32, 116, 105, 109, 101, 115]


44..1133 DDaattaabbaassee

SWI-Prolog offers  three different database mechanisms.   The first  one
is  the common  assert/retract  mechanism  for manipulating  the  clause
database.    As facts  and clauses  asserted using  assert/1  or one  of
its  derivatives become  part of  the program  these predicates  compile
the term  given to them.    retract/1 and retractall/1  have to unify  a
term and  therefore have to  decompile the program.   For these  reasons
the assert/retract  mechanism is  expensive.   On the  other hand,  once
compiled, queries to the database are faster than  querying the recorded
database discussed below.  See also dynamic/1.

The second way of  storing arbitrary terms in the database is  using the
``recorded database''.   In  this database terms  are associated with  a
_k_e_y.  A key can be an atom, integer or term.   In the last case only the
functor and  arity determine the  key.   Each key has  a chain of  terms
associated with it.   New terms  can be added either  at the head or  at
the tail  of this  chain.   This mechanism  is considerably faster  than
the assert/retract mechanism as terms are not compiled,  but just copied
into the heap.

The third mechanism is a special purpose one.   It associates an integer
or atom with  a key, which is  an atom, integer or  term.  Each key  can
only have  one atom or integer  associated with it.   It is faster  than
the mechanisms  described above, but  can only be  used to store  simple
status information like counters, etc.


aabboolliisshh((_:_P_r_e_d_i_c_a_t_e_I_n_d_i_c_a_t_o_r))
    Removes  all clauses of a  predicate with functor _F_u_n_c_t_o_r and  arity
    _A_r_i_t_y  from  the  database.    All  predicate  attributes  (dynamic,
    multifile,  index, etc.)  are  reset to their defaults.   Abolishing
    an  imported predicate only removes  the import link; the  predicate
    will keep its old definition in its definition module.

    According  to the  ISO standard,  abolish/1 can only  be applied  to
    dynamic  procedures.    This is  odd, as  for  dealing with  dynamic
    procedures  there  is  already  retract/1 and  retractall/1.     The
    abolish/1  predicate has been introduced in DEC-10  Prolog precisely
    for dealing with  static procedures.  In SWI-Prolog, abolish/1 works
    on static procedures, unless the prolog flag iso is set to true.

    It  is advised  to use  retractall/1 for  erasing all  clauses of  a
    dynamic predicate.


aabboolliisshh((_+_N_a_m_e_, _+_A_r_i_t_y))
    Same  as abolish(Name/Arity).   The predicate abolish/2 conforms  to
    the Edinburgh standard, while abolish/1 is ISO compliant.


rreeddeeffiinnee__ssyysstteemm__pprreeddiiccaattee((_+_H_e_a_d))
    This  directive  may be  used  both in  module  user and  in  normal
    modules to redefine  any system predicate.  If the system definition
    is  redefined in  module user,  the  new definition  is the  default
    definition  for  all sub-modules.    Otherwise  the redefinition  is
    local  to the module.   The system definition remains in the  module
    system.

    Redefining   system   predicate   facilitates  the   definition   of
    compatibility packages.  Use in other context is discouraged.


rreettrraacctt((_+_T_e_r_m))
    When  _T_e_r_m  is an  atom  or a  term  it is  unified with  the  first
    unifying  fact or clause  in the database.   The  fact or clause  is
    removed from the database.


rreettrraaccttaallll((_+_H_e_a_d))
    All  facts or  clauses in the  database for  which the _h_e_a_d  unifies
    with _H_e_a_d are removed.


aasssseerrtt((_+_T_e_r_m))
    Assert  a fact or clause in the  database.  _T_e_r_m is asserted  as the
    last fact or clause of the corresponding predicate.


aasssseerrttaa((_+_T_e_r_m))
    Equivalent  to assert/1,  but _T_e_r_m  is asserted as  first clause  or
    fact of the predicate.


aasssseerrttzz((_+_T_e_r_m))
    Equivalent to assert/1.


aasssseerrtt((_+_T_e_r_m_, _-_R_e_f_e_r_e_n_c_e))
    Equivalent  to  assert/1, but  _R_e_f_e_r_e_n_c_e is  unified  with a  unique
    reference  to the asserted clause.  This key can later be  used with
    clause/3 or erase/1.


aasssseerrttaa((_+_T_e_r_m_, _-_R_e_f_e_r_e_n_c_e))
    Equivalent  to assert/2,  but _T_e_r_m  is asserted as  first clause  or
    fact of the predicate.


aasssseerrttzz((_+_T_e_r_m_, _-_R_e_f_e_r_e_n_c_e))
    Equivalent to assert/2.


rreeccoorrddaa((_+_K_e_y_, _+_T_e_r_m_, _-_R_e_f_e_r_e_n_c_e))
    Assert  _T_e_r_m  in the  recorded  database  under key  _K_e_y.    _K_e_y  is
    an  integer,  atom or  term.   _R_e_f_e_r_e_n_c_e  is unified  with a  unique
    reference to the record (see erase/1).


rreeccoorrddaa((_+_K_e_y_, _+_T_e_r_m))
    Equivalent to recorda(Key, Value,  _).


rreeccoorrddzz((_+_K_e_y_, _+_T_e_r_m_, _-_R_e_f_e_r_e_n_c_e))
    Equivalent to recorda/3, but  puts the _T_e_r_m at the tail of the terms
    recorded under _K_e_y.


rreeccoorrddzz((_+_K_e_y_, _+_T_e_r_m))
    Equivalent to recordz(Key, Value,  _).


rreeccoorrddeedd((_+_K_e_y_, _-_V_a_l_u_e_, _-_R_e_f_e_r_e_n_c_e))
    Unify  _V_a_l_u_e  with the  first  term recorded  under _K_e_y  which  does
    unify.    _R_e_f_e_r_e_n_c_e  is  unified with  the  memory location  of  the
    record.


rreeccoorrddeedd((_+_K_e_y_, _-_V_a_l_u_e))
    Equivalent to recorded(Key, Value,  _).


eerraassee((_+_R_e_f_e_r_e_n_c_e))
    Erase  a  record or  clause from  the  database.   _R_e_f_e_r_e_n_c_e  is  an
    integer  returned by  recorda/3 or  recorded/3, clause/3,  assert/2,
    asserta/2  or assertz/2.    Other integers might  conflict with  the
    internal  consistency of the system.  Erase can only be  called once
    on  a record or clause.  A second call also might  conflict with the
    internal consistency of the system.


ffllaagg((_+_K_e_y_, _-_O_l_d_, _+_N_e_w))
    _K_e_y is an atom,  integer or term.  As with the recorded database, if
    _K_e_y is a term,  only the name and arity are used to locate the flag.
    Unify  _O_l_d with the old  value associated with _K_e_y.   If the key  is
    used  for the first time  _O_l_d is unified with  the integer 0.   Then
    store  the value  of _N_e_w, which  should be an  integer, float,  atom
    or  arithmetic expression, under  _K_e_y.   flag/3 is a fast  mechanism
    for  storing simple facts  in the  database.   The flag database  is
    shared  between threads and updates  are atomic, making it  suitable
    for generating unique integer counters.


44..1133..11 UUppddaattee vviieeww

Traditionally,  Prolog systems  used  the _i_m_m_e_d_i_a_t_e  _u_p_d_a_t_e _v_i_e_w:    new
clauses  became   visible  to   predicates  backtracking  over   dynamic
predicates   immediately   and  retracted   clauses   became   invisible
immediately.

Starting  with SWI-Prolog  3.3.0  we  adhere the  _l_o_g_i_c_a_l  _u_p_d_a_t_e  _v_i_e_w,
where backtrackable predicates that enter the definition  of a predicate
will not  see any changes  (either caused by  assert/1 or retract/1)  to
the  predicate.    This view  is the  ISO  standard, the  most  commonly
used and  the most  `safe'.   Logical  updates are  realised by  keeping
reference-counts on  predicates and  _g_e_n_e_r_a_t_i_o_n information on  clauses.
Each change  to the database  causes an increment  of the generation  of
the database.  Each  goal is tagged with the generation in which  it was
started.  Each  clause is flagged with the generation it was  created as
well as  the generation  it was  erased.   Only  clauses with  `created'
...`erased' interval  that encloses the generation  of the current  goal
are considered visible.


44..1133..22 IInnddeexxiinngg ddaattaabbaasseess

By  default,   SWI-Prolog,  as   most  other  implementations,   indexes
predicates  on their  first argument.    SWI-Prolog  allows indexing  on
other and multiple arguments using the declaration index/1.

For advanced database indexing, it defines hash_term/2:


hhaasshh__tteerrmm((_+_T_e_r_m_, _-_H_a_s_h_K_e_y))
    If  _T_e_r_m is a ground term (see ground/1), _H_a_s_h_K_e_y is unified  with a
    positive integer value that  may be used as a hash-key to the value.
    If  _T_e_r_m is not ground, the predicate succeeds  immediately, leaving
    _H_a_s_h_K_e_y an unbound variable.

    This  predicate  may be  used to  build hash-tables  as  well as  to
    exploit argument-indexing to find complex terms more quickly.

    The  hash-key does not rely on temporary information  like addresses
    of atoms and  may be assumed constant over different invocations and
    versions of SWI-Prolog.  The hash_term/2 predicate is cycle-safe.


44..1144 DDeeccllaarriinngg pprreeddiiccaatteess pprrooppeerrttiieess

This  section  describes  directives  which  manipulate   attributes  of
predicate  definitions.     The  functors  dynamic/1,   multifile/1  and
discontiguous/1  are  operators  of  priority  1150  (see  op/3),  which
implies  the  list of  predicates  they  involve  can just  be  a  comma
separated list:

:- dynamic
        foo/0,
        baz/2.

On SWI-Prolog all  these directives are just  predicates.  This  implies
they can also  be called by a program.   Do not rely on this  feature if
you want to maintain portability to other Prolog implementations.


ddyynnaammiicc _+_N_a_m_e_/_+_A_r_i_t_y_, _._._.
    Informs the interpreter  that the definition of the predicate(s) may
    change  during  execution (using  assert/1 and/or  retract/1).    In
    the  multi-threaded version, the  clauses of dynamic predicates  are
    shared  between the threads.  The  directive thread_local/1 provides
    an  alternative where each threads  has its own clause-list for  the
    predicate.   Dynamic predicates can be turned into static ones using
    compile_predicates/1.


ccoommppiillee__pprreeddiiccaatteess((_:_L_i_s_t_O_f_N_a_m_e_A_r_i_t_y))
    Compile  a list of specified  dynamic predicates (see dynamic/1  and
    assert/1)  into  normal static  predicates.    This call  tells  the
    Prolog  environment  the  definition  will not  change  anymore  and
    further  calls  to assert/1  or retract/1  on  the named  predicates
    raise  a permission error.  This predicate is designed to  deal with
    parts  of the  program that  is generated  at runtime  but does  not
    change during the remainder of the program execution.


mmuullttiiffiillee _+_N_a_m_e_/_+_A_r_i_t_y_, _._._.
    Informs  the system that the  specified predicate(s) may be  defined
    over  more than one  file.  This  stops consult/1 from redefining  a
    predicate when a new definition is found.


ddiissccoonnttiigguuoouuss _+_N_a_m_e_/_+_A_r_i_t_y_, _._._.
    Informs  the system that the  clauses of the specified  predicate(s)
    might not be together in the source file.  See also style_check/1.


iinnddeexx((_+_H_e_a_d))
    Index  the clauses  of the predicate  with the  same name and  arity
    as  _H_e_a_d  on the  specified arguments.    _H_e_a_d is  a  term of  which
    all  arguments  are  either  `1' (denoting  `index  this  argument')
    or  `0'  (denoting `do  not index  this argument').    Indexing  has
    no  implications  for the  semantics of  a  predicate, only  on  its
    performance.    If  indexing is  enabled on  a  predicate a  special
    purpose  algorithm  is used  to select  candidate  clauses based  on
    the  actual arguments of  the goal.   This algorithm checks  whether
    indexed  arguments might  unify in  the clause  head.   Only  atoms,
    integers  and compound  terms are  considered.   Compound terms  are
    indexed  on the combination  of their name and  arity.  Indexing  is
    very useful for predicates with many clauses representing facts.

    Due to the  representation technique used at most 4 arguments can be
    indexed.  All  indexed arguments should be in the first 32 arguments
    of  the predicate.    If  more than  4 arguments  are specified  for
    indexing only the first  4 will be accepted.  Arguments above 32 are
    ignored for indexing.

    Indexing   as  specified  by  this   predicate  uses  a  quick   but
    linear  scan.     Without explicit  specification  the  system  uses
    an  algorithm  depending  on the  structure  of the  first  argument
    and  number  of clauses,  In  particular,  for predicates  that  can
    be  indexed  on  the  first argument  and  have  many  clauses,  the
    system  will use  an  automatically resizing  hash-table to  provide
    access  time  independent from  the  number of  clauses.    If---for
    example---one  wants  to  represents  sub-types using  a  fact  list
    `sub_type(Sub, Super)' that  should be used  both to determine  sub-
    and super types one should declare sub_type/2 as follows:

    :- index(sub_type(1, 1)).

    sub_type(horse, animal).
    ...
    ...

    Note that this  type of indexing makes selecting clauses much faster
    but  remains _l_i_n_e_a_r  with respect  to the number  of clauses,  while
    hashing  as described  with  hash/1 provides  constant access  time.
    See also hash/1 and hash_term/2.


hhaasshh((_+_H_e_a_d))
    Index  the given predicate by hashing  on the first argument.   This
    is done by default  on any predicate with more than 5 clauses having
    a  first argument  that  can be  indexed and  at most  two that  can
    not  be indexed.   On dynamic  predicates the hash-table is  resized
    as  the number  of clauses  grows,  providing roughly  constant-time
    access  regardless of the number  of clauses predicates that can  be
    indexed  on the first argument.   See also index/1,  hash_term/2 and
    predicate_property/2.


44..1155 EExxaammiinniinngg tthhee pprrooggrraamm


ccuurrrreenntt__aattoomm((_-_A_t_o_m))
    Successively unifies _A_t_o_m with  all atoms known to the system.  Note
    that  current_atom/1 always succeeds if  _A_t_o_m is instantiated to  an
    atom.


ccuurrrreenntt__bblloobb((_?_B_l_o_b_, _?_T_y_p_e))
    Examine the type or  enumerate blobs of the given _T_y_p_e.  Typed blobs
    are  supported through  the foreign language  interface for  storing
    arbitrary  BLOBS  (Binary  Large  Object)  or  handles  to  external
    entities.  See section 9.6.6 for details.


ccuurrrreenntt__ffuunnccttoorr((_?_N_a_m_e_, _?_A_r_i_t_y))
    Successively unifies _N_a_m_e  with the name and _A_r_i_t_y with the arity of
    functors known to the system.


ccuurrrreenntt__ffllaagg((_-_F_l_a_g_K_e_y))
    Successively  unifies  _F_l_a_g_K_e_y with  all keys  used  for flags  (see
    flag/3).


ccuurrrreenntt__kkeeyy((_-_K_e_y))
    Successively  unifies  _K_e_y  with  all keys  used  for  records  (see
    recorda/3, etc.).


ccuurrrreenntt__pprreeddiiccaattee((_?_N_a_m_e_, _?_H_e_a_d))
    Successively  unifies _N_a_m_e  with  the name  of predicates  currently
    defined and _H_e_a_d with  the most general term built from _N_a_m_e and the
    arity of the  predicate.  This predicate succeeds for all predicates
    defined  in the specified module, imported  to it, or in one  of the
    modules from which the predicate will be imported if it is called.


ccuurrrreenntt__pprreeddiiccaattee((_:_N_a_m_e_/_A_r_i_t_y))
    ISO   compliant  implementation   of  current_predicate/2.       Un-
    like    current_predicate/2,    the   current   implementation    of
    current_predicate/1  does  not  consider  predicates   that  can  be
    autoloaded `current'.


pprreeddiiccaattee__pprrooppeerrttyy((_:_H_e_a_d_, _?_P_r_o_p_e_r_t_y))
    True  if _H_e_a_d  refers  to a  predicate that  has property  _P_r_o_p_e_r_t_y.
    Can  be used  to test whether  a predicate  has a certain  property,
    obtain  all properties known  for _H_e_a_d,  find all predicates  having
    _p_r_o_p_e_r_t_y  or  even obtaining  all  information available  about  the
    current program.  _P_r_o_p_e_r_t_y is one of:

    bbuuiilltt__iinn
         Is true  if the predicate  is locked  as a built-in  predicate.
         This implies it  cannot be redefined  in its definition  module
         and it can normally not be seen in the tracer.

    ddyynnaammiicc
         Is true  if assert/1 and  retract/1 may be  used to modify  the
         predicate.  This property is set using dynamic/1.

    eexxppoorrtteedd
         Is true if the predicate  is in the public list of  the context
         module.

    iimmppoorrtteedd__ffrroomm((_M_o_d_u_l_e))
         Is true if  the predicate is  imported into the context  module
         from module _M_o_d_u_l_e.

    ffiillee((_F_i_l_e_N_a_m_e))
         Unify _F_i_l_e_N_a_m_e with  the name of the  source file in which  the
         predicate is defined.  See also source_file/2.

    ffoorreeiiggnn
         Is true if the predicate is defined in the C language.

    iinnddeexxeedd((_H_e_a_d))
         Predicate is indexed (see index/1) according to _H_e_a_d.   _H_e_a_d is
         a term  whose name and  arity are  identical to the  predicate.
         The arguments are unified  with `1' for indexed arguments,  `0'
         otherwise.

    iinntteerrpprreetteedd
         Is true if the predicate is defined in Prolog.   We return true
         on this because, although the code is actually  compiled, it is
         completely transparent, just like interpreted code.

    lliinnee__ccoouunntt((_L_i_n_e_N_u_m_b_e_r))
         Unify _L_i_n_e_N_u_m_b_e_r with  the line number  of the first clause  of
         the predicate.   Fails if the predicate is not  associated with
         a file.  See also source_file/2.

    mmuullttiiffiillee
         Is true there  may be multiple  (or no) file providing  clauses
         for the predicate.  This property is set using multifile/1.

    nnooddeebbuugg
         Details of the predicate are  not shown by the debugger.   This
         is the default  for built-in predicates.   User predicates  can
         be compiled this way using the Prolog flag generate_debug_info.

    nnoottrraaccee
         Do not show ports of this predicate in the debugger.

    nnuummbbeerr__ooff__ccllaauusseess((_C_l_a_u_s_e_C_o_u_n_t))
         Unify _C_l_a_u_s_e_C_o_u_n_t to the number of clauses  associated with the
         predicate.  Fails for foreign predicates.

    tthhrreeaadd__llooccaall
         If true  (only  possible on  the multi-threaded  version)  each
         thread has its  own clauses for the  predicate.  This  property
         is set using thread_local/1.

    ttrraannssppaarreenntt
         Is true  if the  predicate  is declared  transparent using  the
         module_transparent/1 declaration.

    uunnddeeffiinneedd
         Is  true if  a procedure  definition  block for  the  predicate
         exists, but there are no clauses for it and it  is not declared
         dynamic or multifile.  This is true if the  predicate occurs in
         the body of a loaded predicate, an attempt to call  it has been
         made via one of  the meta-call predicates or the  predicate had
         a definition in  the past.   See the library package check  for
         example usage.

    vvoollaattiillee
         If  true, the  clauses  are not  saved  into a  saved-state  by
         qsave_program/[1,2].  This property is set using volatile/1.


ddwwiimm__pprreeddiiccaattee((_+_T_e_r_m_, _-_D_w_i_m))
    `Do What I Mean'  (`dwim') support predicate.  _T_e_r_m is a term, which
    name  and arity  are used  as a predicate  specification.   _D_w_i_m  is
    instantiated  with the  most general  term built from  _N_a_m_e and  the
    arity  of a defined predicate  that matches the predicate  specified
    by  _T_e_r_m  in the  `Do What  I Mean'  sense.    See dwim_match/2  for
    `Do  What I Mean' string matching.   Internal system predicates  are
    not generated,  unless style_check(+dollar)is active.   Backtracking
    provides all alternative matches.


ccllaauussee((_?_H_e_a_d_, _?_B_o_d_y))
    True  if  _H_e_a_d can  be unified  with  a clause  head and  _B_o_d_y  with
    the  corresponding  clause  body.    Gives  alternative  clauses  on
    backtracking.    For  facts  _B_o_d_y is  unified  with the  atom  _t_r_u_e.
    Normally  clause/2  is   used  to  find  clause  definitions  for  a
    predicate,  but it can  also be used to  find clause heads for  some
    body template.


ccllaauussee((_?_H_e_a_d_, _?_B_o_d_y_, _?_R_e_f_e_r_e_n_c_e))
    Equivalent  to  clause/2,   but  unifies  _R_e_f_e_r_e_n_c_e  with  a  unique
    reference to the  clause (see also assert/2, erase/1).  If _R_e_f_e_r_e_n_c_e
    is  instantiated to a reference the  clause's head and body will  be
    unified with  _H_e_a_d and _B_o_d_y.


nntthh__ccllaauussee((_?_P_r_e_d_, _?_I_n_d_e_x_, _?_R_e_f_e_r_e_n_c_e))
    Provides  access to  the clauses  of a predicate  using their  index
    number.    Counting  starts at  1.   If  _R_e_f_e_r_e_n_c_e  is specified  it
    unifies  _P_r_e_d with the  most general term  with the same  name/arity
    as  the predicate  and _I_n_d_e_x  with the index-number  of the  clause.
    Otherwise  the name  and arity  of _P_r_e_d  are used  to determine  the
    predicate.    If _I_n_d_e_x is  provided _R_e_f_e_r_e_n_c_e  will be unified  with
    the  clause  reference.   If  _I_n_d_e_x  is unbound,  backtracking  will
    yield  both the  indices and the  references of  all clauses of  the
    predicate.  The following example finds the 2nd clause of member/2:

    ?- nth_clause(member(_,_), 2, Ref), clause(Head, Body, Ref).

    Ref = 160088
    Head = system : member(G575, [G578|G579])
    Body = member(G575, G579)


ccllaauussee__pprrooppeerrttyy((_+_C_l_a_u_s_e_R_e_f_, _-_P_r_o_p_e_r_t_y))
    Queries  properties  of   a  clause.     _C_l_a_u_s_e_R_e_f  is  a  reference
    to   a   clause   as   produced   by   clause/3,   nth_clause/3   or
    prolog_frame_attribute/3.  _P_r_o_p_e_r_t_y is one of the following:

    ffiillee((_F_i_l_e_N_a_m_e))
         Unify _F_i_l_e_N_a_m_e with  the name of the  source file in which  the
         clause is defined.  Fails if the clause is not  associated to a
         file.

    lliinnee__ccoouunntt((_L_i_n_e_N_u_m_b_e_r))
         Unify _L_i_n_e_N_u_m_b_e_r with the line number of the clause.   Fails if
         the clause is not associated to a file.

    ffaacctt
         True if the clause has no body.

    eerraasseedd
         True if  the  clause has  been erased,  but  not yet  reclaimed
         because it is referenced.


44..1166 IInnppuutt aanndd oouuttppuutt

SWI-Prolog provides two  different packages for input  and output.   The
native  I/O system  is  based on  the  ISO standard  predicates  open/3,
close/1 and  friends.   Being more widely portable  and equipped with  a
clearer and  more robust specification,  new code  is encouraged to  use
these predicates for manipulation of I/O streams.

Section 4.16.2  describes tell/1,  see/1 and friends,  providing I/O  in
the spirit  of the outdated  Edinburgh standard.   These predicates  are
layered  on  top of  the  ISO  predicates.    Both  packages  are  fully
integrated; the user may switch freely between them.


44..1166..11 IISSOO IInnppuutt aanndd OOuuttppuutt SSttrreeaammss

The  predicates described  in this  section provide  ISO compliant  I/O,
where streams are  explicitly created using the  predicate open/3.   The
resulting  stream  identifier is  then  passed  as a  parameter  to  the
reading and writing  predicates to specify the source or  destination of
the data.

This schema  is not  vulnerable to  filename and  stream ambiguities  as
well as changes  to the working directory.   New code is advised to  use
these predicates to manage input and output streams.


ooppeenn((_+_S_r_c_D_e_s_t_, _+_M_o_d_e_, _-_S_t_r_e_a_m_, _+_O_p_t_i_o_n_s))
    ISO  compliant predicate  to open  a stream.   _S_r_c_D_e_s  is either  an
    atom,  specifying a file, or a term `pipe(Command)', like  see/1 and
    tell/1.    _M_o_d_e is  one of  read,  write, append  or update.    Mode
    append  opens the file for writing, positioning the  file-pointer at
    the  end.  Mode update  opens the file for writing,  positioning the
    file-pointer  at the  beginning of the  file without truncating  the
    file.  _S_t_r_e_a_m is  either a variable, in which case it is bound to an
    integer identifying the stream,  or an atom, in which case this atom
    will  be the stream  identifier.  The  _O_p_t_i_o_n_s list can contain  the
    following options:

    ttyyppee((_T_y_p_e))
         Using type text (default), Prolog will write a  text-file in an
         operating-system compatible way.   Using type binary the  bytes
         will be read or written without any translation.   See also the
         option encoding.

    aalliiaass((_A_t_o_m))
         Gives the  stream a name.   Below  is an example.   Be  careful
         with  this  option  as stream-names  are  global.     See  also
         set_stream/2.

         ?- open(data, read, Fd, [alias(input)]).

                 ...,
                 read(input, Term),
                 ...

    eennccooddiinngg((_E_n_c_o_d_i_n_g))
         Define the encoding used  for reading and writing text  to this
         stream.   The default  encoding for type  text is derived  from
         the Prolog  flag  encoding.   For  binary  streams the  default
         encoding  is octet.     For details  on  encoding  issues,  see
         section 2.17.1.

    bboomm((_B_o_o_l))
         Check for a BOM (_B_y_t_e _O_r_d_e_r _M_a_r_k_e_r) or write one.   If omitted,
         the default  is true for  mode read and  false for mode  write.
         See also stream_property/2 and especially section  2.17.1.1 for
         a discussion on this feature.

    eeooff__aaccttiioonn((_A_c_t_i_o_n))
         Defines  what  happens  if the  end  of  the  input  stream  is
         reached.   Action eof_code makes get0/1  and friends return  -1
         and read/1 and friends return the atom end_of_file.  Repetitive
         reading keeps yielding the  same result.  Action error  is like
         eof_code, but repetitive  reading will  raise an error.    With
         action reset,  Prolog will  examine the file  again and  return
         more data if the file has grown.

    bbuuffffeerr((_B_u_f_f_e_r_i_n_g))
         Defines output  buffering.    The atom  full (default)  defines
         full buffering, line buffering  by line, and false implies  the
         stream is  fully unbuffered.   Smaller  buffering is useful  if
         another process or the user is waiting for the output  as it is
         being produced.   See also flush_output/[0,1].  This option  is
         not an ISO option.

    cclloossee__oonn__aabboorrtt((_B_o_o_l))
         If  true (default),  the  stream is  closed  on an  abort  (see
         abort/0).   If  false, the  stream is  not closed.    If it  is
         an output  stream,  it will  be flushed  however.   Useful  for
         logfiles and if  the stream is  associated to a process  (using
         the pipe/1 construct).

    lloocckk((_L_o_c_k_i_n_g_M_o_d_e))
         Try to obtain a lock on the open file.  Default  is none, which
         does not lock the file.   The value read or shared  means other
         processes may  read the  file, but  not write  it.   The  value
         write or  exclusive means no  other process  may read or  write
         the file.

         Locks are  acquired through  the POSIX  function fcntl()  using
         the command F_SETLKW, which makes  a blocked call wait for  the
         lock to  be  released.    Please note  that fcntl()  locks  are
         _a_d_v_i_s_o_r_y and therefore  only other applications using the  same
         advisory locks  honour your  lock.   As there  are many  issues
         around locking  in  Unix, especially  related  to NFS  (network
         file  system), please  study  the  fcntl() manual  page  before
         trusting your locks!

         The lock option is a SWI-Prolog extension.

    The  option reposition is not supported in SWI-Prolog.   All streams
    connected to a file may be repositioned.


ooppeenn((_+_S_r_c_D_e_s_t_, _+_M_o_d_e_, _?_S_t_r_e_a_m))
    Equivalent to open/4 with an empty option-list.


ooppeenn__nnuullll__ssttrreeaamm((_?_S_t_r_e_a_m))
    Open  a stream that produces no output.  All counting  functions are
    enabled  on such a stream.   An attempt  to read from a  null-stream
    will  immediately signal  end-of-file.   Similar to Unix  /dev/null.
    _S_t_r_e_a_m can be an atom, giving the null-stream an alias name.


cclloossee((_+_S_t_r_e_a_m))
    Close the specified stream.   If _S_t_r_e_a_m is not open an error message
    is  displayed.  If the closed stream is the current input  or output
    stream the terminal is made the current input or output.


cclloossee((_+_S_t_r_e_a_m_, _+_O_p_t_i_o_n_s))
    Provides  close(_S_t_r_e_a_m_, _[_f_o_r_c_e_(_t_r_u_e_)_]) as the  only option.   Called
    this  way, any resource error  (such as write-errors while  flushing
    the output buffer) are ignored.


ssttrreeaamm__pprrooppeerrttyy((_?_S_t_r_e_a_m_, _?_S_t_r_e_a_m_P_r_o_p_e_r_t_y))
    ISO  compatible predicate for querying  status of open I/O  streams.
    _S_t_r_e_a_m_P_r_o_p_e_r_t_y is one of:

    aalliiaass((_A_t_o_m))
         If _A_t_o_m is bound,  test of the stream has the  specified alias.
         Otherwise unify _A_t_o_m with the first alias of the stream.

    bbuuffffeerr((_B_u_f_f_e_r_i_n_g))
         SWI-Prolog  extension  to query  the  buffering  mode  of  this
         stream.   _B_u_f_f_e_r_i_n_g is one of  full, line or  false.  See  also
         open/4.

    bboomm((_B_o_o_l))
         If present  and  true, a  BOM (_B_y_t_e  _O_r_d_e_r  _M_a_r_k) was  detected
         while opening the file  for reading or a BOM was  written while
         opening the stream.  See section 2.17.1.1 for details.

    eennccooddiinngg((_E_n_c_o_d_i_n_g))
         Query the encoding  used for text.   See section 2.17.1 for  an
         overview of wide character and encoding issues in SWI-Prolog.

    eenndd__ooff__ssttrreeaamm((_E))
         If _S_t_r_e_a_m is  an input stream,  unify _E with  one of the  atoms
         not, at or past.  See also at_end_of_stream/[0,1].

    eeooff__aaccttiioonn((_A))
         Unify _A with one of  eof_code,  reset or error.  See  open/4 for
         details.

    ffiillee__nnaammee((_A_t_o_m))
         If _S_t_r_e_a_m is  associated to a file,  unify _A_t_o_m to the name  of
         this file.

    ffiillee__nnoo((_I_n_t_e_g_e_r))
         If  the stream  is  associated  with a  POSIX  file-descriptor,
         unify  _I_n_t_e_g_e_r  with   the  descriptor  number.      SWI-Prolog
         extension used  primarily  for integration  with foreign  code.
         See also Sfileno() from SWI-Stream.h.

    iinnppuutt
         True if _S_t_r_e_a_m has mode read.

    mmooddee((_I_O_M_o_d_e))
         Unify  _I_O_M_o_d_e to  the  mode given  to  open/4 for  opening  the
         stream.   Values are:  read,  write, append and the  SWI-Prolog
         extension update.

    oouuttppuutt
         True if _S_t_r_e_a_m has mode write, append or update.

    ppoossiittiioonn((_T_e_r_m))
         Unify  _T_e_r_m  with the  current  stream-position.     A  stream-
         position is an opaque term whose fields can  be extracted using
         stream_position_data/3.  See also set_stream_position/2.

    rreeppoossiittiioonn((_B_o_o_l))
         Unify _B_o_o_l  with _t_r_u_e  if  the position  of the  stream can  be
         set (see seek/4).   It  is assumed the  position can be set  if
         the stream  has a  _s_e_e_k_-_f_u_n_c_t_i_o_n and is  not based  on a  POSIX
         file-descriptor that is not associated to a regular file.

    rreepprreesseennttaattiioonn__eerrrroorrss((_M_o_d_e))
         Determines behaviour of  character output if the stream  cannot
         represent a  character.   For  example, an  ISO Latin-1  stream
         cannot represent  cyrillic characters.   The  behaviour is  one
         of error (throw and  I/O error exception), prolog (write  \...\
         escape code or  xml (write &#...; XML  character entity).   The
         initial mode is prolog  for the user streams and error  for all
         other streams.  See also section 2.17.1 and set_stream/2.

    ttyyppee((_T))
         Unify _B_o_o_l with text or binary.

    ttttyy((_B_o_o_l))
         This property is reported  with _B_o_o_l equals true if  the stream
         is associated with a terminal.  See also set_stream/2.


ccuurrrreenntt__ssttrreeaamm((_?_O_b_j_e_c_t_, _?_M_o_d_e_, _?_S_t_r_e_a_m))
    The  predicate current_stream/3 is  used to access  the status of  a
    stream as well as  to generate all open streams.  _O_b_j_e_c_t is the name
    of the file opened  if the stream refers to an open file, an integer
    file-descriptor  if  the  stream  encapsulates  an  operating-system
    stream  or the atom  [] if the stream  refers to some other  object.
    _M_o_d_e is one of read or write.


iiss__ssttrreeaamm((_+_T_e_r_m))
    True  if  _T_e_r_m is  a  stream name  or  valid stream  handle.    This
    predicate  realises a safe test for the existence of a  stream alias
    or handle.


sseett__ssttrreeaamm__ppoossiittiioonn((_+_S_t_r_e_a_m_, _+_P_o_s))
    Set  the  current  position  of _S_t_r_e_a_m  to  _P_o_s.    _P_o_s  is  a  term
    as  returned by stream_property/2 using the position(_P_o_s)  property.
    See also seek/4.


ssttrreeaamm__ppoossiittiioonn__ddaattaa((_?_F_i_e_l_d_, _+_P_o_s_i_t_i_o_n_, _-_D_a_t_a))
    Extracts  information  from  the  opaque  stream  position  term  as
    returned  by  stream_property/2  requesting  the  position(_P_o_s_i_t_i_o_n)
    property.        _F_i_e_l_d   is  one   of   line_count,   line_position,
    char_count or  byte_count.   See  also line_count/2, line_position/2,
    character_count/2 and byte_count/2.


sseeeekk((_+_S_t_r_e_a_m_, _+_O_f_f_s_e_t_, _+_M_e_t_h_o_d_, _-_N_e_w_L_o_c_a_t_i_o_n))
    Reposition the current point  of the given _S_t_r_e_a_m.  _M_e_t_h_o_d is one of
    bof,  _c_u_r_r_e_n_t or _e_o_f, indicating positioning relative to  the start,
    current  point or  end of  the underlying  object.   _N_e_w_L_o_c_a_t_i_o_n  is
    unified with the new offset, relative to the start of the stream.

    If  the seek  modifies the  current  location, the  line number  and
    character position in the line are set to 0.

    If the stream  cannot be repositioned, a reposition error is raised.
    The  predicate seek/4 is  compatible to  Quintus Prolog, though  the
    error  conditions  and  signalling  is  ISO compliant.     See  also
    stream_property/2 and  set_stream_position/2.   Please note that  the
    use of seek/4 on  non-binary files (see open/4) is of limited use as
    the referred positions are byte offsets.


sseett__ssttrreeaamm((_+_S_t_r_e_a_m_, _+_A_t_t_r_i_b_u_t_e))
    Modify an attribute  of an existing stream.  _A_t_t_r_i_b_u_t_e specifies the
    stream property to set.  See also stream_property/2 and open/4.

    aalliiaass((_A_l_i_a_s_N_a_m_e))
         Set the alias  of an already created  stream.  If _A_l_i_a_s_N_a_m_e  is
         the name of  one of the standard  streams is used, this  stream
         is rebound.  Thus,  set_stream(S, current_input)is the  same as
         set_input/1 and by setting the alias of a stream to user_input,
         etc. all  user terminal input is  read from this  stream.   See
         also interactor/0.

    bbuuffffeerr((_B_u_f_f_e_r_i_n_g))
         Set  the   buffering  mode  of   an  already  created   stream.
         Buffering is one of full, line or false.

    cclloossee__oonn__aabboorrtt((_B_o_o_l))
         Determine whether or not the  stream is closed by abort/0.   By
         default streams are closed.

    eennccooddiinngg((_A_t_o_m))
         Defines the mapping between bytes and character  codes used for
         the stream.  See section 2.17.1 for supported encodings.

    eeooff__aaccttiioonn((_A_c_t_i_o_n))
         Set end-of-file handling to one of eof_code, reset or error.

    ttiimmeeoouutt((_S_e_c_o_n_d_s))
         This option can be  used to make streams generate  an exception
         if it  takes longer than  _S_e_c_o_n_d_s before  any new data  arrives
         at  the  stream.    The  value  _i_n_f_i_n_i_t_e  (default)  makes  the
         stream block  indefinitely.   Like wait_for_input/3, this  call
         only  applies  to streams  that  support  the  select()  system
         call.   For  further information  about  timeout handling,  see
         wait_for_input/3.  The exception is of the form

             error(timeout_error_(_r_e_a_d_, _S_t_r_e_a_m_)_, __)

    rreeccoorrdd__ppoossiittiioonn((_B_o_o_l))
         Do/do  not  record   the  line-count  and  line-position   (see
         line_count/2 and line_position/2).

    rreepprreesseennttaattiioonn__eerrrroorrss((_M_o_d_e))
         Change the  behaviour  when writing  characters to  the  stream
         that  cannot  be  represented  by  the  encoding.     See  also
         stream_property/2 and section 2.17.1.

    ffiillee__nnaammee((_F_i_l_e_N_a_m_e))
         Set the file name associated to this stream.  This  call can be
         used to set the file for error-locations  if _S_t_r_e_a_m corresponds
         to _F_i_l_e_N_a_m_e and  is not obtained  by opening the file  directly
         but, for example, through a network service.

    ttttyy((_B_o_o_l))
         Modify whether Prolog  thinks there is  a terminal (i.e.  human
         interaction) connected  to this stream.    On Unix systems  the
         initial value  comes from isatty().   On  Windows, the  initial
         user streams are supposed to be associated to a terminal.   See
         also stream_property/2.


sseett__pprroolloogg__IIOO((_+_I_n_, _+_O_u_t_, _+_E_r_r_o_r))
    Prepare  the   given  streams  for  interactive  behaviour  normally
    associated  to  the  terminal.     _I_n  becomes  the  user_input  and
    current_input  of  the calling  thread.    _O_u_t  becomes  user_output
    and  current_output.    If _E_r_r_o_r  equals  _O_u_t an  unbuffered  stream
    is  associated to  the same  destination and  linked to  user_error.
    Otherwise  _E_r_r_o_r is used for  user_error.   Output buffering for  _O_u_t
    is  set  to line  and buffering  on _E_r_r_o_r  is disabled.    See  also
    prolog/0  and set_stream/2.  The  _c_l_i_b package provides the  library
    prolog_server creating  a TCP/IP server for creating an  interactive
    session to Prolog.


44..1166..22 EEddiinnbbuurrgghh--ssttyyllee II//OO

The  package for  implicit  input  and output  destination  is  (almost)
compatible to Edinburgh  DEC-10 and C-Prolog.   The reading and  writing
predicates  refer to  resp.    the  _c_u_r_r_e_n_t  input- and  output  stream.
Initially these  streams are  connected to  the terminal.   The  current
output stream is  changed using tell/1 or  append/1.  The current  input
stream  is changed  using  see/1.   The  streams  current value  can  be
obtained using telling/1 for output- and seeing/1 for input streams.

Source  and   destination  are   either  a  file,   user,   or  a   term
`pipe(_C_o_m_m_a_n_d)'.  The reserved stream name user refers  to the terminal.
In the predicate descriptions below we will  call the source/destination
argument  `_S_r_c_D_e_s_t'.    Below are  some examples  of  source/destination
specifications.

         ?- see(data).        % Start reading from file `data'.
         ?- tell(user).       % Start writing to the terminal.
         ?- tell(pipe(lpr)).  % Start writing to the printer.

Another example  of using  the pipe/1 construct  is shown  below.   Note
that  the  pipe/1  construct  is  not  part  of  Prolog's  standard  I/O
repertoire.

getwd(Wd) :-
        seeing(Old), see(pipe(pwd)),
        collect_wd(String),
        seen, see(Old),
        atom_codes(Wd, String).

collect_wd([C|R]) :-
        get0(C), C \== -1, !,
        collect_wd(R).
collect_wd([]).


CCoommppaattiibbiilliittyy nnootteess

Unlike Edinburgh  Prolog systems, telling/1  and seeing/1 do not  return
the filename of the current input/output, but  the stream-identifier, to
ensure the design pattern below works under all circumstances.

        ...,
        telling(Old), tell(x),
        ...,
        told, tell(Old),
        ...,

The predicates tell/1 and see/1 first check for  user, the pipe(_c_o_m_m_a_n_d)
and  a stream-handle.    Otherwise, if  the argument  is an  atom it  is
first compared  to open streams  associated to a  file with _e_x_a_c_t_l_y  the
same name.   If  such a stream,  created using  tell/1 or see/1  exists,
output (input) is switch to the open stream.  Otherwise  a file with the
specified name is opened.

The  behaviour  is  compatible  to  Edinburgh  Prolog.     This  is  not
without problems.  Changing directory, non-file  streams, multiple names
referring to  the same file  easily lead to unexpected  behaviour.   New
code,  especially when managing  multiple I/O  channels should  consider
using the ISO I/O predicates defined in section 4.16.1.


sseeee((_+_S_r_c_D_e_s_t))
    Open  _S_r_c_D_e_s_t  for  reading  and  make it  the  current  input  (see
    set_input/1).    If  _S_r_c_D_e_s_t is  a  stream-handle, just  makes  this
    stream  the current input.   See the introduction of section  4.16.2
    for details.


tteellll((_+_S_r_c_D_e_s_t))
    Open  _S_r_c_D_e_s_t  for  writing and  make  it  the current  output  (see
    set_output/1).    If _S_r_c_D_e_s_t  is a  stream-handle,  just makes  this
    stream  the current output.  See the introduction of  section 4.16.2
    for details.


aappppeenndd((_+_F_i_l_e))
    Similar  to tell/1,  but positions the  file pointer  at the end  of
    _F_i_l_e  rather than truncating an existing  file.  The pipe  construct
    is not accepted by this predicate.


sseeeeiinngg((_?_S_r_c_D_e_s_t))
    Same  as  current_input/1,  except  that user  is  returned  if  the
    current  input  is the  stream user_input  to improve  compatibility
    with   traditional   Edinburgh   I/O.  See   the   introduction   of
    section 4.16.2 for details.


tteelllliinngg((_?_S_r_c_D_e_s_t))
    Same   as  current_output/1,  except   that  user  is  returned   if
    the   current   output  is   the  stream   user_output  to   improve
    compatibility  with traditional Edinburgh I/O. See  the introduction
    of section 4.16.2 for details.


sseeeenn
    Close  the  current input  stream.   The  new  input stream  becomes
    _u_s_e_r___i_n_p_u_t.


ttoolldd
    Close  the current  output stream.   The  new output stream  becomes
    _u_s_e_r___o_u_t_p_u_t.


44..1166..33 SSwwiittcchhiinngg BBeettwweeeenn EEddiinnbbuurrgghh aanndd IISSOO II//OO

The predicates  below can be  used for  switching between the  implicit-
and the explicit stream based I/O predicates.


sseett__iinnppuutt((_+_S_t_r_e_a_m))
    Set  the current input  stream to become _S_t_r_e_a_m.   Thus,  open(file,
    read, Stream), set_input(Stream) is equivalent to see(file).


sseett__oouuttppuutt((_+_S_t_r_e_a_m))
    Set  the  current  output  stream  to  become  _S_t_r_e_a_m.     See  also
    with_output_to/2.


ccuurrrreenntt__iinnppuutt((_-_S_t_r_e_a_m))
    Get  the current input stream.   Useful to get access to  the status
    predicates associated with streams.


ccuurrrreenntt__oouuttppuutt((_-_S_t_r_e_a_m))
    Get the current output stream.


44..1166..44 WWrriittee oonnttoo aattoommss,, ccooddee--lliissttss,, eettcc..


wwiitthh__oouuttppuutt__ttoo((_+_O_u_t_p_u_t_, _:_G_o_a_l))
    Run  _G_o_a_l  as  once/1,  while  characters  written  to  the  current
    output  is sent to  _O_u_t_p_u_t.   The predicate is SWI-Prolog  specific,
    inspired  by  various posts  to  the mailinglist.    It  provides  a
    flexible  replacement for predicates  such as sformat/3,  swritef/3,
    term_to_atom/2, atom_number/2 converting numbers to atoms, etc.  The
    predicate format/3 accepts the same terms as output argument.

    Applications  should  generally  avoid creating  atoms  by  breaking
    and  concatenating  other atoms  as the  creation  of large  numbers
    of  intermediate atoms  generally leads  to  poor performance,  even
    more  so in  multi-threaded applications.   This predicate  supports
    creating  difference-lists from  character  data efficiently.    The
    example  below defines the DCG rule term//1 to insert a term  in the
    output:

    term(Term, In, Tail) :-
            with_output_to(codes(In, Tail), write(Term)).

    ?- phrase(term(hello), X).

    X = [104, 101, 108, 108, 111]

    AA SSttrreeaamm hhaannddllee oorr aalliiaass
         Temporary  switch   current   output  to   the  given   stream.
         Redirection  using  with_output_to/2  guarantees  the  original
         output is restored, also if _G_o_a_l fails or  raises an exception.
         See also call_cleanup/2.

    aattoomm((_-_A_t_o_m))
         Create an atom  from the emitted characters.   Please note  the
         remark above.

    ssttrriinngg((_-_S_t_r_i_n_g))
         Create a string-object as defined in section 4.23.

    ccooddeess((_-_C_o_d_e_s))
         Create a list of  character codes from the emitted  characters,
         similar to atom_codes/2.

    ccooddeess((_-_C_o_d_e_s_, _-_T_a_i_l))
         Create a list of character codes as a difference-list.

    cchhaarrss((_-_C_h_a_r_s))
         Create a  list of  one-character-atoms codes  from the  emitted
         characters, similar to atom_chars/2.

    cchhaarrss((_-_C_h_a_r_s_, _-_T_a_i_l))
         Create a list of one-character-atoms as a difference-list.


44..1177 SSttaattuuss ooff ssttrreeaammss


wwaaiitt__ffoorr__iinnppuutt((_+_L_i_s_t_O_f_S_t_r_e_a_m_s_, _-_R_e_a_d_y_L_i_s_t_, _+_T_i_m_e_O_u_t))
    Wait  for input on  one of the  streams in _L_i_s_t_O_f_S_t_r_e_a_m_s and  return
    a  list  of  streams  on  which input  is  available  in  _R_e_a_d_y_L_i_s_t.
    wait_for_input/3 waits  for at most  _T_i_m_e_O_u_t seconds.   _T_i_m_e_o_u_t  may
    be  specified as  a floating  point number to  specify fractions  of
    a  second.    If  _T_i_m_e_o_u_t  equals  infinite,  wait_for_input/3 waits
    indefinitely.

    This  predicate can be used  to implement timeout while reading  and
    to  handle  input from  multiple  sources.   The  following  example
    will  wait for input from the  user and an explicitly opened  second
    terminal.  On return, _I_n_p_u_t_s may hold user or _P_4 or both.

    ?- open('/dev/ttyp4', read, P4),
       wait_for_input([user, P4], Inputs, 0).

    This  predicate  relies  on  the select()  call  on  most  operating
    systems.  On  Unix this call is implemented for any stream referring
    to  a file-handle,  which implies  all OS-based  streams:   sockets,
    terminals,  pipes, etc.   On non-Unix systems select() is  generally
    only  implemented for socket-based  streams.   See also socket  from
    the clib package.

    Note  that wait_for_input/3 returns streams that have data  waiting.
    This  does not mean you can, for example, call read/2 on  the stream
    without  blocking as the stream might hold an incomplete term.   The
    predicate  set_stream/2 using  the  option timeout(_S_e_c_o_n_d_s)  can  be
    used  to  make the  stream  generate an  exception  if no  new  data
    arrives  for within the timeout.  Suppose two  processes communicate
    by  exchanging Prolog terms.   The  following code makes the  server
    immune for clients that write an incomplete term:

            ...,
            tcp_accept(Server, Socket, _Peer),
            tcp_open(Socket, In, Out),
            set_stream(In, timeout(10)),
            catch(read(In, Term), _, (close(Out), close(In), fail)),
            ...,


bbyyttee__ccoouunntt((_+_S_t_r_e_a_m_, _-_C_o_u_n_t))
    Byte-position  in _S_t_r_e_a_m.   For binary streams  this is the same  as
    character_count/2.   For text files the number may be  different due
    to  multi-byte encodings  or additional record  separators (such  as
    Control-M in Windows).


cchhaarraacctteerr__ccoouunntt((_+_S_t_r_e_a_m_, _-_C_o_u_n_t))
    Unify  _C_o_u_n_t with the  current character index.   For input  streams
    this  is the number  of characters read since  the open, for  output
    streams  this is the number of characters written.   Counting starts
    at 0.


lliinnee__ccoouunntt((_+_S_t_r_e_a_m_, _-_C_o_u_n_t))
    Unify  _C_o_u_n_t with the  number of  lines read or  written.   Counting
    starts at 1.


lliinnee__ppoossiittiioonn((_+_S_t_r_e_a_m_, _-_C_o_u_n_t))
    Unify  _C_o_u_n_t with the position on the current line.  Note  that this
    assumes  the position is 0 after the  open.  Tabs are assumed  to be
    defined on each  8-th character and backspaces are assumed to reduce
    the count by one, provided it is positive.


ffiilleeeerrrroorrss((_-_O_l_d_, _+_N_e_w))
    Define error behaviour  on errors when opening a file for reading or
    writing.   Valid values are the  atoms on (default) and off.   First
    _O_l_d  is unified with the current value.   Then the new value  is set
    to _N_e_w.

    With  the introduction of exception-handling,  it is advised to  use
    catch/3  to catch possibly  file-errors and act  accordingly.   Note
    that if _f_i_l_e_e_r_r_o_r_s is off, _n_o exception is generated.


44..1188 PPrriimmiittiivvee cchhaarraacctteerr II//OO

See section 4.2 for an overview of supported character representations.


nnll
    Write  a newline character  to the current output  stream.  On  Unix
    systems nl/0 is equivalent to put(10).


nnll((_+_S_t_r_e_a_m))
    Write a newline to _S_t_r_e_a_m.


ppuutt((_+_C_h_a_r))
    Write  _C_h_a_r  to  the  current  output  stream,  _C_h_a_r  is  either  an
    integer-expression  evaluating to  a character  code or  an atom  of
    one  character.   Depreciated.   New code  should use  put_char/1 or
    put_code/1.


ppuutt((_+_S_t_r_e_a_m_, _+_C_h_a_r))
    Write _C_h_a_r to _S_t_r_e_a_m.  See put/1 for details.


ppuutt__bbyyttee((_+_B_y_t_e))
    Write a single byte  to the output.  _B_y_t_e must be an integer between
    0 and 255.


ppuutt__bbyyttee((_+_S_t_r_e_a_m_, _+_B_y_t_e))
    Write a single byte  to a stream.  _B_y_t_e must be an integer between 0
    and 255.


ppuutt__cchhaarr((_+_C_h_a_r))
    Write  a  character to  the  current output,  obeying  the  encoding
    defined for the current  output stream.  Note that this may raise an
    exception if the encoding of _S_t_r_e_a_m cannot represent _C_h_a_r.


ppuutt__cchhaarr((_+_S_t_r_e_a_m_, _+_C_h_a_r))
    Write  a  character to  _S_t_r_e_a_m,  obeying  the encoding  defined  for
    _S_t_r_e_a_m.   Note that this may  raise an exception if the  encoding of
    _S_t_r_e_a_m cannot represent _C_h_a_r.


ppuutt__ccooddee((_+_C_o_d_e))
    Similar  to put_char/1,  but using  a _c_h_a_r_a_c_t_e_r  _c_o_d_e.    _C_o_d_e is  a
    non-negative integer.   Note that this may raise an exception if the
    encoding of _S_t_r_e_a_m cannot represent _C_o_d_e.


ppuutt__ccooddee((_+_S_t_r_e_a_m_, _+_C_o_d_e))
    Same as put_code/1 but directing _C_o_d_e to _S_t_r_e_a_m.


ttaabb((_+_A_m_o_u_n_t))
    Writes  _A_m_o_u_n_t  spaces  on  the  current  output  stream.     _A_m_o_u_n_t
    should  be an expression that  evaluates to a positive integer  (see
    section 4.26).


ttaabb((_+_S_t_r_e_a_m_, _+_A_m_o_u_n_t))
    Writes _A_m_o_u_n_t spaces to _S_t_r_e_a_m.


fflluusshh__oouuttppuutt
    Flush  pending output on current  output stream.   flush_output/0 is
    automatically  generated by  read/1 and derivatives  if the  current
    input stream is user and the cursor is not at the left margin.


fflluusshh__oouuttppuutt((_+_S_t_r_e_a_m))
    Flush  output on the specified stream.  The stream must be  open for
    writing.


ttttyyfflluusshh
    Flush pending output on stream _u_s_e_r.  See also flush_output/[0,1].


ggeett__bbyyttee((_-_B_y_t_e))
    Read the current input  stream and unify the next byte with _B_y_t_e (an
    integer between 0 and 255.  _B_y_t_e is unified with -1 on end of file.


ggeett__bbyyttee((_+_S_t_r_e_a_m_, _-_B_y_t_e))
    Read  the next byte from _S_t_r_e_a_m, returning an integer between  0 and
    255.


ggeett__ccooddee((_-_C_o_d_e))
    Read  the current  input stream  and unify _C_o_d_e  with the  character
    code  of the  next character.   _C_o_d_e is  unified with  -1 on end  of
    file.  See also get_char/1.


ggeett__ccooddee((_+_S_t_r_e_a_m_, _-_C_o_d_e))
    Read the next character-code from _S_t_r_e_a_m.


ggeett__cchhaarr((_-_C_h_a_r))
    Read  the  current  input  stream  and  unify  _C_h_a_r  with  the  next
    character  as a  one-character-atom.   See  also atom_chars/2.    On
    end-of-file, _C_h_a_r is unified to the atom end_of_file.


ggeett__cchhaarr((_+_S_t_r_e_a_m_, _-_C_h_a_r))
    Unify   _C_h_a_r   with   the   next  character   from   _S_t_r_e_a_m   as   a
    one-character-atom.  See also get_char/2, get_byte/2 and get_code/2.


ggeett00((_-_C_h_a_r))
    Edinburgh  version  of the  ISO  get_code/1 predicate.    Note  that
    Edinburgh  prolog  didn't  support  wide  characters  and  therefore
    technically  speaking get0/1 should have been  mapped to get_byte/1.
    The intention of get0/1 however is to read character codes.


ggeett00((_+_S_t_r_e_a_m_, _-_C_h_a_r))
    Edinburgh  version  of  the ISO  get_code/2  predicate.    See  also
    get0/1.


ggeett((_-_C_h_a_r))
    Read  the  current   input  stream  and  unify  the  next  non-blank
    character with _C_h_a_r.  _C_h_a_r is unified with -1 on end of file.


ggeett((_+_S_t_r_e_a_m_, _-_C_h_a_r))
    Read the next non-blank character from _S_t_r_e_a_m.


ppeeeekk__bbyyttee((_-_B_y_t_e))
    Reads  the next input byte  like get_byte/1, but does not remove  it
    from the input stream.


ppeeeekk__bbyyttee((_+_S_t_r_e_a_m_, _-_B_y_t_e))
    Reads  the next input byte  like get_byte/2, but does not remove  it
    from the stream.


ppeeeekk__ccooddee((_-_C_o_d_e))
    Reads  the next input code  like get_code/1, but does not remove  it
    from the input stream.


ppeeeekk__ccooddee((_+_S_t_r_e_a_m_, _-_C_o_d_e))
    Reads  the next input code  like get_code/2, but does not remove  it
    from the stream.


ppeeeekk__cchhaarr((_-_C_h_a_r))
    Reads the  next input character like get_char/1,  but does not remove
    it from the input stream.


ppeeeekk__cchhaarr((_+_S_t_r_e_a_m_, _-_C_h_a_r))
    Reads the  next input character like get_char/2,  but does not remove
    it from the stream.


sskkiipp((_+_C_o_d_e))
    Read the input until  _C_h_a_r or the end of the file is encountered.  A
    subsequent  call to get_code/1 will  read the first character  after
    _C_o_d_e.


sskkiipp((_+_S_t_r_e_a_m_, _+_C_o_d_e))
    Skip input (as skip/1) on _S_t_r_e_a_m.


ggeett__ssiinnggllee__cchhaarr((_-_C_o_d_e))
    Get  a single character from input stream `user' (regardless  of the
    current  input stream).   Unlike get_code/1 this predicate does  not
    wait  for a  return.   The  character is  not echoed  to the  user's
    terminal.   This predicate is meant for keyboard menu selection etc.
    If SWI-Prolog was  started with the -tty option this predicate reads
    an  entire line of input  and returns the first non-blank  character
    on  this line,  or the  character code  of the newline  (10) if  the
    entire line consisted of blank characters.


aatt__eenndd__ooff__ssttrreeaamm
    Succeeds  after the last character  of the current input stream  has
    been  read.    Also succeeds  if  there is  no valid  current  input
    stream.


aatt__eenndd__ooff__ssttrreeaamm((_+_S_t_r_e_a_m))
    Succeeds  after the last character of  the named stream is read,  or
    _S_t_r_e_a_m is not a  valid input stream.  The end-of-stream test is only
    available  on buffered  input stream (unbuffered  input streams  are
    rarely used, see open/4).


ccooppyy__ssttrreeaamm__ddaattaa((_+_S_t_r_e_a_m_I_n_, _+_S_t_r_e_a_m_O_u_t_, _+_L_e_n))
    Copy  _L_e_n codes from  stream _S_t_r_e_a_m_I_n to _S_t_r_e_a_m_O_u_t.   Note that  the
    copy  is  done using  the  semantics of  get_code/2 and  put_code/2,
    taking  care of possibly recoding that needs take place  between two
    text files.  See section 2.17.1.


ccooppyy__ssttrreeaamm__ddaattaa((_+_S_t_r_e_a_m_I_n_, _+_S_t_r_e_a_m_O_u_t))
    Copy data all (remaining) data from stream _S_t_r_e_a_m_I_n to _S_t_r_e_a_m_O_u_t.


rreeaadd__ppeennddiinngg__iinnppuutt((_+_S_t_r_e_a_m_I_n_, _-_C_o_d_e_s_, _?_T_a_i_l))
    Read  input pending in  the input buffer  of _S_t_r_e_a_m_I_n and return  it
    in  the difference list _C_o_d_e_s-_T_a_i_l.   I.e. the available  characters
    codes  are used to  create the list _C_o_d_e_s  ending in the tail  _T_a_i_l.
    This  predicate is  intended  for efficient  unbuffered copying  and
    filtering of input coming from network connections or devices.

    The following code  fragment realises efficient non-blocking copy of
    data  from an  input- to an  output stream.   The  at_end_of_stream/1
    call  checks for  end-of-stream and fills  the input  buffer.   Note
    that  the use of a  get_code/2 and put_code/2 based loop requires  a
    flush_output/1 call  after _e_a_c_h put_code/2.   The copy_stream_data/2
    does  not allow for inspection of  the copied data and suffers  from
    the same buffering issues.

    copy(In, Out) :-
            repeat,
                (   at_end_of_stream(In)
                ->  !
                ;   read_pending_input(In, Chars, []),
                    format(Out, '~s', [Chars]),
                    flush_output(Out),
                    fail
                ).


44..1199 TTeerrmm rreeaaddiinngg aanndd wwrriittiinngg

This section  describes the basic term  reading and writing  predicates.
The  predicates  format/[1,2] and  writef/2  provide  formatted  output.
Writing  to  Prolog  datastructures  such  as  atoms  or  code-lists  is
supported by with_output_to/2and format/3.

There are  two ways  to manipulate  the output  format.   The  predicate
print/[1,2]  may  be  programmed  using  portray/1.      The  format  of
floating point  numbers may  be manipulated  using the  prolog_flag  (see
current_prolog_flag/2) float_format.

Reading  is  sensitive   to  the  prolog_flag  character_escapes,   which
controls  the interpretation  of the  \ character  in  quoted atoms  and
strings.


wwrriittee__tteerrmm((_+_T_e_r_m_, _+_O_p_t_i_o_n_s))
    The  predicate  write_term/2  is  the generic  form  of  all  Prolog
    term-write predicates.  Valid options are:

    aattttrriibbuutteess((_A_t_o_m))
         Define how attributed variables (see section 6.1)  are written.
         The default is determined by  the prolog flag write_attributes.
         Defined values are  ignore (ignore the attribute), dots  (write
         the attributes  as {...}),  write (simply  hand the  attributes
         recursively to write_term/2) and  portray (hand the  attributes
         to attr_portray_hook/2).

    bbaacckkqquuootteedd__ssttrriinngg((_B_o_o_l))
         If true,  write a string  object (see  section 4.23) as  `...`.
         The default depends on the prolog flag with the same name.

    cchhaarraacctteerr__eessccaappeess((_B_o_o_l))
         If true,  and  quoted(_t_r_u_e) is  active,  special characters  in
         quoted atoms and  strings are emitted as ISO  escape-sequences.
         Default is taken from the reference module (see below).

    iiggnnoorree__ooppss((_B_o_o_l))
         If true, the generic term-representation (<_f_u_n_c_t_o_r>(<_a_r_g_s> ...))
         will be  used for  all terms,  Otherwise (default),  operators,
         list-notation and  {}/1  will be  written using  their  special
         syntax.

    mmaaxx__ddeepptthh((_I_n_t_e_g_e_r))
         If the term is nested deeper than _I_n_t_e_g_e_r,  print the remainder
         as eclipse (...).  A 0 (zero) value (default)  imposes no depth
         limit.  This option  also delimits the number of printed  for a
         list.  Example:

         ?- write_term(a(s(s(s(s(0)))), [a,b,c,d,e,f]), [max_depth(3)]).
         a(s(s(...)), [a, b|...])

         Yes

         Used  by   the   top-level  and   debugger  to   limit   screen
         output.   See also the  prolog-flags toplevel_print_options  and
         debugger_print_options.

    mmoodduullee((_M_o_d_u_l_e))
         Define the reference module  (default user).  This  defines the
         default value for  the character_escapes option as well as  the
         operator definitions to use.  See also op/3.

    nnuummbbeerrvvaarrss((_B_o_o_l))
         If true, terms  of the format $VAR(N), where  <_N> is a positive
         integer,  will be  written as  a variable  name.   If  _N is  an
         atom it is written  without quotes.  This extension  allows for
         writing variables  with user-provided  names.   The default  is
         false.  See also numbervars/3.

    ppoorrttrraayy((_B_o_o_l))
         If true, the  hook portray/1 is  called before printing a  term
         that is not  a variable.   If portray/1  succeeds, the term  is
         considered printed.  See  also print/1.  The default  is false.
         This option is an extension to the ISO write_term options.

    pprriioorriittyy((_I_n_t_e_g_e_r))
         An  integer  between  0  and  1200  represeting   the  `context
         priority'.   Default is  1200.   Can be  used to write  partial
         terms appearing as the argument to an operator.  For example:

                 format('~w = ', [VarName]),
                 write_term(Value, [quoted(true), priority(699)])

    qquuootteedd((_B_o_o_l))
         If true, atoms and  functors that needs quotes will  be quoted.
         The default is false.


wwrriittee__tteerrmm((_+_S_t_r_e_a_m_, _+_T_e_r_m_, _+_O_p_t_i_o_n_s))
    As  write_term/2,  but output  is sent  to  _S_t_r_e_a_m rather  than  the
    current output.


wwrriittee__ccaannoonniiccaall((_+_T_e_r_m))
    Write  _T_e_r_m  on  the current  output  stream using  standard  paren-
    thesised  prefix notation  (i.e.,  ignoring operator  declarations).
    Atoms  that  need  quotes are  quoted.    Terms  written  with  this
    predicate  can always be read  back, regardless of current  operator
    declarations.      Equivalent  to  write_term/2  using  the  options
    ignore_ops,  quoted  and  numbervars after  numbervars/4  using  the
    singletons option.

    Note  that due to the use of numbervars/4, non-ground terms  must be
    written  using a  _s_i_n_g_l_e write_canonical/1 call.   This  used to  be
    the  case anyhow,  as garbage collection  between multiple calls  to
    one  of the write predicates can change  the _G<_N_N_N> identity of the
    variables.


wwrriittee__ccaannoonniiccaall((_+_S_t_r_e_a_m_, _+_T_e_r_m))
    Write _T_e_r_m in canonical form on _S_t_r_e_a_m.


wwrriittee((_+_T_e_r_m))
    Write  _T_e_r_m  to the  current output,  using  brackets and  operators
    where  appropriate.     See  current_prolog_flag/2  for  controlling
    floating point output format.


wwrriittee((_+_S_t_r_e_a_m_, _+_T_e_r_m))
    Write _T_e_r_m to _S_t_r_e_a_m.


wwrriitteeqq((_+_T_e_r_m))
    Write  _T_e_r_m  to the  current output,  using  brackets and  operators
    where  appropriate.    Atoms that  need quotes  are quoted.    Terms
    written  with this predicate can  be read back with read/1  provided
    the currently active operator declarations are identical.


wwrriitteeqq((_+_S_t_r_e_a_m_, _+_T_e_r_m))
    Write _T_e_r_m to _S_t_r_e_a_m, inserting quotes.


pprriinntt((_+_T_e_r_m))
    Prints  _T_e_r_m on the  current output stream  similar to write/1,  but
    for each (sub)term  of _T_e_r_m first the dynamic predicate portray/1 is
    called.   If this predicate succeeds _p_r_i_n_t assumes the (sub)term has
    been written.  This allows for user defined term writing.


pprriinntt((_+_S_t_r_e_a_m_, _+_T_e_r_m))
    Print _T_e_r_m to _S_t_r_e_a_m.


ppoorrttrraayy((_+_T_e_r_m))
    A dynamic predicate, which  can be defined by the user to change the
    behaviour  of print/1 on (sub)terms.   For each subterm  encountered
    that is not  a variable print/1 first calls portray/1 using the term
    as  argument.    For lists  only the  list as  a whole  is given  to
    portray/1.   If portray succeeds  print/1 assumes the term has  been
    written.


rreeaadd((_-_T_e_r_m))
    Read  the next Prolog term from  the current input stream and  unify
    it  with _T_e_r_m.  On a syntax error read/1 displays an  error message,
    attempts  to  skip  the erroneous  term  and  fails.    On  reaching
    end-of-file _T_e_r_m is unified with the atom end_of_file.


rreeaadd((_+_S_t_r_e_a_m_, _-_T_e_r_m))
    Read _T_e_r_m from _S_t_r_e_a_m.


rreeaadd__ccllaauussee((_-_T_e_r_m))
    Equivalent   to   read/1,   but  warns   the  user   for   variables
    only   occurring  once   in  a   term  (singleton   variables,   see
    section  2.15.1.5)  which   do  not  start  with  an  underscore  if
    style_check(singleton)  is active (default).    Used to read  Prolog
    source files  (see consult/1).  New code should use read_term/2 with
    the option singletons(warning).


rreeaadd__ccllaauussee((_+_S_t_r_e_a_m_, _-_T_e_r_m))
    Read a clause from _S_t_r_e_a_m.  See read_clause/1.


rreeaadd__tteerrmm((_-_T_e_r_m_, _+_O_p_t_i_o_n_s))
    Read  a term from the current  input stream and unify the term  with
    _T_e_r_m.    The  reading is  controlled  by options  from the  list  of
    _O_p_t_i_o_n_s.   If this list is  empty, the behaviour is the same  as for
    read/1.   The options are upward compatible to Quintus Prolog.   The
    argument order is  according to the ISO standard.  Syntax-errors are
    always reported using exception-handling (see catch/3).  Options:

    bbaacckkqquuootteedd__ssttrriinngg((_B_o_o_l))
         If true,  read `...`  to a  string object  (see section  4.23).
         The default depends on the prolog flag with the same name.

    cchhaarraacctteerr__eessccaappeess((_B_o_o_l))
         Defines  how  to  read  \  escape-sequences  in  quoted  atoms.
         See the  prolog-flags character_escapes, current_prolog_flag/2.
         (SWI-Prolog).

    ccoommmmeennttss((_-_C_o_m_m_e_n_t_s))
         Unify _C_o_m_m_e_n_t_s with a list of _P_o_s_i_t_i_o_n-_C_o_m_m_e_n_t,  where _P_o_s_i_t_i_o_n
         is  a   stream-position  object   (see  stream_position_data/3)
         indicating  the   start  of   a  comment  and   _C_o_m_m_e_n_t  is   a
         string-object containing  the  text including  delimiters of  a
         comment.   It returns all  comments from where  the read_term/2
         call started upto the end of the term read.

    ddoouubbllee__qquuootteess((_B_o_o_l))
         Defines  how to  read  "..." strings.    See  the  prolog-flags
         double_quotes, current_prolog_flag/2.  (SWI-Prolog).

    mmoodduullee((_M_o_d_u_l_e))
         Specify  _M_o_d_u_l_e  for  operators,   character_escapes   flag  and
         double_quotes flag.  The  value of the latter two is  overruled
         if  the corresponding  read_term/3  option  is provided.     If
         no module  is specified, the  current `source-module' is  used.
         (SWI-Prolog).

    ssiinngglleettoonnss((_V_a_r_s))
         As variable_names,  but only  reports  the variables  occurring
         only  once in  the  _T_e_r_m read.     Variables starting  with  an
         underscore (`_')  are not included  in this  list.   (ISO).  If
         _V_a_r_s is the constant warning, singleton variables  are reported
         using print_message/2.

    ssyynnttaaxx__eerrrroorrss((_A_t_o_m))
         If error  (default),  throw and  exception on  a syntax  error.
         Other values  are fail, which  causes a  message to be  printed
         using print_message/2, after which  the predicate fails,  quiet
         which causes  the predicate  to fail silently  and dec10  which
         causes syntax errors to be printed, after which read_term/[2,3]
         continues reading the next term.   Using dec10, read_term/[2,3]
         never fails.  (Quintus, SICStus).

    ssuubbtteerrmm__ppoossiittiioonnss((_T_e_r_m_P_o_s))
         Describes the  detailed layout of  the term.   The formats  for
         the various types of terms  if given below.  All  positions are
         character positions.    If  the input  is related  to a  normal
         stream,  these  positions are  relative  to the  start  of  the
         input, when  reading from  the terminal, they  are relative  to
         the start of the term.

         FFrroomm--TToo
             Used for primitive types (atoms, numbers, variables).

         ssttrriinngg__ppoossiittiioonn((_F_r_o_m_, _T_o))
             Used  to indicate  the  position of  a string  enclosed  in
             double quotes (").

         bbrraaccee__tteerrmm__ppoossiittiioonn((_F_r_o_m_, _T_o_, _A_r_g))
             Term  of  the form  {...},  as used  in  DCG  rules.    _A_r_g
             describes the argument.

         lliisstt__ppoossiittiioonn((_F_r_o_m_, _T_o_, _E_l_m_s_, _T_a_i_l))
             A list.  _E_l_m_s describes the  positions of the elements.  If
             the list specifies the tail as |<_T_a_i_l_T_e_r_m>, _T_a_i_l is unified
             with  the term-position  of the  tail,  otherwise with  the
             atom none.

         tteerrmm__ppoossiittiioonn((_F_r_o_m_, _T_o_, _F_F_r_o_m_, _F_T_o_, _S_u_b_P_o_s))
             Used  for a compound  term not matching  one of the  above.
             _F_F_r_o_m  and  _F_T_o  describe  the  position  of  the  functor.
             _S_u_b_P_o_s  is a  list,  each element  of  which describes  the
             term-position of the corresponding subterm.

    tteerrmm__ppoossiittiioonn((_P_o_s))
         Unifies _P_o_s with the starting  position of the term read.   _P_o_s
         if of the same format as use by stream_property/2.

    vvaarriiaabblleess((_V_a_r_s))
         Unify  _V_a_r_s  with a  list  of  variables  in the  term.     The
         variables appear in  the order they have  been read.  See  also
         term_variables/2.  (ISO).

    vvaarriiaabbllee__nnaammeess((_V_a_r_s))
         Unify _V_a_r_s with a list  of `_N_a_m_e = _V_a_r', where _N_a_m_e is  an atom
         describing the variable name and _V_a_r is a  variable that shares
         with the corresponding variable in _T_e_r_m.  (ISO).


rreeaadd__tteerrmm((_+_S_t_r_e_a_m_, _-_T_e_r_m_, _+_O_p_t_i_o_n_s))
    Read term with options from _S_t_r_e_a_m.  See read_term/2.


rreeaadd__hhiissttoorryy((_+_S_h_o_w_, _+_H_e_l_p_, _+_S_p_e_c_i_a_l_, _+_P_r_o_m_p_t_, _-_T_e_r_m_, _-_B_i_n_d_i_n_g_s))
    Similar  to read_term/2 using the option variable_names, but  allows
    for history  substitutions.  read_history/6is used  by the top level
    to  read the user's actions.   _S_h_o_w is  the command the user  should
    type  to show  the saved  events.   _H_e_l_p is  the command  to get  an
    overview  of the capabilities.   _S_p_e_c_i_a_l is a list of commands  that
    are  not saved in the  history.  _P_r_o_m_p_t  is the first prompt  given.
    Continuation  prompts for  more  lines are  determined by  prompt/2.
    A  %w  in the  prompt  is substituted  by  the event  number.    See
    section 2.7 for available substitutions.

    SWI-Prolog calls read_history/6 as follows:

    read_history(h, '!h', [trace], '%w ?- ', Goal, Bindings)


pprroommpptt((_-_O_l_d_, _+_N_e_w))
    Set  prompt associated  with read/1  and its  derivatives.   _O_l_d  is
    first  unified with the current prompt.  On success the  prompt will
    be  set to _N_e_w if  this is an atom.   Otherwise an error message  is
    displayed.   A prompt  is printed if one  of the read predicates  is
    called  and the cursor is  at the left margin.   It is also  printed
    whenever  a newline is given and  the term has not been  terminated.
    Prompts are only printed when the current input stream is _u_s_e_r.


pprroommpptt11((_+_P_r_o_m_p_t))
    Sets  the prompt for the next line  to be read.   Continuation lines
    will be read using the prompt defined by prompt/2.


44..2200 AAnnaallyyssiinngg aanndd CCoonnssttrruuccttiinngg TTeerrmmss


ffuunnccttoorr((_?_T_e_r_m_, _?_F_u_n_c_t_o_r_, _?_A_r_i_t_y))
    True  if _T_e_r_m is a  term with functor _F_u_n_c_t_o_r  and arity _A_r_i_t_y.   If
    _T_e_r_m  is a  variable  it is  unified with  a new  term holding  only
    variables.    functor/3 silently  fails on  instantiation faults  If
    _T_e_r_m  is an atom or  number, _F_u_n_c_t_o_r will  be unified with _T_e_r_m  and
    arity will be unified with the integer 0 (zero).


aarrgg((_?_A_r_g_, _+_T_e_r_m_, _?_V_a_l_u_e))
    _T_e_r_m  should be instantiated  to a term,  _A_r_g to an integer  between
    1  and  the  arity of  _T_e_r_m.    _V_a_l_u_e  is  unified with  the  _A_r_g-th
    argument  of _T_e_r_m.   _A_r_g may also  be unbound.   In this case  _V_a_l_u_e
    will  be unified  with the  successive arguments of  the term.    On
    successful  unification, _A_r_g  is unified  with the argument  number.
    Backtracking  yields alternative  solutions.    The predicate  arg/3
    fails  silently if  _A_r_g= 0 or _A_r_g > _a_r_i_t_y and raises  the exception
    domain_error(not_less_then_zero, Arg)if _A_r_g <0.


_?_T_e_r_m =.. _?_L_i_s_t
    _L_i_s_t  is a list which head is the functor of _T_e_r_m and  the remaining
    arguments are the arguments  of the term.  Each of the arguments may
    be  a variable,  but not  both.   This predicate  is called  `Univ'.
    Examples:

    ?- foo(hello, X) =.. List.

    List = [foo, hello, X]

    ?- Term =.. [baz, foo(1)]

    Term = baz(foo(1))


nnuummbbeerrvvaarrss((_+_T_e_r_m_, _+_S_t_a_r_t_, _-_E_n_d))
    Unify  the free variables  of _T_e_r_m with a  term $VAR(_N), where _N  is
    the  number of  the variable.   Counting starts  at _S_t_a_r_t.   _E_n_d  is
    unified  with the number that should be given to the  next variable.
    Example:

    ?- numbervars(foo(A, B, A), 0, End).

    A = '$VAR'(0)
    B = '$VAR'(1)
    End = 2

    See also the numbervars option to write_term/3 and numbervars/4.


nnuummbbeerrvvaarrss((_+_T_e_r_m_, _+_S_t_a_r_t_, _-_E_n_d_, _+_O_p_t_i_o_n_s))
    As numbervars/3, but providing the following options:

    ffuunnccttoorr__nnaammee((_+_A_t_o_m))
         Name of the functor to use instead of $VAR.

    aattttvvaarr((_+_A_c_t_i_o_n))
         What to do if  an attributed variable is encountered.   Options
         are skip,  which causes numbervars/3  to ignore the  attributed
         variable,  bind  which causes  it  to  thread it  as  a  normal
         variable and assign the next '$VAR'(N) term to  it or (default)
         error which raises the a type_error exception.

    ssiinngglleettoonnss((_+_B_o_o_l))
         If   true  (default   false),   numbervars/4   does   singleton
         detection.   Singleton variables are unified with  '$VAR'('_'),
         causing  them  to  be  printed  as   _  by  write_term/2  using
         the  numbervars  option.      This   option  is  exploited   by
         portray_clause/2 and write_canonical/2.


tteerrmm__vvaarriiaabblleess((_+_T_e_r_m_, _-_L_i_s_t))
    Unify  _L_i_s_t with  a list of  variables, each  sharing with a  unique
    variable of _T_e_r_m.  See also term_variables/3.  For example:

    ?- term_variables(a(X, b(Y, X), Z), L).

    L = [G367, G366, G371]
    X = G367
    Y = G366
    Z = G371


tteerrmm__vvaarriiaabblleess((_+_T_e_r_m_, _-_L_i_s_t_, _?_T_a_i_l))
    Difference list  version of term_variables/2.  I.e.  _T_a_i_l is the tail
    of the variable-list _L_i_s_t.


ccooppyy__tteerrmm((_+_I_n_, _-_O_u_t))
    Create  a version  if _I_n  with renamed (fresh)  variables and  unify
    it  to  _O_u_t.   Attributed  variables (see  section  6.1) have  their
    attributed  copied.    The  implementation of  copy_term/2 can  deal
    with  infinite  trees  (cyclic  terms).     As  pure  Prolog  cannot
    distinguish a ground  term from another ground term with exactly the
    same  structure, ground  sub-terms are  _s_h_a_r_e_d between  _I_n and  _O_u_t.
    Sharing  ground terms  does affect  setarg/3.   SWI-Prolog  provides
    duplicate_term/2 to create a true copy of a term.


44..2200..11 NNoonn--llooggiiccaall ooppeerraattiioonnss oonn tteerrmmss

Prolog is not capable  to _m_o_d_i_f_y instantiated parts of a term.   Lacking
that capability makes that language much safer,  but unfortunately there
are problems that suffer severely in terms of time  and/or memory usage.
Always try hard to avoid the use of these primitives, but  they can be a
good alternative  to using dynamic  predicates.   See also section  6.3,
discussing the use of global variables.


sseettaarrgg((_+_A_r_g_, _+_T_e_r_m_, _+_V_a_l_u_e))
    Extra-logical  predicate.     Assigns the  _A_r_g-th  argument  of  the
    compound  term _T_e_r_m with the given _V_a_l_u_e.  The assignment  is undone
    if  backtracking brings the  state back into  a position before  the
    setarg/3 call.  See also nb_setarg/3.

    This  predicate may  be used  for destructive  assignment to  terms,
    using  them as an  extra-logical storage  bin.   Always try hard  to
    avoid  the use of  setarg/3 as  it is not  supported by many  Prolog
    systems  and one has to be very careful about unexpected  copying as
    well as unexpected not copying of terms.


nnbb__sseettaarrgg((_+_A_r_g_, _+_T_e_r_m_, _+_V_a_l_u_e))
    Assigns  the _A_r_g-th  argument  of the  compound term  _T_e_r_m with  the
    given  _V_a_l_u_e  as setarg/3,  but on  backtracking  the assignment  is
    _n_o_t  reversed.    If _T_e_r_m  is  not atomic,  it  is duplicated  using
    duplicate_term/2.     This  predicate uses  the  same  technique  as
    nb_setval/2.   We therefore refer to the  description of nb_setval/2
    for  details  on  non-backtrackable  assignment  of  terms.     This
    predicate is  compatible to GNU-Prolog setarg(_A_,_T_,_V_,_f_a_l_s_e), removing
    the  type-restriction on _V_a_l_u_e.   See also  nb_linkarg/3.  Below  is
    an  example for counting the  number of solutions of  a goal.   Note
    that  this implementation is  thread-safe, reentrant and capable  of
    handling  exceptions.  Realising  these features with a  traditional
    implementation  based  on  assert/retract  or flag/3  is  much  more
    complicated.

    :- module_transparent succeeds_n_times/2.

    succeeds_n_times(Goal, Times) :-
            Counter = counter(0),
            (   Goal,
                arg(1, Counter, N0),
                N is N0 + 1,
                nb_setarg(1, Counter, N),
                fail
            ;   arg(1, Counter, Times)
            ).


nnbb__lliinnkkaarrgg((_+_A_r_g_, _+_T_e_r_m_, _+_V_a_l_u_e))
    As  nb_setarg/3,  but like nb_linkval/2 it does _n_o_t duplicate  _V_a_l_u_e.
    Use with  extreme care and consult the documentation of nb_linkval/2
    before use.


dduupplliiccaattee__tteerrmm((_+_I_n_, _-_O_u_t))
    Version  of copy_term/2 that also copies ground terms  and therefore
    ensures  destructive  modification using  setarg/3 does  not  affect
    the  copy.    See  also nb_setval/2,  nb_linkval/2, nb_setarg/3  and
    nb_linkarg/3.


44..2211 AAnnaallyyssiinngg aanndd CCoonnssttrruuccttiinngg AAttoommss

These  predicates  convert   between  Prolog  constants  and  lists   of
character codes.  The predicates atom_codes/2, number_codes/2 and name/2
behave the same when  converting from a constant to a list  of character
codes.     When converting  the  other  way  around,  atom_codes/2  will
generate an  atom, number_codes/2  will generate a  number or  exception
and name/2 will return a number if possible and an atom otherwise.

The  ISO  standard  defines atom_chars/2  to  describe  the  `broken-up'
atom  as a  list of  one-character atoms  instead of  a  list of  codes.
Up-to version  3.2.x, SWI-Prolog's  atom_chars/2 behaved, compatible  to
Quintus and  SICStus Prolog,  like atom_codes.    As of 3.3.x  SWI-Prolog
atom_codes/2 and atom_chars/2are compliant to the ISO standard.

To  ease  the pain  of  all  variations in  the  Prolog  community,  all
SWI-Prolog predicates  behave as  flexible as  possible.   This  implies
the  `list-side' accepts  either  a code-list  or  a char-list  and  the
`atom-side' accept all atomic types (atom, number and string).


aattoomm__ccooddeess((_?_A_t_o_m_, _?_S_t_r_i_n_g))
    Convert  between an atom and a list of character codes.   If _A_t_o_m is
    instantiated,  if will be translated into a list of  character codes
    and  the result  is unified with  _S_t_r_i_n_g.   If _A_t_o_m  is unbound  and
    _S_t_r_i_n_g  is a list of character  codes, it will _A_t_o_m will  be unified
    with an atom constructed from this list.


aattoomm__cchhaarrss((_?_A_t_o_m_, _?_C_h_a_r_L_i_s_t))
    As  atom_codes/2, but  _C_h_a_r_L_i_s_t  is a  list of  one-character  atoms
    rather than a list of character codes.

    ?- atom_chars(hello, X).

    X = [h, e, l, l, o]


cchhaarr__ccooddee((_?_A_t_o_m_, _?_C_o_d_e))
    Convert   between  character  and   character  code  for  a   single
    character.


nnuummbbeerr__cchhaarrss((_?_N_u_m_b_e_r_, _?_C_h_a_r_L_i_s_t))
    Similar  to  atom_chars/2,  but  converts between  a number  and  its
    representation  as a  list of  one-character atoms.    Fails with  a
    representation_error  if _N_u_m_b_e_r  is unbound  and  _C_h_a_r_L_i_s_t does  not
    describe a number.


nnuummbbeerr__ccooddeess((_?_N_u_m_b_e_r_, _?_C_o_d_e_L_i_s_t))
    As number_chars/2, but converts to a list  of character codes rather
    than one-character atoms.   In the mode -, +, both predicates behave
    identically to improve handling of non-ISO source.


aattoomm__nnuummbbeerr((_?_A_t_o_m_, _?_N_u_m_b_e_r))
    Realises  the popular combination of atom_codes/2 and number_codes/2
    to  convert  between  atom and  number  (integer  or float)  in  one
    predicate, avoiding the intermediate list.


nnaammee((_?_A_t_o_m_O_r_I_n_t_, _?_S_t_r_i_n_g))
    _S_t_r_i_n_g  is a list of character  codes representing the same text  as
    _A_t_o_m.  Each of  the arguments may be a variable, but not both.  When
    _S_t_r_i_n_g  is bound  to an  character code list  describing an  integer
    and  _A_t_o_m  is a  variable  _A_t_o_m will  be  unified with  the  integer
    value  described by _S_t_r_i_n_g  (e.g.   `name(N, "300"), 400 is N + 100'
    succeeds).


tteerrmm__ttoo__aattoomm((_?_T_e_r_m_, _?_A_t_o_m))
    True if _A_t_o_m describes  a term that unifies with _T_e_r_m.  When _A_t_o_m is
    instantiated _A_t_o_m is converted  and then unified with _T_e_r_m.  If _A_t_o_m
    has no  valid syntax, a syntax_errorexception is  raised.  Otherwise
    _T_e_r_m is ``written'' on _A_t_o_m using write/1.


aattoomm__ttoo__tteerrmm((_+_A_t_o_m_, _-_T_e_r_m_, _-_B_i_n_d_i_n_g_s))
    Use  _A_t_o_m as  input to  read_term/2 using the  option variable_names
    and  return  the read  term in  _T_e_r_m and  the  variable bindings  in
    _B_i_n_d_i_n_g_s.   _B_i_n_d_i_n_g_s is a list of _N_a_m_e =_V_a_r couples, thus providing
    access  to the actual  variable names.   See  also read_term/2.   If
    _A_t_o_m has no valid syntax, a syntax_error exception is raised.


aattoomm__ccoonnccaatt((_?_A_t_o_m_1_, _?_A_t_o_m_2_, _?_A_t_o_m_3))
    _A_t_o_m_3  forms the concatenation of _A_t_o_m_1 and _A_t_o_m_2.  At least  two of
    the  arguments must be instantiated  to atoms, integers or  floating
    point  numbers.    For  ISO  compliance,  the  instantiation-pattern
    -, -, +  is allowed  too, non-deterministically  splitting the  3-th
    argument  into two parts  (as append/3  does for lists).   See  also
    string_concat/3.


ccoonnccaatt__aattoomm((_+_L_i_s_t_, _-_A_t_o_m))
    _L_i_s_t  is  a  list of  atoms,  integers  or floating  point  numbers.
    Succeeds  if  _A_t_o_m can  be unified  with  the concatenated  elements
    of  _L_i_s_t.    If _L_i_s_t  has exactly  2 elements  it  is equivalent  to
    atom_concat/3, allowing for variables in the list.


ccoonnccaatt__aattoomm((_?_L_i_s_t_, _+_S_e_p_a_r_a_t_o_r_, _?_A_t_o_m))
    Creates  an  atom just  like  concat_atom/2, but  inserts  _S_e_p_a_r_a_t_o_r
    between each pair of atoms.  For example:

    ?- concat_atom([gnu, gnat], ', ', A).

    A = 'gnu, gnat'

    This  predicate can  also be  used to split  atoms by  instantiating
    _S_e_p_a_r_a_t_o_r and _A_t_o_m:

    ?- concat_atom(L, -, 'gnu-gnat').

    L = [gnu, gnat]


aattoomm__lleennggtthh((_+_A_t_o_m_, _-_L_e_n_g_t_h))
    True  if _A_t_o_m is an atom of _L_e_n_g_t_h characters long.   This predicate
    also  works for  strings (see  section 4.23).   If  the prolog  flag
    iso  is _n_o_t  set, it  also accepts integers  and floats,  expressing
    the  number of characters  output when given  to write/1 as well  as
    code-lists and character-lists, expressing the length of the list.


aattoomm__pprreeffiixx((_+_A_t_o_m_, _+_P_r_e_f_i_x))
    True if _A_t_o_m starts  with the characters from _P_r_e_f_i_x.  Its behaviour
    is equivalent to ?- sub_atom(Atom, 0, _, _, Prefix).  Depreciated.


ssuubb__aattoomm((_+_A_t_o_m_, _?_B_e_f_o_r_e_, _?_L_e_n_, _?_A_f_t_e_r_, _?_S_u_b))
    ISO  predicate  for breaking  atoms.    It maintains  the  following
    relation:  _S_u_b is  a sub-atom of _A_t_o_m that starts at _B_e_f_o_r_e, has _L_e_n
    characters and _A_t_o_m contains _A_f_t_e_r characters after the match.

    ?- sub_atom(abc, 1, 1, A, S).

    A = 1, S = b

    The implementation  minimises non-determinism and creation of atoms.
    This  is a very flexible predicate  that can do search, prefix-  and
    suffix-matching, etc.


44..2222 CChhaarraacctteerr pprrooppeerrttiieess

SWI-Prolog   offers  two   comprehensive  predicates   for   classifying
characters and character-codes.  These predicates are  defined as built-
in predicates  to exploit the  C-character classification's handling  of
_l_o_c_a_l_e (handling of  local character-sets).  These predicates  are fast,
logical and deterministic if applicable.

In addition, there is the library ctype providing  compatibility to some
other Prolog  systems.   The predicates of this  library are defined  in
terms of code_type/2.


cchhaarr__ttyyppee((_?_C_h_a_r_, _?_T_y_p_e))
    Tests or generates  alternative _T_y_p_es or _C_h_a_rs.  The character-types
    are inspired by the standard C <ctype.h>  primitives.

    aallnnuumm
         _C_h_a_r is a letter (upper- or lowercase) or digit.

    aallpphhaa
         _C_h_a_r is a letter (upper- or lowercase).

    ccssyymm
         _C_h_a_r  is  a  letter   (upper-  or  lowercase),  digit   or  the
         underscore  (_).     These  are  valid  C-  and  Prolog  symbol
         characters.

    ccssyymmff
         _C_h_a_r is a letter  (upper- or lowercase) or the  underscore (_).
         These are valid first characters for C- and Prolog symbols

    aasscciiii
         _C_h_a_r is a 7-bits ASCII character (0..127).

    wwhhiittee
         _C_h_a_r is a space or tab.  E.i. white space inside a line.

    ccnnttrrll
         _C_h_a_r is an ASCII control-character (0..31).

    ddiiggiitt
         _C_h_a_r is a digit.

    ddiiggiitt((_W_e_i_g_t_h))
         _C_h_a_r is a digit with value  _W_e_i_g_t_h.  I.e. char_type(X, digit(6)
         yields _X = '6'.  Useful for parsing numbers.

    xxddiiggiitt((_W_e_i_g_t_h))
         _C_h_a_r  is  a  hexa-decimal  digit  with  value  _W_e_i_g_t_h.     I.e.
         char_type(a, xdigit(X) yields _X  = '10'.    Useful for  parsing
         numbers.

    ggrraapphh
         _C_h_a_r produces  a visible mark  on a  page when printed.    Note
         that the space is not included!

    lloowweerr
         _C_h_a_r is a lower-case letter.

    lloowweerr((_U_p_p_e_r))
         _C_h_a_r is a  lower-case version of _U_p_p_e_r.   Only true if _C_h_a_r  is
         lowercase and _U_p_p_e_r uppercase.

    ttoo__lloowweerr((_U_p_p_e_r))
         _C_h_a_r is  a lower-case version  of _U_p_p_e_r.   For non-letters,  or
         letter without case,  _C_h_a_r and  _L_o_w_e_r are the same.   See  also
         upcase_atom/2 and downcase_atom/2.

    uuppppeerr
         _C_h_a_r is an upper-case letter.

    uuppppeerr((_L_o_w_e_r))
         _C_h_a_r is an upper-case version  of _L_o_w_e_r.  Only true if  _C_h_a_r is
         uppercase and _L_o_w_e_r lowercase.

    ttoo__uuppppeerr((_L_o_w_e_r))
         _C_h_a_r is an  upper-case version of _L_o_w_e_r.   For non-letters,  or
         letter without case,  _C_h_a_r and  _L_o_w_e_r are the same.   See  also
         upcase_atom/2 and downcase_atom/2.

    ppuunncctt
         _C_h_a_r is  a punctuation character.   This  is a graph  character
         that is not a letter or digit.

    ssppaaccee
         _C_h_a_r  is some  form  of layout  character  (tab,  vertical-tab,
         newline, etc.).

    eenndd__ooff__ffiillee
         _C_h_a_r is -1.

    eenndd__ooff__lliinnee
         _C_h_a_r ends a line (ASCII: 10..13).

    nneewwlliinnee
         _C_h_a_r is a the newline character (10).

    ppeerriioodd
         _C_h_a_r counts as the end of a sentence (.,!,?).

    qquuoottee
         _C_h_a_r is a quote-character (", ', `).

    ppaarreenn((_C_l_o_s_e))
         _C_h_a_r is  an  open-parenthesis and  _C_l_o_s_e is  the  corresponding
         close-parenthesis.


ccooddee__ttyyppee((_?_C_o_d_e_, _?_T_y_p_e))
    As  char_type/2,  but uses character-codes rather than  one-character
    atoms.     Please note  that  both  predicates are  as  flexible  as
    possible.    They handle  either representation if  the argument  is
    instantiated  and  only will  instantiate with  an  integer code  or
    one-character  atom depending  of the version  used.   See also  the
    prolog-flag double_quotes, atom_chars/2 and atom_codes/2.


44..2222..11 CCaassee ccoonnvveerrssiioonn

There is nothing in  the Prolog standard for converting case  in textual
data.    The SWI-Prolog  predicates code_type/2 and  char_type/2 can  be
used to test  and convert individual characters.   We have started  some
additional support:


ddoowwnnccaassee__aattoomm((_+_A_n_y_C_a_s_e_, _-_L_o_w_e_r_C_a_s_e))
    Converts  the characters  of _A_n_y_C_a_s_e  into lowercase  as char_type/2
    does  (i.e. based on  the defined _l_o_c_a_l_e  if Prolog provides  locale
    support  on the  hosting platform)  and unifies  the lowercase  atom
    with _L_o_w_e_r_C_a_s_e.


uuppccaassee__aattoomm((_+_A_n_y_C_a_s_e_, _-_U_p_p_e_r_C_a_s_e))
    Converts, similar to downcase_atom/2, an atom to upper-case.


44..2222..22 LLaanngguuaaggee ssppeecciiffiicc ccoommppaarriissoonn

This  section  deals  with  predicates  for   language  specific  string
comparison operations.


ccoollllaattiioonn__kkeeyy((_+_A_t_o_m_, _-_K_e_y))
    Create  a _K_e_y from _A_t_o_m for locale specific comparison.  The  key is
    defined  such that if the key of atom A preceeds the  key of atom B
    in the  standard order of terms, A is alphabetically smaller than B
    using the sort order of the current locale.

    The   predicate  collation_key/2  is  used  by   locale_sort/2  from
    library(sort).   Please examine the  implementation of locale_sort/2
    as an example of using this call.

    The  _K_e_y  is  an  implementation defined  and  generally  unreadable
    string.


llooccaallee__ssoorrtt((_+_L_i_s_t_, _-_S_o_r_t_e_d))
    Sort  a list of atoms using the current  locale.  _L_i_s_t is a  list of
    atoms or string objects  (see section 4.23).  _S_o_r_t_e_d is unified with
    a  list containing all  atoms of  _L_i_s_t, sorted to  the rules of  the
    current locale.  See also collation_key/2 and setlocale/3.


44..2233 RReepprreesseennttiinngg tteexxtt iinn ssttrriinnggss

SWI-Prolog  supports the  data type  _s_t_r_i_n_g.   Strings  are  a time  and
space efficient mechanism to handle text in Prolog.   Strings are stored
as  a byte  array  on the  global (term)  stack  and thus  destroyed  on
backtracking and reclaimed by the garbage collector.

Strings were  added to  SWI-Prolog based on  an early  draft of the  ISO
standard,  offering a mechanism  to represent  temporary character  data
efficiently.    As  SWI-Prolog  strings can  handle  0-bytes,  they  are
frequently used through  the foreign language interface (section 9)  for
storing arbitrary byte-sequences.

Starting with version  3.3, SWI-Prolog offers garbage collection  on the
atom-space as well as  representing 0-bytes in atoms.   Although strings
and  atoms still  have  different  features,  new code  should  consider
using atoms to  avoid too many representations  for text as well as  for
compatibility to other  Prolog implementations.   Below are some of  the
differences:

  o _c_r_e_a_t_i_o_n
    Creating  strings  is fast,  as the  data is  simply  copied to  the
    global  stack.   Atoms are  unique and  therefore more expensive  in
    terms  of memory  and time to  create.   On the other  hand, if  the
    same  text has  to be  represented  multiple times,  atoms are  more
    efficient.

  o _d_e_s_t_r_u_c_t_i_o_n
    Backtracking destroys strings at  no cost.  They are cheap to handle
    by  the garbage  collector, but  it should be  noted that  extensive
    use  of strings will cause many  garbage collections.  Atom  garbage
    collection is generally faster.

String objects  by default have no  lexical representation and thus  can
only  be created  using  the predicates  below  or through  the  foreign
language interface (See  chapter 9.  There  are two ways to make  read/1
read text into  strings, both controlled through  Prolog flags.  One  is
by setting the double_quotes flag to string and the other  is by setting
the backquoted_string flag  to true.   In latter case, `Hello world`  is
read into a  string and write_term/2 prints strings between  back-quotes
if quoted  is true.    This flag  provides compatibility  to LPA  Prolog
string handling.


ssttrriinngg__ttoo__aattoomm((_?_S_t_r_i_n_g_, _?_A_t_o_m))
    Logical  conversion between  a string  and an atom.    At least  one
    of  the two arguments  must be instantiated.   _A_t_o_m  can also be  an
    integer or floating point number.


ssttrriinngg__ttoo__lliisstt((_?_S_t_r_i_n_g_, _?_L_i_s_t))
    Logical  conversion  between  a  string  and  a  list  of  character
    codes  characters.    At least  one  of the  two arguments  must  be
    instantiated.


ssttrriinngg__lleennggtthh((_+_S_t_r_i_n_g_, _-_L_e_n_g_t_h))
    Unify  _L_e_n_g_t_h  with  the number  of  characters  in _S_t_r_i_n_g.     This
    predicate  is  functionally  equivalent  to atom_length/2  and  also
    accepts atoms, integers and floats as its first argument.


ssttrriinngg__ccoonnccaatt((_?_S_t_r_i_n_g_1_, _?_S_t_r_i_n_g_2_, _?_S_t_r_i_n_g_3))
    Similar  to atom_concat/3,  but the unbound argument will  be unified
    with  a string object rather  than an atom.   Also, if both  _S_t_r_i_n_g_1
    and  _S_t_r_i_n_g_2 are  unbound and _S_t_r_i_n_g_3  is bound to  text, it  breaks
    _S_t_r_i_n_g_3,  unifying the start with  _S_t_r_i_n_g_1 and the end with  _S_t_r_i_n_g_2
    as  append does  with lists.   Note  that this  is not  particularly
    fast  on long  strings as  for each redo  the system  has to  create
    two  entirely new strings, while the list equivalent only  creates a
    single new list-cell and moves some pointers around.


ssuubb__ssttrriinngg((_+_S_t_r_i_n_g_, _?_S_t_a_r_t_, _?_L_e_n_g_t_h_, _?_A_f_t_e_r_, _?_S_u_b))
    _S_u_b  is a substring of _S_t_r_i_n_g starting at _S_t_a_r_t, with  length _L_e_n_g_t_h
    and  _S_t_r_i_n_g has _A_f_t_e_r  characters left  after the match.   See  also
    sub_atom/5.


44..2244 OOppeerraattoorrss

Operators  are  defined  to  improve  the  readability  of  source-code.
For  example, without  operators, to  write  2*3+4*5 one  would have  to
write +(*(2,3),*(4,5)).    In Prolog,  a number of  operators have  been
predefined.   All operators, except for  the comma (,) can be  redefined
by the user.

Some care  has to  be taken  before defining  new operators.    Defining
too many  operators might  make your  source `natural'  looking, but  at
the same  time lead  to hard to  understand the  limits of your  syntax.
To ease  the pain, as  of SWI-Prolog 3.3.0,  operators are local to  the
module  in which  they are  defined.   Operators  can  be exported  from
modules using a  term op(_P_r_e_c_e_d_e_n_c_e_, _T_y_p_e_,  _N_a_m_e) in the export list  as
specified by module/2.  This is an extension  specific to SWI-Prolog and
the advised mechanism of portability is not an important concern.

The  module-table of  the module  user  acts as  default table  for  all
modules and can be  modified explicitly from inside a module  to achieve
compatibility to other Prolog systems:

:- module(prove,
          [ prove/1
          ]).

:- op(900, xfx, user:(=>)).

Unlike  what many  users  think,  operators  and quoted  atoms  have  no
relation:  defining  an atom as an  operator does nnoott influence  parsing
characters into atoms and  quoting an atom does nnoott stop it  from acting
as an operator.   To stop an atom  acting as an operator, enclose  it in
braces like this:  (myop).


oopp((_+_P_r_e_c_e_d_e_n_c_e_, _+_T_y_p_e_, _:_N_a_m_e))
    Declare  _N_a_m_e  to  be  an  operator of  type  _T_y_p_e  with  precedence
    _P_r_e_c_e_d_e_n_c_e.    _N_a_m_e  can also  be a  list of  names,  in which  case
    all  elements of the  list are declared  to be identical  operators.
    _P_r_e_c_e_d_e_n_c_e  is an integer between 0 and 1200.  Precedence  0 removes
    the  declaration.   _T_y_p_e is one  of:   xf, yf, xfx,  xfy, yfx,  yfy,
    fy  or fx.   The `f'  indicates the position  of the functor,  while
    x  and y  indicate the position  of the  arguments.   `y' should  be
    interpreted  as ``on this position  a term with precedence lower  or
    equal to the precedence  of the functor should occur''.  For `x' the
    precedence  of the argument must be strictly lower.   The precedence
    of  a term is  0, unless  its principal functor  is an operator,  in
    which  case the precedence  is the precedence of  this operator.   A
    term enclosed in brackets (...) has precedence 0.

    The  predefined operators are  shown in  table 4.1.   Note that  all
    operators can be redefined by the user.
     ______________________________________________________________
     | 1200 |xfx  |-->, :-                                        |
     | 1200 | fx  |:-, ?-                                         |
     | 1150 | fx  |dynamic,  discontiguous, initialization,  mod- |
     |      |     |ule_transparent,    multifile,     thread_local,|
     |      |     |volatile                                       |

     | 1100 |xfy  |;, |                                           |
     | 1050 |xfy  |->, op*->                                      |
     | 1000 |xfy  |,                                              |
     |  954 |xfy  |\                                              |
     |  900 | fy  |\+                                             |
     |  900 | fx  |~                                              |
     |  700 |xfx  |<, =, =.., =@=,  =:=, =<, ==, =\=,  >, >=, @<, |
     |      |     |@=<, @>, @>=, \=, \==, is                      |

     |  600 |xfy  |:                                              |
     |  500 | yfx |+, -, /\, \/, xor                              |
     |  500 | fx  |+, -, ?, \                                     |
     |  400 | yfx |*, /, //, rdiv, <<, >>, mod, rem               |
     |  200 |xfx  |**                                             |
     |__200_|xfy__|^______________________________________________|_

                      Table 4.1:  System operators


ccuurrrreenntt__oopp((_?_P_r_e_c_e_d_e_n_c_e_, _?_T_y_p_e_, _?_:_N_a_m_e))
    True  if _N_a_m_e is currently defined as an operator of type  _T_y_p_e with
    precedence _P_r_e_c_e_d_e_n_c_e.  See also op/3.


44..2255 CChhaarraacctteerr CCoonnvveerrssiioonn

Although I  wouldn't really know  for what you would  like to use  these
features, they are provided for ISO compliance.


cchhaarr__ccoonnvveerrssiioonn((_+_C_h_a_r_I_n_, _+_C_h_a_r_O_u_t))
    Define  that term-input (see  read_term/3)  maps each character  read
    as  _C_h_a_r_I_n to the character _C_h_a_r_O_u_t.   Character conversion is  only
    executed  if the prolog-flag char_conversion is set to true  and not
    inside  quoted  atoms or  strings.    The  initial table  maps  each
    character onto itself.  See also current_char_conversion/2.


ccuurrrreenntt__cchhaarr__ccoonnvveerrssiioonn((_?_C_h_a_r_I_n_, _?_C_h_a_r_O_u_t))
    Queries    the   current   character   conversion-table.         See
    char_conversion/2 for details.


44..2266 AArriitthhmmeettiicc

Arithmetic can be  divided into some special purpose integer  predicates
and  a series  of general  predicates for  integer,  floating point  and
rational arithmetic as  appropriate.  The general arithmetic  predicates
all handle _e_x_p_r_e_s_s_i_o_n_s.   An expression is  either a simple number or  a
_f_u_n_c_t_i_o_n.  The  arguments of a function are expressions.   The functions
are described in section 4.26.2.3.


44..2266..11 SSppeecciiaall ppuurrppoossee iinntteeggeerr aarriitthhmmeettiicc

The predicates in  this section provide more logical operations  between
integers.  They  are not covered by the ISO standard, although  they are
`part of the community' and found as either library  or built-in in many
other Prolog systems.


bbeettwweeeenn((_+_L_o_w_, _+_H_i_g_h_, _?_V_a_l_u_e))
    _L_o_w  and _H_i_g_h are  integers, _H_i_g_h >=_L_o_w.   If  _V_a_l_u_e is an  integer,
    _L_o_w=< _V_a_l_u_e=< _H_i_g_h.   When _V_a_l_u_e  is a  variable it is  successively
    bound  to  all integers  between  _L_o_w and  _H_i_g_h.    If _H_i_g_h  is  inf
    or  infinite between/3 is  true iff _V_a_l_u_e>= _L_o_w,  a feature that  is
    particularly  interesting  for generating  integers from  a  certain
    value.


ssuucccc((_?_I_n_t_1_, _?_I_n_t_2))
    True  if  _I_n_t_2 = _I_n_t_1+ 1  and  _I_n_t_1>=0.     At  least  one  of  the
    arguments must be  instantiated to a natural number.  This predicate
    raises the domain-error not_less_than_zero if called with a negative
    integer.   E.g. succ(_X_, _0)  fails silently and succ(_X_, _-_1) raises  a
    domain-error.


pplluuss((_?_I_n_t_1_, _?_I_n_t_2_, _?_I_n_t_3))
    True  if _I_n_t_3 =_I_n_t_1 +_I_n_t_2.    At least two  of the  three arguments
    must be instantiated to integers.


44..2266..22 GGeenneerraall ppuurrppoossee aarriitthhmmeettiicc

The  general   arithmetic  predicates   are  optionally  compiled   (see
set_prolog_flag/2  and  the   -O  command  line   option).      Compiled
arithmetic reduces global  stack requirements and improves  performance.
Unfortunately compiled arithmetic cannot  be traced, which is why  it is
optional.


_+_E_x_p_r_1 > _+_E_x_p_r_2
    True if expression _E_x_p_r_1 evaluates to a larger number than _E_x_p_r_2.


_+_E_x_p_r_1 < _+_E_x_p_r_2
    True if expression _E_x_p_r_1 evaluates to a smaller number than _E_x_p_r_2.


_+_E_x_p_r_1 =< _+_E_x_p_r_2
    True  if expression _E_x_p_r_1 evaluates to a smaller or equal  number to
    _E_x_p_r_2.


_+_E_x_p_r_1 >= _+_E_x_p_r_2
    True  if expression _E_x_p_r_1 evaluates to  a larger or equal number  to
    _E_x_p_r_2.


_+_E_x_p_r_1 =\= _+_E_x_p_r_2
    True if expression _E_x_p_r_1 evaluates to a number non-equal to _E_x_p_r_2.


_+_E_x_p_r_1 =:= _+_E_x_p_r_2
    True if expression _E_x_p_r_1 evaluates to a number equal to  _E_x_p_r_2.


_-_N_u_m_b_e_r iiss _+_E_x_p_r
    True  if _N_u_m_b_e_r has successfully  been unified with the number  _E_x_p_r
    evaluates to.   If _E_x_p_r evaluates to a float that can be represented
    using  an integer (i.e,  the value is  integer and within the  range
    that can be  described by Prolog's integer representation),  _E_x_p_r is
    unified with the integer value.

    Note  that normally, is/2 should be used with unbound  left operand.
    If equality is to be tested, =:=/2 should be used.  For example:

             ?- 1 is sin(pi/2).   Fails!.   sin(pi/2) evaluates
                                  to the float  1.0, which does
                                  not unify with the integer 1.
             ?- 1 =:= sin(pi/2).  Succeeds as expected.


44..2266..22..11 AArriitthhmmeettiicc ttyyppeess

SWI-Prolog defines the following numeric types:

  o _i_n_t_e_g_e_r
    If  SWI-Prolog is built using the _G_N_U _m_u_l_t_i_p_l_e  _p_r_e_c_i_s_i_o_n _a_r_i_t_h_m_e_t_i_c
    _l_i_b_r_a_r_y  (GMP), integer  arithmetic is _u_n_b_o_u_n_d_e_d,  which means  that
    the  size of integers is limited by available memory only.   Without
    GMP,  SWI-Prolog  integers are  64-bits,  regardless of  the  native
    integer  size  of  the  platform.    The  type  of  integer  support
    can  be detected  using the  Prolog flags  bounded, min_integer  and
    max_integer.   As the use of GMP  is default, most of the  following
    descriptions assume unbounded integer arithmetic.

    Internally,  SWI-Prolog has  three integer  representations.   Small
    integers   (defined  by  the  Prolog  flag  max_tagged_integer)  are
    encoded  directly.  Larger integers are represented as  64-bit value
    on  the  global stack.    Integers that  do not  fit  in 64-bit  are
    represented as serialised GNU MPZ structures on the global stack.

  o _r_a_t_i_o_n_a_l _n_u_m_b_e_r
    Rational  numbers  (Q) are  quotients of  two  integers.   Rational
    arithmetic  is only provided if GMP  is used (see above).   Rational
    numbers  are currently not  supported by  a Prolog type.   They  are
    represented  by the compound  term rdiv(_N_,_M). Rational numbers  that
    are  returned from is/2  are _c_a_n_o_n_i_c_a_l, which  means M  is positive
    and  N  and  M have  no  common divisors.    Rational  numbers  are
    introduced  in the computation  using the rational/1,  rationalize/1
    or  the  rdiv/2  (rational  division)  function.    Using  the  same
    functor  for  rational division  and representing  rational  numbers
    allow  for passing rational numbers between computations as  well as
    to format/3 for printing.

    On  the long  term it is  likely that  rational numbers will  become
    _a_t_o_m_i_c  as  well as  subtype of  _n_u_m_b_e_r.    User code  that  creates
    or  inspects the  rdiv(_M_,_N)  terms will  not be  portable to  future
    versions.     Rationals  are  created using  one  of  the  functions
    mentioned above and inspected using rational/3.

  o _f_l_o_a_t
    Floating point numbers  are represented using the C-type double.  On
    most today platforms these are 64-bit IEEE floating point numbers.

Arithmetic functions that require integer arguments accept,  in addition
to integers,  rational numbers with  denominator `1' and floating  point
numbers that can be  accurately converted to integers.  If  the required
argument is  a float  the argument  is converted to  float.   Note  that
conversion of integers  to floating point numbers may raise  an overflow
exception.   In  all other cases,  arguments are  converted to the  same
type using the order below.

    integer ! rational number ! floating point number


44..2266..22..22 RRaattiioonnaall nnuummbbeerr eexxaammpplleess

The use  of rational numbers  with unbounded  integers allows for  exact
integer  or _f_i_x_e_d  _p_o_i_n_t  arithmetic under  the  addition,  subtraction,
multiplication  and division.    To exploit  rational arithmetic  rdiv/2
should  be used  instead  of `/'  and  floating  point numbers  must  be
converted to  rational using  rational/1.   Omitting  the rational/1  on
floats  will  convert a  rational  operand  to float  and  continue  the
arithmetic using floating point numbers.  Here are some examples.

              A is 2 rdiv 6                  A = 1 rdiv 3
              A is 4 rdiv 3 + 1              A = 7 rdiv 3
              A is 4 rdiv 3 + 1.5            A = 2.83333
              A is 4 rdiv 3 + rational(1.5)  A = 17 rdiv 6

Note that  floats cannot  represent all  decimal numbers exactly.    The
function  rational/1 creates  an _e_x_a_c_t  equivalent of  the float,  while
rationalize/1  creates  a  rational number  that  is  within  the  float
rounding error from the original float.  Please  check the documentation
of these functions for details and examples.

Rational  numbers can  be  printed  as decimal  numbers  with  arbitrary
precision using the format/3 floating point conversion:

?- A is 4 rdiv 3 + rational(1.5),
   format('~50f~n', [A]).
2.83333333333333333333333333333333333333333333333333

A = 17 rdiv 6


44..2266..22..33 AArriitthhmmeettiicc FFuunnccttiioonnss

Arithmetic functions  are terms  which are evaluated  by the  arithmetic
predicates described in  section 4.26.2.   SWI-Prolog tries to hide  the
difference  between integer  arithmetic  and floating  point  arithmetic
from the Prolog user.  Arithmetic is done as  integer arithmetic as long
as possible and  converted to floating point arithmetic whenever  one of
the arguments  or the combination of  them requires it.   If a  function
returns  a floating  point  value which  is  whole it  is  automatically
transformed into  an integer.    There are  four types  of arguments  to
functions:

       _E_x_p_r       Arbitrary   expression,   returning  either   a
                  floating point value or an integer.
       _I_n_t_E_x_p_r    Arbitrary expression  that  must evaluate  into
                  an integer.
       _R_a_t_E_x_p_r    Arbitrary expression that must evaluate  into a
                  rational number.
       _F_l_o_a_t_E_x_p_r  Arbitrary expression that must evaluate  into a

                  floating point.

For  systems using  bounded integer  arithmetic  (default is  unbounded,
see section 4.26.2.1  for details), integer operations that  would cause
overflow automatically convert to floating point arithmetic.


- _+_E_x_p_r
    _R_e_s_u_l_t =-_E_x_p_r


_+_E_x_p_r_1 + _+_E_x_p_r_2
    _R_e_s_u_l_t =_E_x_p_r_1 +_E_x_p_r_2


_+_E_x_p_r_1 - _+_E_x_p_r_2
    _R_e_s_u_l_t =_E_x_p_r_1 -_E_x_p_r_2


_+_E_x_p_r_1 * _+_E_x_p_r_2
    _R_e_s_u_l_t =_E_x_p_r_1_*Expr2


_+_E_x_p_r_1 / _+_E_x_p_r_2
    _R_e_s_u_l_t = _E_x_p_r_1=_E_x_p_r_2 The the flag  iso is true,  both arguments are
    converted  to float  and the  return value is  a float.    Otherwise
    (default),  if both arguments are integers the operation  returns an
    integer if the division  is exact.  If at least one of the arguments
    is  rational  and  the  other argument  is  integer,  the  operation
    returns  a rational number.  In all other cases the return  value is
    a float.  See also ///2 and rdiv/2.


_+_I_n_t_E_x_p_r_1 mmoodd _+_I_n_t_E_x_p_r_2
    Modulo:   _R_e_s_u_l_t = _I_n_t_E_x_p_r_1 - (_I_n_t_E_x_p_r_1 // _I_n_t_E_x_p_r_2)  * _I_n_t_E_x_p_r_2 The
    function  mod/2  is  implemented using  the  C  % operator.     It's
    behaviour with negative values is illustrated in the table below.

                              2  =   17  mod   5
                              2  =   17  mod  -5
                             -2  =  -17  mod   5
                             -2  =  -17  mod  -5


_+_I_n_t_E_x_p_r_1 rreemm _+_I_n_t_E_x_p_r_2
    Remainder of division:  _R_e_s_u_l_t is float_fractional_part(_I_n_t_E_x_p_r_1/_I_n_t_E_x_p_r_2)


_+_I_n_t_E_x_p_r_1 // _+_I_n_t_E_x_p_r_2
    Integer division:  _R_e_s_u_l_t is truncate(_E_x_p_r_1/_E_x_p_r_2)


_+_R_a_t_E_x_p_r rrddiivv _+_R_a_t_E_x_p_r
    Rational  number  division.    This function  is only  available  if
    SWI-Prolog  has been  compiled with  rational number support.    See
    section 4.26.2.2 for details.


aabbss((_+_E_x_p_r))
    Evaluate _E_x_p_r and return the absolute value of it.


ssiiggnn((_+_E_x_p_r))
    Evaluate to -1 if _E_x_p_r <0, 1 if _E_x_p_r >0 and 0 if _E_x_p_r =0.


mmaaxx((_+_E_x_p_r_1_, _+_E_x_p_r_2))
    Evaluates  to the largest of both  _E_x_p_r_1 and _E_x_p_r_2.  Both  arguments
    are  compared after  converting to  the  same type,  but the  return
    value  is in the original type.   For example, max(2.5,  3) compares
    the  two values after converting  to float, but returns the  integer
    3.


mmiinn((_+_E_x_p_r_1_, _+_E_x_p_r_2))
    Evaluates to the smallest  of both _E_x_p_r_1 and _E_x_p_r_2.  See max/2 for a
    description of type-handling.


.((_+_I_n_t_, _[_]))
    A  list of one element evaluates to  the element.  This  implies "a"
    evaluates  to the  character  code of  the letter  `a' (97).    This
    option  is available for  compatibility only.   It will not work  if
    `style_check(+string)'  is active as  "a" will  then be  transformed
    into a string object.   The recommended way to specify the character
    code of the letter `a' is 0'a.


rraannddoomm((_+_I_n_t_E_x_p_r))
    Evaluates to  a random integer _i for which 0=< i< _I_n_t_E_x_p_r.  The seed
    of  this random  generator is  determined by the  system clock  when
    SWI-Prolog was started.


rroouunndd((_+_E_x_p_r))
    Evaluates _E_x_p_r and rounds the result to the nearest integer.


iinntteeggeerr((_+_E_x_p_r))
    Same as round/1 (backward compatibility).


ffllooaatt((_+_E_x_p_r))
    Translate  the result to a floating point number.   Normally, Prolog
    will  use integers  whenever possible.    When used  around the  2nd
    argument  of is/2, the result will  be returned as a floating  point
    number.  In other contexts, the operation has no effect.


rraattiioonnaall((_+_E_x_p_r))
    Convert  the _E_x_p_r to  a rational  number or integer.   The  function
    returns  the input on integers and  rational numbers.  For  floating
    point  numbers, the returned rational number _e_x_a_c_t_l_y  represents the
    float.    As  floats cannot  exactly represent  all decimal  numbers
    the  results may  be surprising.    In the  examples below,  doubles
    can  represent 0.25 and  the result is as  expected, in contrast  to
    the  result of rational(_0_._1).   The function rationalize/1  remedies
    this.   See section 4.26.2.2 for more information on rational number
    support.

    ?- A is rational(0.25).

    A is 1 rdiv 4
    ?- A is rational(0.1).
    A = 3602879701896397 rdiv 36028797018963968


rraattiioonnaalliizzee((_+_E_x_p_r))
    Convert  the _E_x_p_r to  a rational  number or integer.   The  function
    is  similar to rational/1,  but the result  is only accurate  within
    the rounding error  of floating point numbers, generally producing a
    much smaller denominator.

    ?- A is rationalize(0.25).

    A = 1 rdiv 4
    ?- A is rationalize(0.1).

    A = 1 rdiv 10


ffllooaatt__ffrraaccttiioonnaall__ppaarrtt((_+_E_x_p_r))
    Fractional   part  of  a  floating-point   number.     Negative   if
    _E_x_p_r   is  negative,  rational   if  _E_x_p_r  is  rational  and  0   if
    _E_x_p_r   is  integer.     The  following  relation  is   always  true:
    Xisfloatfractionalpart(X)+ floatintegerpart(X).


ffllooaatt__iinntteeggeerr__ppaarrtt((_+_E_x_p_r))
    Integer  part  of  floating-point  number.    Negative  if  _E_x_p_r  is
    negative, _E_x_p_r if _E_x_p_r is integer.


ttrruunnccaattee((_+_E_x_p_r))
    Truncate  _E_x_p_r to  an integer.    If _E_x_p_r>= 0  this is  the same  as
    floor(_E_x_p_r).   For  _E_x_p_r< 0 this is  the same as  ceil(_E_x_p_r).  E.i.
    truncate rounds towards zero.


fflloooorr((_+_E_x_p_r))
    Evaluates  _E_x_p_r and returns the largest integer smaller or  equal to
    the result of the evaluation.


cceeiilliinngg((_+_E_x_p_r))
    Evaluates  _E_x_p_r and returns the smallest integer larger or  equal to
    the result of the evaluation.


cceeiill((_+_E_x_p_r))
    Same as ceiling/1 (backward compatibility).


_+_I_n_t_E_x_p_r >> _+_I_n_t_E_x_p_r
    Bitwise  shift  _I_n_t_E_x_p_r_1  by  _I_n_t_E_x_p_r_2  bits to  the  right.     The
    operation   performs  _a_r_i_t_h_m_e_t_i_c  _s_h_i_f_t,  which  implies   that  the
    inserted  most  significant bits  are copies  of  the original  most
    significant bit.


_+_I_n_t_E_x_p_r << _+_I_n_t_E_x_p_r
    Bitwise shift _I_n_t_E_x_p_r_1 by _I_n_t_E_x_p_r_2 bits to the left.


_+_I_n_t_E_x_p_r \/ _+_I_n_t_E_x_p_r
    Bitwise `or' _I_n_t_E_x_p_r_1 and _I_n_t_E_x_p_r_2.


_+_I_n_t_E_x_p_r /\ _+_I_n_t_E_x_p_r
    Bitwise `and' _I_n_t_E_x_p_r_1 and _I_n_t_E_x_p_r_2.


_+_I_n_t_E_x_p_r xxoorr _+_I_n_t_E_x_p_r
    Bitwise `exclusive or' _I_n_t_E_x_p_r_1 and _I_n_t_E_x_p_r_2.


\ _+_I_n_t_E_x_p_r
    Bitwise  negation.   The returned value  is the one's complement  of
    _I_n_t_E_x_p_r.


ssqqrrtt((_+_E_x_p_r))
    _R_e_s_u_l_t =square root of _E_x_p_r


ssiinn((_+_E_x_p_r))
    _R_e_s_u_l_t =sine of _E_x_p_r.  _E_x_p_r is the angle in radians.


ccooss((_+_E_x_p_r))
    _R_e_s_u_l_t =cosine of _E_x_p_r.  _E_x_p_r is the angle in radians.


ttaann((_+_E_x_p_r))
    _R_e_s_u_l_t =tangus of _E_x_p_r.  _E_x_p_r is the angle in radians.


aassiinn((_+_E_x_p_r))
    _R_e_s_u_l_t =inverse sine of _E_x_p_r.  _R_e_s_u_l_t is the angle in radians.


aaccooss((_+_E_x_p_r))
    _R_e_s_u_l_t =inverse cosine of _E_x_p_r.  _R_e_s_u_l_t is the angle in radians.


aattaann((_+_E_x_p_r))
    _R_e_s_u_l_t =inverse tangus of _E_x_p_r.  _R_e_s_u_l_t is the angle in radians.


aattaann((_+_Y_E_x_p_r_, _+_X_E_x_p_r))
    _R_e_s_u_l_t = inverse tangus of _Y_E_x_p_r / _X_E_x_p_r.   _R_e_s_u_l_t is  the angle in
    radians.    The return  value is  in the  range [-pi:::pi].   Used  to
    convert between rectangular and polar coordinate system.


lloogg((_+_E_x_p_r))
    _R_e_s_u_l_t =natural logarithm of _E_x_p_r


lloogg1100((_+_E_x_p_r))
    _R_e_s_u_l_t =10 base logarithm of _E_x_p_r


eexxpp((_+_E_x_p_r))
    _R_e_s_u_l_t =e to the power _E_x_p_r


_+_E_x_p_r_1 ** _+_E_x_p_r_2
    _R_e_s_u_l_t = _E_x_p_r_1 to the power _E_x_p_r_2.    With  unbounded  integers and
    integer  values  for _E_x_p_r_1  and a  non-negative integer  _E_x_p_r_2,  the
    result is always integer.


_+_E_x_p_r_1 ^ _+_E_x_p_r_2
    Same as **/2.  (backward compatibility).


ppii
    Evaluates to the mathematical constant pi (3.141593).


ee
    Evaluates to the mathematical constant e (2.718282).


ccppuuttiimmee
    Evaluates  to a floating  point number expressing  the cpu time  (in
    seconds)  used by Prolog  up till  now.   See also statistics/2  and
    time/1.

BBiittvveeccttoorr ffuunnccttiioonnss

The  functions below  are  not  covered by  the  standard.    The  msb/1
function is compatible  to hProlog.   The others are private  extensions
that improve handling of ---unbounded--- integers as bit-vectors.


mmssbb((_+_I_n_t_E_x_p_r))
    Return  the largest integer N  such that (IntExpr >> N) /\ 1 =:= 1.
    This  is the (zero-origin)  index of the  most significant 1 bit  in
    the  value of _I_n_t_E_x_p_r,  which must evaluate  to a positive  integer.
    Errors for 0, negative integers, and non-integers.


llssbb((_+_I_n_t_E_x_p_r))
    Return  the smallest integer N such that (IntExpr >> N) /\ 1 =:= 1.
    This  is the (zero-origin) index of  the least significant 1 bit  in
    the  value of IntExpr,  which must evaluate  to a positive  integer.
    Errors for 0, negative integers, and non-integers.


ppooppccoouunntt((_+_I_n_t_E_x_p_r))
    Return  the  number  of  1s  in the  binary  representation  of  the
    non-negative integer _I_n_t_E_x_p_r.


44..2277 AAddddiinngg AArriitthhmmeettiicc FFuunnccttiioonnss

Prolog predicates  can be given  the role of arithmetic  function.   The
last  argument is  used  to  return the  result,  the  arguments  before
the last  are the  inputs.   Arithmetic  functions are  added using  the
predicate arithmetic_function/1, which takes  the head as its  argument.
Arithmetic  functions  are module  sensitive,  that  is  they  are  only
visible from the module  in which the function is defined  and declared.
Global  arithmetic  functions  should be  defined  and  registered  from
module user.   Global definitions can  be overruled locally in  modules.
The built-in functions described above can be redefined as well.


aarriitthhmmeettiicc__ffuunnccttiioonn((_+_H_e_a_d))
    Register  a Prolog  predicate as an  arithmetic function (see  is/2,
    >/2 , etc.).   The Prolog  predicate should  have one more  argument
    than  specified by _H_e_a_d, which it either a term _N_a_m_e_/_A_r_i_t_y,  an atom
    or  a complex term.   This last argument  is an unbound variable  at
    call  time and  should  be instantiated  to an  integer or  floating
    point  number.    The other  arguments  are the  parameters.    This
    predicate  is  module  sensitive  and will  declare  the  arithmetic
    function  only for the context  module, unless declared from  module
    user.  Example:

    1 ?- [user].
    :- arithmetic_function(mean/2).

    mean(A, B, C) :-
            C is (A+B)/2.
    user compiled, 0.07 sec, 440 bytes.

    Yes
    2 ?- A is mean(4, 5).

    A = 4.500000


ccuurrrreenntt__aarriitthhmmeettiicc__ffuunnccttiioonn((_?_H_e_a_d))
    Successively unifies all  arithmetic functions that are visible from
    the context module with _H_e_a_d.


44..2288 BBuuiilltt--iinn lliisstt ooppeerraattiioonnss

Most  list operations  are defined  in the  library  lists described  in
section 11.1.  Some that are implemented with  more low-level primitives
are built-in and described here.


iiss__lliisstt((_+_T_e_r_m))
    True if _T_e_r_m is  bound to the empty list ([]) or a term with functor
    `.'  and arity 2 and the second argument is a list.   This predicate
    acts as if defined by the following definition:

    is_list(X) :-
            var(X), !,
            fail.
    is_list([]).
    is_list([_|T]) :-
            is_list(T).


mmeemmbbeerrcchhkk((_?_E_l_e_m_, _+_L_i_s_t))
    Equivalent to member/2, but leaves no choice point.


lleennggtthh((_?_L_i_s_t_, _?_I_n_t))
    True if _I_n_t represents  the number of elements of list _L_i_s_t.  Can be
    used to create a list holding only variables.


ssoorrtt((_+_L_i_s_t_, _-_S_o_r_t_e_d))
    True  if _S_o_r_t_e_d  can be  unified with  a list  holding the  elements
    of  _L_i_s_t, sorted to the standard  order of terms (see section  4.6).
    Duplicates  are removed.  The implementation is in C,  using _n_a_t_u_r_a_l
    _m_e_r_g_e _s_o_r_t


mmssoorrtt((_+_L_i_s_t_, _-_S_o_r_t_e_d))
    Equivalent to sort/2, but does not remove duplicates.


kkeeyyssoorrtt((_+_L_i_s_t_, _-_S_o_r_t_e_d))
    List  is a proper list whose elements are Key-Value, that  is, terms
    whose  principal  functor is  (-)/2,  whose  first argument  is  the
    sorting  key, and whose second argument is the satellite data  to be
    carried  along with  the key.   keysort/2  sorts _L_i_s_t like  msort/2,
    but  only compares  the  keys.   It  is used  to sort  terms not  on
    standard  order, but  on any criterion  that can  be expressed on  a
    multi-dimensional scale.   Sorting on more than one criterion can be
    done  using terms as keys,  putting the first criterion as  argument
    1,  the second as argument 2,  etc.  The order of  multiple elements
    that have the same  _K_e_y is not changed.  The implementation is in C,
    using _n_a_t_u_r_a_l _m_e_r_g_e _s_o_r_t.


pprreeddssoorrtt((_+_P_r_e_d_, _+_L_i_s_t_, _-_S_o_r_t_e_d))
    Sorts  similar to sort/2, but determines  the order of two terms  by
    calling  _P_r_e_d(-_D_e_l_t_a, +_E_1, +_E_2).   This call  must unify _D_e_l_t_a  with
    one  of <, >  or =.   If built-in predicate  compare/3 is used,  the
    result is the same as sort/2.  See also keysort/2.


mmeerrggee((_+_L_i_s_t_1_, _+_L_i_s_t_2_, _-_L_i_s_t_3))
    _L_i_s_t_1  and _L_i_s_t_2 are  lists, sorted to  the standard order of  terms
    (see  section 4.6).    _L_i_s_t_3 will be  unified with  an ordered  list
    holding  both the elements of _L_i_s_t_1  and _L_i_s_t_2.  Duplicates are  nnoott
    removed.


mmeerrggee__sseett((_+_S_e_t_1_, _+_S_e_t_2_, _-_S_e_t_3))
    _S_e_t_1  and _S_e_t_2 are lists without duplicates, sorted to  the standard
    order  of terms.    _S_e_t_3  is unified  with an  ordered list  without
    duplicates holding the union of the elements of _S_e_t_1 and _S_e_t_2.


44..2299 FFiinnddiinngg aallll SSoolluuttiioonnss ttoo aa GGooaall


ffiinnddaallll((_+_T_e_m_p_l_a_t_e_, _+_G_o_a_l_, _-_B_a_g))
    Creates  a list of the instantiations _T_e_m_p_l_a_t_e gets  successively on
    backtracking  over _G_o_a_l and unifies the  result with _B_a_g.   Succeeds
    with  an  empty  list  if _G_o_a_l  has  no  solutions.    findall/3  is
    equivalent  to  bagof/3  with  all free  variables  bound  with  the
    existential  operator (^), except that  bagof/3 fails when goal  has
    no solutions.


bbaaggooff((_+_T_e_m_p_l_a_t_e_, _+_G_o_a_l_, _-_B_a_g))
    Unify  _B_a_g  with the  alternatives of  _T_e_m_p_l_a_t_e,  if _G_o_a_l  has  free
    variables   besides  the  one  sharing  with  _T_e_m_p_l_a_t_e   bagof  will
    backtrack  over the alternatives  of these free variables,  unifying
    _B_a_g with the  corresponding alternatives of _T_e_m_p_l_a_t_e.  The construct
    +Var^Goal  tells bagof not to  bind _V_a_r in _G_o_a_l.   bagof/3 fails  if
    _G_o_a_l has no solutions.

    The  example below  illustrates bagof/3  and the  ^ operator.    The
    variable bindings are printed together on one line to save paper.

    2 ?- listing(foo).

    foo(a, b, c).
    foo(a, b, d).
    foo(b, c, e).
    foo(b, c, f).
    foo(c, c, g).

    Yes
    3 ?- bagof(C, foo(A, B, C), Cs).

    A = a, B = b, C = G308, Cs = [c, d] ;
    A = b, B = c, C = G308, Cs = [e, f] ;
    A = c, B = c, C = G308, Cs = [g] ;

    No
    4 ?- bagof(C, A^foo(A, B, C), Cs).

    A = G324, B = b, C = G326, Cs = [c, d] ;
    A = G324, B = c, C = G326, Cs = [e, f, g] ;

    No
    5 ?-


sseettooff((_+_T_e_m_p_l_a_t_e_, _+_G_o_a_l_, _-_S_e_t))
    Equivalent  to bagof/3, but sorts the  result using sort/2 to get  a
    sorted list of alternatives without duplicates.


44..3300 IInnvvookkiinngg PPrreeddiiccaatteess oonn aallll MMeemmbbeerrss ooff aa LLiisstt

All the predicates in this section call a predicate on  all members of a
list or until the  predicate called fails.  The predicate is  called via
call/[2..], which  implies common arguments can be  put in front of  the
arguments obtained from the list(s).  For example:

?- maplist(plus(1), [0, 1, 2], X).

X = [1, 2, 3]

we will phrase this as ``_P_r_e_d_i_c_a_t_e is applied on ...''


mmaapplliisstt((_+_P_r_e_d_, _+_L_i_s_t))
    _P_r_e_d  is applied successively on each element of _L_i_s_t until  the end
    of the list or _P_r_e_d fails.  In the latter case the maplist/2 fails.


mmaapplliisstt((_+_P_r_e_d_, _?_L_i_s_t_1_, _?_L_i_s_t_2))
    Apply  _P_r_e_d  on all  successive  pairs of  elements from  _L_i_s_t_1  and
    _L_i_s_t_2.    Fails if  _P_r_e_d can  not be  applied to a  pair.   See  the
    example above.


mmaapplliisstt((_+_P_r_e_d_, _?_L_i_s_t_1_, _?_L_i_s_t_2_, _?_L_i_s_t_3))
    Apply  _P_r_e_d on all successive triples of elements from  _L_i_s_t_1, _L_i_s_t_2
    and  _L_i_s_t_3.  Fails if _P_r_e_d can not be applied to a triple.   See the
    example above.


ssuubblliisstt((_+_P_r_e_d_, _+_L_i_s_t_1_, _?_L_i_s_t_2))
    Unify  _L_i_s_t_2 with  a list  of all elements  of _L_i_s_t_1  to which  _P_r_e_d
    applies.


44..3311 FFoorraallll


ffoorraallll((_+_C_o_n_d_, _+_A_c_t_i_o_n))
    For  all alternative bindings  of _C_o_n_d  _A_c_t_i_o_n can be  proven.   The
    example  verifies that all arithmetic  statements in the list _L  are
    correct.  It does not say which is wrong if one proves wrong.

    ?- forall(member(Result = Formula, [2 = 1 + 1, 4 = 2 * 2]),
                     Result =:= Formula).


44..3322 FFoorrmmaatttteedd WWrriittee

The  current  version   of  SWI-Prolog  provides  two  formatted   write
predicates.    The  first  is writef/[1,2],  which  is  compatible  with
Edinburgh C-Prolog.   The  second is  format/[1,2], which is  compatible
with Quintus  Prolog.   We hope  the Prolog  community will once  define
a  standard formatted  write predicate.    If you  want performance  use
format/[1,2] as this predicate is defined in  C. Otherwise compatibility
reasons might tell you which predicate to use.


44..3322..11 WWrriitteeff


wwrriitteellnn((_+_T_e_r_m))
    Equivalent to write(Term), nl.


wwrriitteeff((_+_A_t_o_m))
    Equivalent to writef(Atom, []).


wwrriitteeff((_+_F_o_r_m_a_t_, _+_A_r_g_u_m_e_n_t_s))
    Formatted  write.      _F_o_r_m_a_t  is  an  atom  whose  characters  will
    be   printed.     _F_o_r_m_a_t  may  contain  certain   special  character
    sequences   which  specify   certain  formatting  and   substitution
    actions.    _A_r_g_u_m_e_n_t_s then  provides all  the terms  required to  be
    output.

    Escape sequences to generate a single special character:

             __________________________________________________
             | \n   |Output  a  newline  character  (see  also |
             |      |nl/[0,1])                                 |
             | \l   |Output a line separator (same as \n)      |

             | \r   |Output   a   carriage-return    character |
             |      |(ASCII 13)                                |
             | \t   |Output the ASCII character TAB (9)        |
             | \\   |The character \ is output                 |
             | \%   |The character % is output                 |
             | \nnn |where <_n_n_n>  is an  integer  (1-3 digits) |
             |      |the character  with character  code <_n_n_n> |

             |______|is_output_(NB_:_<_n_n_n>_is_read_as_ddeecciimmaall)_|

    Note  that  \l,   \nnn  and  \\  are  interpreted  differently  when
    character-escapes are in effect.  See section 2.15.1.2.

    Escape  sequences to include  arguments from _A_r_g_u_m_e_n_t_s.   Each  time
    a  %  escape sequence  is found  in _F_o_r_m_a_t  the  next argument  from
    _A_r_g_u_m_e_n_t_s is formatted according to the specification.

              _________________________________________________%t

              | %w  print/1 the next item (mnemonic:  term)   |    |

              | %q  |write/1the next item                     |

              |     |writeq/1the next item                    |
              | %d  |Write the term,  ignoring operators.  See|
              |     |also  write_term/2.      Mnemonic:    old|
              | %p  |Edinburgh display/1.                     |

              |     |print/1the next item (identical to %t)   |
              | %n  |Put the  next item as  a character (i.e.,|

              |     |it is a character code)                  |
              | %r  |Write the  next item  N times where  N is|
              |     |the second item (an integer)             |
              | %s  |Write the  next item  as a String  (so it|
              |     |must be a list of characters)            |
              | %f  |Perform a ttyflush/0 (no items used)     |
              | %Nc |Write  the   next  item  Centered  in  N |

              |     |columns.                                 |
              | %Nl |Write the next  item Left justified in N |
              |     |columns.                                 |
              | %Nr |Write the next item Right justified in N |
              |     |columns.   N is a decimal number with  at|
              |     |least  one digit.   The  item must  be an|
              |_____|atom,_integer,_float_or_string.__________|_


sswwrriitteeff((_-_S_t_r_i_n_g_, _+_F_o_r_m_a_t_, _+_A_r_g_u_m_e_n_t_s))
    Equivalent to writef/2,  but ``writes'' the result on _S_t_r_i_n_g instead
    of the current output stream.  Example:

    ?- swritef(S, '%15L%w', ['Hello', 'World']).

    S = "Hello          World"


sswwrriitteeff((_-_S_t_r_i_n_g_, _+_F_o_r_m_a_t))
    Equivalent to swritef(String, Format, []).


44..3322..22 FFoorrmmaatt


ffoorrmmaatt((_+_F_o_r_m_a_t))
    Defined as `format(Format) :- format(Format, []).'


ffoorrmmaatt((_+_F_o_r_m_a_t_, _+_A_r_g_u_m_e_n_t_s))
    _F_o_r_m_a_t   is  an  atom,  list   of  character  codes,  or   a  Prolog
    string.   _A_r_g_u_m_e_n_t_s  provides the arguments  required by the  format
    specification.   If  only one argument is  required and this is  not
    a  list of character codes the argument  need not be put in  a list.
    Otherwise the arguments are put in a list.

    Special sequences start  with the tilde (~), followed by an optional
    numeric  argument, followed by a character describing the  action to
    be  undertaken.  A numeric argument is either a sequence  of digits,
    representing  a  positive decimal  number, a  sequence `<_c_h_a_r_a_c_t_e_r>,
    representing the character  code value of the character (only useful
    for  ~t) or a asterisk  (*), in when  the numeric argument is  taken
    from  the next  argument of  the argument  list, which  should be  a
    positive integer.

    Numeric  conversion (d, D,  e, E, f, g  and G) accept an  arithmetic
    expression  as argument.    This  is introduced  to handle  rational
    numbers  transparently (see  section 4.26.2.2.   The floating  point
    conversions  allow  for unlimited  precision for  printing  rational
    numbers in decimal form.

      ~  Output the tilde itself.

      a  Output the next argument, which  must be an atom.   This option
         is equivalent to  ww, except for  that it requires the  argument
         to be an atom.

      c  Interpret the next argument as an character code and  add it to
         the output.   This argument should  be an integer in the  range
         [0, ..., 255] (including 0 and 255).

      d  Output  next argument  as  a decimal  number.    It  should  be
         an integer.    If  a numeric  argument is  specified  a dot  is
         inserted _a_r_g_u_m_e_n_t  positions from the  right (useful for  doing
         fixed point arithmetic with integers, such as  handling amounts
         of money).

      D  Same as dd, but  makes large values easier to read  by inserting
         a comma every three digits left to the dot or right.

      e  Output next argument as a floating point  number in exponential
         notation.    The  numeric  argument  specifies  the  precision.
         Default is  6 digits.   Exact representation  depends on the  C
         library function printf().   This function is invoked  with the
         format %.<_p_r_e_c_i_s_i_o_n>e.

      E  Equivalent  to ee,  but  outputs a  capital  E to  indicate  the
         exponent.

      f  Floating point  in non-exponential  notation.    See C  library
         function printf().

      g  Floating point in ee or ff notation, whichever is shorter.

      G  Floating point in EE or ff notation, whichever is shorter.

      i  Ignore  next argument  of  the  argument  list.    Produces  no
         output.

      k  Give the next argument to (write_canonical/1).

      n  Output a newline character.

      N  Only output  a newline  if the  last character  output on  this
         stream was not a newline.  Not properly implemented yet.

      p  Give the next argument to print/1.

      q  Give the next argument to writeq/1.

      r  Print integer  in radix the  numeric argument  notation.   Thus
         ~16r prints its argument  hexadecimal.  The argument  should be
         in the range [2; :::;36].   Lower case letters are used for  digits
         above 9.

      R  Same as rr, but uses upper case letters for digits above 9.

      s  Output text  from a list  of character codes  or a string  (see
         string/1 and section 4.23) from the next argument.

      @  Interpret the next argument as  a goal and execute it.   Output
         written to the current_output stream is inserted at this place.
         Goal is  called in the  module calling format/3.   This  option
         is not  present  in the  original  definition by  Quintus,  but
         supported by some other Prolog systems.

      t  All remaining space between 2 tab stops  is distributed equally
         over ~t  statements  between the  tab  stops.   This  space  is
         padded with  spaces by  default.   If an  argument is  supplied
         this is taken  to be the character  code of the character  used
         for padding.  This  can be used to do left or  right alignment,
         centering, distributing, etc.   See also  ~| and ~+ to set  tab
         stops.  A tab stop is assumed at the start of each line.

      |  Set a  tab stop on  the current position.    If an argument  is
         supplied set  a  tab stop  on the  position  of that  argument.
         This  will  cause  all  ~t's  to  be  distributed  between  the
         previous and this tab stop.

      +  Set a tab stop relative  to the current position.   Further the
         same as ~|.

      w  Give the next argument to write/1.

      W  Give the  next two argument  to write_term/2.   This option  is
         SWI-Prolog specific.

    Example:

    simple_statistics :-
        <obtain statistics>         % left to the user
        format('~tStatistics~t~72|~n~n'),
        format('Runtime: ~`.t ~2f~34|  Inferences: ~`.t ~D~72|~n',
                                                [RunT, Inf]),
        ....

    Will output

                                 Statistics

    Runtime: .................. 3.45  Inferences: .......... 60,345


ffoorrmmaatt((_+_O_u_t_p_u_t_, _+_F_o_r_m_a_t_, _+_A_r_g_u_m_e_n_t_s))
    As  format/2, but write  the output on  the given _O_u_t_p_u_t.   The  de-
    facto  standard only allows _O_u_t_p_u_t to  be a stream.  The  SWI-Prolog
    implementation  allows  all  valid  arguments for  with_output_to/2.
    For example:

    ?- format(atom(A), '~D', [1000000]).
    A = '1,000,000'


44..3322..33 PPrrooggrraammmmiinngg FFoorrmmaatt


ffoorrmmaatt__pprreeddiiccaattee((_+_C_h_a_r_, _+_H_e_a_d))
    If  a sequence ~c (tilde, followed by some character) is  found, the
    format  derivatives will first check whether the user has  defined a
    predicate  to handle the format.   If  not, the built in  formatting
    rules  described above are  used.   _C_h_a_r is  either an ascii  value,
    or  a one character atom,  specifying the letter to be  (re)defined.
    _H_e_a_d  is a  term, whose  name and  arity are used  to determine  the
    predicate  to call  for  the redefined  formatting character.    The
    first  argument to  the  predicate is  the numeric  argument of  the
    format  command, or the  atom default if  no argument is  specified.
    The  remaining arguments  are filled from  the argument  list.   The
    example  below redefines ~n to produce _A_r_g times return  followed by
    linefeed (so a (Grr.)  DOS machine is happy with the output).

    :- format_predicate(n, dos_newline(_Arg)).

    dos_newline(default) :- !,
            dos_newline(1).
    dos_newline(N) :-
            (   N > 0
            ->  write('\r\n'),
                N2 is N - 1,
                dos_newline(N2)
            ;   true
            ).


ccuurrrreenntt__ffoorrmmaatt__pprreeddiiccaattee((_?_C_o_d_e_, _?_:_H_e_a_d))
    Enumerates  all  user-defined  format  predicates.     _C_o_d_e  is  the
    character  code  of the  format character.    _H_e_a_d  is unified  with
    a  term with  the same  name and  arity as the  predicate.   If  the
    predicate  does not reside  in module user,  _H_e_a_d is qualified  with
    the definition module of the predicate.


44..3333 TTeerrmmiinnaall CCoonnttrrooll

The  following  predicates  form  a  simple  access   mechanism  to  the
Unix  termcap library  to provide  terminal independent  I/O for  screen
terminals.  These  predicates are only available on Unix machines.   The
SWI-Prolog Windows consoles accepts the ANSI escape sequences.


ttttyy__ggeett__ccaappaabbiilliittyy((_+_N_a_m_e_, _+_T_y_p_e_, _-_R_e_s_u_l_t))
    Get  the  capability named  _N_a_m_e  from the  termcap  library.    See
    termcap(5)  for the capability  names.   _T_y_p_e specifies the type  of
    the  expected result, and is one of string, number or bool.   String
    results  are returned as  an atom, number  result as an integer  and
    bool  results as the atom on or off.   If an option cannot  be found
    this predicate fails  silently.  The results are only computed once.
    Successive queries on the same capability are fast.


ttttyy__ggoottoo((_+_X_, _+_Y))
    Goto  position  (_X, _Y) on  the screen.    Note  that the  predicates
    line_count/2  and  line_position/2 will  not  have  a  well  defined
    behaviour while using this predicate.


ttttyy__ppuutt((_+_A_t_o_m_, _+_L_i_n_e_s))
    Put  an  atom  via the  termcap  library  function tputs().     This
    function  decodes padding  information  in the  strings returned  by
    tty_get_capability/3 and  should be  used to  output these  strings.
    _L_i_n_e_s is the number  of lines affected by the operation, or 1 if not
    applicable (as in almost all cases).


sseett__ttttyy((_-_O_l_d_S_t_r_e_a_m_, _+_N_e_w_S_t_r_e_a_m))
    Set  the  output  stream,  used by  tty_put/2  and tty_goto/2  to  a
    specific stream.  Default is user_output.


ttttyy__ssiizzee((_-_R_o_w_s_, _-_C_o_l_u_m_n_s))
    Determine the size of the terminal.  Platforms:

    UUnniixx  If  the  system  provides _i_o_c_t_l  calls  for  this,  these  are
         used and tty_size/2 properly  reflects the actual size after  a
         user resize  of the window.    As a fallback,  the system  uses
         tty_get_capability/3 using li  and co  capabilities.   In  this
         case the reported size reflects the size at the  first call and
         is not updated after a user-initiated resize of the terminal.

    WWiinnddoowwss  Getting  the   size  of  the   terminal  is  provided   for
         plwin.exe.   The  requested  value reflects  the current  size.
         For the multi-threaded  version the console that is  associated
         with the user_input stream is used.


44..3344 OOppeerraattiinngg SSyysstteemm IInntteerraaccttiioonn


sshheellll((_+_C_o_m_m_a_n_d_, _-_S_t_a_t_u_s))
    Execute  _C_o_m_m_a_n_d on the operating system.   _C_o_m_m_a_n_d is given to  the
    Bourne  shell (/bin/sh).  _S_t_a_t_u_s is unified with the exit  status of
    the command.

    On  _W_i_n_3_2  systems,  shell/[1,2]  executes  the  command  using  the
    CreateProcess()  API and  waits for the  command to  terminate.   If
    the  command  ends with  a &  sign,  the command  is  handed to  the
    WinExec()  API, which does not wait  for the new task to  terminate.
    See  also  win_exec/2  and  win_shell/2.     Please  note  that  the
    CreateProcess()  API does nnoott imply the Windows  command interpreter
    (command.exe  on  Windows  95/98  and  cmd.exe  on  Windows-NT)  and
    therefore  commands  built-in to  the command-interpreter  can  only
    be   activated  using  the  command  interpreter.      For  example:
    'command.exe /C copy file1.txt file2.txt'


sshheellll((_+_C_o_m_m_a_n_d))
    Equivalent to `shell(Command, 0)'.


sshheellll
    Start  an  interactive  Unix  shell.     Default  is  /bin/sh,   the
    environment  variable SHELL overrides this  default.  Not  available
    for Win32 platforms.


wwiinn__eexxeecc((_+_C_o_m_m_a_n_d_, _+_S_h_o_w))
    Win32  systems only.    Spawns a  Windows task  without waiting  for
    its  completion.   _S_h_o_w is one of  the Win32 SW_* constants  written
    in  lowercase  without the  SW_*:   hide maximize  minimize  restore
    show showdefault  showmaximized showminimized showminnoactive showna
    shownoactive  shownormal.   In  addition,  iconic is  a synonym  for
    minimize and normal for shownormal


wwiinn__sshheellll((_+_O_p_e_r_a_t_i_o_n_, _+_F_i_l_e_, _+_S_h_o_w))
    Win32  systems only.    Opens the  document _F_i_l_e  using the  windows
    shell-rules  for doing  so.   _O_p_e_r_a_t_i_o_n  is  one of  open, print  or
    explore  or  another operation  registered with  the  shell for  the
    given document-type.   On modern systems it is also possible to pass
    a  URL as _F_i_l_e, opening  the URL in Windows  default browser.   This
    call interfaces to  the Win32 API ShellExecute().  The _S_h_o_w argument
    determines  the initial state  of the opened window  (if any).   See
    win_exec/2 for defined values.


wwiinn__sshheellll((_+_O_p_e_r_a_t_i_o_n_, _+_F_i_l_e))
    Same as win_shell(_O_p_e_r_a_t_i_o_n_, _F_i_l_e_, _n_o_r_m_a_l)


wwiinn__rreeggiissttrryy__ggeett__vvaalluuee((_+_K_e_y_, _+_N_a_m_e_, _-_V_a_l_u_e))
    Win32  systems  only.     Fetches the  value  of  a  Win32  registry
    key.   _K_e_y is an atom  formed as a path-name describing  the desired
    registry  key.    _N_a_m_e is  the desired  attribute name  of the  key.
    _V_a_l_u_e  is unified with the  value.  If  the value is of type  DWORD,
    the  value is  returned as an  integer.   If the  value is a  string
    it  is returned as  a Prolog atom.   Other  types are currently  not
    supported.    The default  `root' is HKEY_CURRENT_USER. Other  roots
    can be  specified explicitly as HKEY_CLASSES_ROOT, HKEY_CURRENT_USER,
    HKEY_LOCAL_MACHINE  or HKEY_USERS.  The  example below  fetches  the
    extension  to use for  Prolog files (see  README.TXT on the  Windows
    version):

    ?- win_registry_get_value('HKEY_LOCAL_MACHINE/Software/SWI/Prolog',
                              fileExtension,
                              Ext).

    Ext = pl


ggeetteennvv((_+_N_a_m_e_, _-_V_a_l_u_e))
    Get  environment variable.    Fails  silently if  the variable  does
    not  exist.     Please  note that  environment  variable  names  are
    case-sensitive on Unix systems and case-insensitive on Windows.


sseetteennvv((_+_N_a_m_e_, _+_V_a_l_u_e))
    Set  an environment variable.   _N_a_m_e and _V_a_l_u_e must be  instantiated
    to  atoms or  integers.   The  environment variable  will be  passed
    to  shell/[0-2] and  can be  requested using  getenv/2.   They  also
    influence  expand_file_name/2.    Environment variables  are  shared
    between  threads.  Depending  on the underlying C library,  setenv/2
    and  unsetenv/1 may not be  thread-safe and may cause memory  leaks.
    Only  changing the environment once  and before starting threads  is
    safe in all versions of SWI-Prolog.


uunnsseetteennvv((_+_N_a_m_e))
    Remove  an environment variable from the environment.   Some systems
    lack  the underlying unsetenv() library function.  On  these systems
    unsetenv/1 sets the variable to the empty string.


sseettllooccaallee((_+_C_a_t_e_g_o_r_y_, _-_O_l_d_, _+_N_e_w))
    Set/Query  the  _l_o_c_a_l_e  setting which  tells  the C-library  how  to
    interpret  text-files, write numbers, dates,  etc.  Category is  one
    of  all, collate, ctype, messages,  monetary, numeric or time.   For
    details,  please consult  the C-library locale  documentation.   See
    also  section  2.17.1.    Please  note  that the  locale  is  shared
    between  all  threads and  thread-safe usage  of  setlocale/3 is  in
    general  not  possible.     Do  locale  operations  before  starting
    threads  or thoroughly  study  threading aspects  of locale  support
    in  your  environment  before use  in  multi-threaded  environments.
    Locale  settings  are  used  by format_time/3,  collation_key/2  and
    locale_sort/2.


uunniixx((_+_C_o_m_m_a_n_d))
    This  predicate comes  from  the Quintus  compatibility library  and
    provides  a partial implementation thereof.   It provides access  to
    some operating system  features and unlike the name suggests, is not
    operating system specific.  Defined _C_o_m_m_a_n_d's are below.

    ssyysstteemm((_+_C_o_m_m_a_n_d))
         Equivalent to calling shell/1.  Use for compatibility only.

    sshheellll((_+_C_o_m_m_a_n_d))
         Equivalent to calling shell/1.  Use for compatibility only.

    sshheellll
         Equivalent to calling shell/0.  Use for compatibility only.

    ccdd
         Equivalent to calling working_directory/2 to the expansion (see
         expand_file_name/2) of ~.  For compatibility only.

    ccdd((_+_D_i_r_e_c_t_o_r_y))
         Equivalent   to  calling   working_directory/2.       Use   for
         compatibility only.

    aarrggvv((_-_A_r_g_v))
         Unify _A_r_g_v with the list of command-line  arguments provides to
         this Prolog run.  Please note that  Prolog system-arguments and
         application arguments are separated  by --.  Integer  arguments
         are  passed as  Prolog  integers,  float arguments  and  Prolog
         floating  point  numbers and  all  other  arguments  as  Prolog
         atoms.  New applications should use the prolog-flag argv.   See
         also prolog prolog-flag argv.

         A  stand-alone program  could  use  the following  skeleton  to
         handle command-line arguments.  See also section 2.10.2.4.

         main :-
                 current_prolog_flag(argv, Argv),
                 append(_PrologArgs, [--|AppArgs], Argv), !,
                 main(AppArgs).


44..3344..11 DDeeaalliinngg wwiitthh ttiimmee aanndd ddaattee

Representing  time in  a computer  system  is surprisingly  complicated.
There are a large number of time representations in  use and the correct
choice depends  on factors such as  compactness, resolution and  desired
operations.   Humans tend to  think about time  in hours, days,  months,
years or  centuries.   Physicists  think about time  in seconds.    But,
a  month does  not  have a  defined  number  of seconds.    Even  a  day
does not  have a defined  number of seconds  as sometimes a  leap-second
is introduced  to synchronise properly  with our earth's  rotation.   At
the same  time, resolution demands  range from better then  pico-seconds
to millions  of years.    Finally, civilizations  have a  wide range  of
calendars.   Although there  exist libraries dealing  with most if  this
complexity,  our desire  to keep  Prolog clean  and lean  stops us  from
fully supporting these.

For human-oriented tasks,  time can be broken into years, months,  days,
hours,  minutes, seconds  and a  timezone.   Physicists  prefer to  have
time in an arithmetic type representing seconds or  fraction thereof, so
basic arithmetic  deal with  comparison and  durations.   An  additional
advantage  of the  physicists approach  is that  it  requires much  less
space.   For these  reasons, SWI-Prolog uses an  arithmetic type as  its
prime time representation.

Many C  libraries deal with time  using fixed-point arithmetic,  dealing
with a large  but finite time interval at  constant resolution.  In  our
opinion using  a floating point  number is a more  natural choice as  we
can use a natural unit and the interface does not need  to be changed if
a higher resolution  is required in the future.   Our unit of choice  is
the second as  it is the scientific unit.   e have placed our  origin at
1970-1-1T0:0:0Z for compatibility with the POSIX notion of  time as well
as with older time support provided by SWI-Prolog.

Where  older versions  of  SWI-Prolog  relied on  the  POSIX  conversion
functions, the current implementation uses libtai  to realise conversion
between  time-stamps and  calendar  dates for  a  period of  10  million
years.


44..3344..11..11 TTiimmee aanndd ddaattee ddaattaa--ssttrruuccttuurreess

We use the following time representations

TTiimmeeSSttaammpp
    A  TimeStamp  is a  floating  point number  expression the  time  in
    seconds since the Epoch at 1970-1-1.

ddaattee((_Y_,_M_,_D_,_H_,_M_n_,_S_,_O_f_f_,_T_Z_,_D_S_T))
    We  call this term a  _d_a_t_e_-_t_i_m_e structure.   The first 5 fields  are
    integers  expressing  the year,  month  (1..12), day  (1..31),  hour
    (0..23),  Minute  (0..59).    The _S  field holds  the  seconds as  a
    floating  point number  between 0.0  and 60.0.   _O_f_f  is an  integer
    representing  the offset relative to  UTC in seconds where  positive
    values  are west of Greenwhich.   If converted from local time  (see
    stamp_date_time/3, _T_Z holds the name of the local timezone.   If the
    timezone  is not known _T_Z is  the atom -.   _D_S_T is true if  daylight
    saving  time applies to the  current time, false if daylight  saving
    time is relevant but not  effective and - if unknown or the timezone
    has no daylight saving time.

ddaattee((_Y_,_M_._D))
    Date  using the  same values as  described above.   Extracted  using
    date_time_value/3.

ttiimmee((_H_,_M_n_,_S))
    Time  using the  same values as  described above.   Extracted  using
    date_time_value/3.


44..3344..11..22 TTiimmee aanndd ddaattee pprreeddiiccaatteess


ggeett__ttiimmee((_-_T_i_m_e_S_t_a_m_p))
    Return  the current time as a _T_i_m_e_S_t_a_m_p.  The granularity  is system
    dependent.  See section 4.34.1.1.


ssttaammpp__ddaattee__ttiimmee((_+_T_i_m_e_S_t_a_m_p_, _-_D_a_t_e_T_i_m_e_, _+_T_i_m_e_Z_o_n_e))
    Convert  a _T_i_m_e_Z_o_n_e  to a  _D_a_t_e_T_i_m_e in  the given  time zone.    See
    section 4.34.1.1 for  details on the data-types.  _T_i_m_e_Z_o_n_e describes
    the timezone for the  conversion.  It is one of local to extract the
    local  time, 'UTC' to extract at  UTC time or an integer  describing
    the seconds west of Greenwhich.


ddaattee__ttiimmee__ssttaammpp((_+_D_a_t_e_T_i_m_e_, _-_T_i_m_e_S_t_a_m_p))
    Compute  the timestamp from a date/9  term.  Values for month,  day,
    hour,  minute or second  need not be  normalized.  This  flexibility
    allows  for easy  computation of  the time  at any  given number  of
    these  units from a given timestamp.  Normalization can  be achieved
    following  this call with stamp_date_time/3.  This example  computes
    the date 200 days after 2006-7-14:

    ?- date_time_stamp(date(2006,7,214,0,0,0,0,-,-), Stamp),
       stamp_date_time(Stamp, D, 0),
       date_time_value(date, D, Date).
    Date = date(2007, 1, 30)


ddaattee__ttiimmee__vvaalluuee((_?_K_e_y_, _+_D_a_t_e_T_i_m_e_, _?_V_a_l_u_e))
    Extract values from a date/9 term.  Provided keys are:

       ______________________________________________________________kkeeyyvvaalluuee
       ____________________________________________________________________________________________________________________________yearCalendar year as an integer

        month            Calendar month as an integer 1..12
        day              Calendar day as an integer 1..31
        hour             Clock hour as an integer 0..23

        minute           Clock minute as an integer 0..59
        second           Clock second as a float 0.0..60.0
        utc_offset       Offset to UTC in seconds (positive is west)
        time_zone        Name of timezone; fails if unknown
        daylight_saving  Bool (true) if dst is effective
        date             Term date(_Y_,_M_,_D)
       _time_____________Term_time(_H_,_M_,_S)____________________________


ffoorrmmaatt__ttiimmee((_+_O_u_t_, _+_F_o_r_m_a_t_, _+_S_t_a_m_p_O_r_D_a_t_e_T_i_m_e))
    Modelled  after POSIX strftime(),  using GNU extensions.   _O_u_t is  a
    destination  as specified with with_output_to/2.  _F_o_r_m_a_t is  an atom
    or string with  the following conversions.  Conversions start with a
    tilde (%) character.

      a  The abbreviated weekday name according to the current locale.

      A  The full weekday name according to the current locale.

      b  The abbreviated month name according to the current locale.

      B  The full month name according to the current locale.

      c  The preferred  date  and time  representation for  the  current
         locale.

      C  The century number (year/100) as a 2-digit integer.

      d  The day of the month as a decimal number (range 01 to 31).

      D  Equivalent  to  %m/%d/%y.      (Yecch    for  Americans   only.
         Americans  should note  that  in  other countries  %d/%m/%y  is
         rather common.   This means that in international  context this
         format is ambigu ous and should not be used.)

      e  Like %d,  the  day of  the month  as a  decimal number,  but  a
         leading zero is replaced by a space.

      E  Modifier.  Not implemented.

      F  Equivalent to %Y-%m-%d (the ISO 8601 date format).

      g  Like  %G,  but without  century,  i.e.,  with  a  2-digit  year
         (00-99).

      G  The ISO  8601  year with  century as  a decimal  number.    The
         4-digit year  corresponding to  the ISO week  number (see  %V).
         This has the  same format and value as  %y, except that if  the
         ISO week  number belongs  to the  previous or  next year,  that
         year is used instead.

      V  The ISO 8601:1988 week number of the current year  as a decimal
         number, range  01 to 53,  where week 1 is  the first week  that
         has at least  4 days in  the current year,  and with Monday  as
         the first day of the week.  See also %U and %W.

      h  Equivalent to %b.

      H  The hour as  a decimal number using  a 24-hour clock (range  00
         to 23).

      I  The hour as  a decimal number using  a 12-hour clock (range  01
         to 12).

      j  The day of the year as a decimal number (range 001 to 366).

      k  The hour (24-hour clock)  as a decimal number (range 0  to 23);
         single digits are preceded by a blank.  (See also %H.)

      l  The hour (12-hour clock)  as a decimal number (range 1  to 12);
         single digits are preceded by a blank.  (See also %I.)

      m  The month as a decimal number (range 01 to 12).

      M  The minute as a decimal number (range 00 to 59).

      n  A newline character.

      O  Modifier.  Not implemented.

      p  Either `AM' or `PM'  according to the given time value,  or the
         corresponding strings for the current locale.   Noon is treated
         as `pm' and midnight as `am'.

      P  Like %p  but in  lowercase:  `am'  or `pm'  or a  corresponding
         string for the current locale.

      r  The time in  a.m. or p.m. notation.   In the POSIX locale  this
         is equivalent to `%I:%M:%S %p'.

      R  The time in 24-hour  notation (%H:%M). For a version  including
         the seconds, see %T below.

      s  The number of seconds  since the Epoch, i.e., since  1970-01-01
         00:00:00 UTC.

      S  The second as  a decimal number (range 00  to 60).  (The  range
         is up to 60 to allow for occasional leap seconds.)

      t  A tab character.

      T  The time in 24-hour notation (%H:%M:%S).

      u  The day of the  week as a decimal,  range 1 to 7, Monday  being
         1.  See also %w.

      U  The week number of the current year as a  decimal number, range
         00 to 53,  starting with the first  Sunday as the first day  of
         week 01.  See also %V and %W.

      w  The day of the  week as a decimal,  range 0 to 6, Sunday  being
         0.  See also %u.

      W  The week number of the current year as a  decimal number, range
         00 to 53,  starting with the first  Monday as the first day  of
         week 01.

      x  The  preferred  date  representation  for  the  current  locale
         without the time.

      X  The  preferred  time  representation  for  the  current  locale
         without the date.

      y  The year  as a decimal  number without a  century (range 00  to
         99).

      Y  The year as a decimal number including the century.

      z  The  time-zone  as hour  offset  from  GMT.  Required  to  emit
         RFC822-conformant dates (using "%a, %d %b %Y %H:%M:%S %z").

      Z  The time zone or name or abbreviation.

      +  The date and time in date(1) format.

      %  A literal `%' character.


ppaarrssee__ttiimmee((_+_T_e_x_t_, _-_S_t_a_m_p))
    Parse  a  textual  time  representation,   producing  a  time-stamp.
    Supported formats for _T_e_x_t are:

                 __________________________________________
                 |__NNaammee__________||EExxaammppllee______________________________________________||
                 ||_RFC_1123_|Fri,_08_Dec_2006_15:29:44_GMT_|


44..3344..22 CCoonnttrroolllliinngg tthhee PLWIN.EXE ccoonnssoollee wwiinnddooww

The Windows executable  PLWIN.EXE console has a number of  predicates to
control the  appearance of  the console.    Being totally  non-portable,
we do  not advice using  it for your  own application,  but use XPCE  or
another  portable GUI  platform instead.    We give  the predicates  for
reference here.


wwiinnddooww__ttiittllee((_-_O_l_d_, _+_N_e_w))
    Unify  _O_l_d with the  title displayed in  the console and change  the
    title to _N_e_w.


wwiinn__wwiinnddooww__ppooss((_+_L_i_s_t_O_f_O_p_t_i_o_n_s))
    Interface  to the  MS-Windows  SetWindowPos() function,  controlling
    size,  position and stacking order of the window.   _L_i_s_t_O_f_O_p_t_i_o_n_s is
    a list that may hold any number of the terms below.

    ssiizzee((_W_, _H))
         Change the  size of  the window.    _W  and _H  are expressed  in
         character-units.

    ppoossiittiioonn((_X_, _Y))
         Change the  top-left corner  of  the window.    The values  are
         expressed in pixel units.

    zzoorrddeerr((_Z_O_r_d_e_r))
         Change the  location  in the  window stacking  order.    Values
         are bottom, top,  topmost and notopmost.   _T_o_p_m_o_s_t windows  are
         displayed above all other windows.

    sshhooww((_B_o_o_l))
         If true, show the window, if false hide the window.

    aaccttiivvaattee
         If present, activate the window.


wwiinn__hhaass__mmeennuu
    True if win_insert_menu/2 and win_insert_menu_item/4are present.


wwiinn__iinnsseerrtt__mmeennuu((_+_L_a_b_e_l_, _+_B_e_f_o_r_e))
    Insert  a new entry  (pulldown) in the  menu.   If the menu  already
    contains  this entry,  nothing  is done.    The _L_a_b_e_l  is the  label
    and  using the  Windows conventions,  a  letter prefixed  with &  is
    underlined  and defines the associated  accelerator key.  _B_e_f_o_r_e  is
    the label before which this one  must be inserted.  Using - adds the
    new  entry at the end (right).   For example, the call below  adds a
    Application entry just before the Help menu.

    win_insert_menu('&Application', '&Help')


wwiinn__iinnsseerrtt__mmeennuu__iitteemm((_+_P_u_l_l_d_o_w_n_, _+_L_a_b_e_l_, _+_B_e_f_o_r_e_, _:_G_o_a_l))
    Add  an  item  to  the  named  _P_u_l_l_d_o_w_n menu.     _L_a_b_e_l  and  _B_e_f_o_r_e
    are  handled  as in  win_insert_menu/2,  but the  label -  inserts  a
    _s_e_p_a_r_a_t_o_r.  _G_o_a_l is called if the user selects the item.


44..3355 FFiillee SSyysstteemm IInntteerraaccttiioonn


aacccceessss__ffiillee((_+_F_i_l_e_, _+_M_o_d_e))
    True  if _F_i_l_e  exists and  can be  accessed by  this prolog  process
    under  mode _M_o_d_e.   _M_o_d_e is  one of the  atoms read, write,  append,
    exist,  none or execute.  _F_i_l_e may also be the name  of a directory.
    Fails  silently otherwise.   access_file(File, none)simply  succeeds
    without testing anything.

    If  `Mode' is write or append,  this predicate also succeeds if  the
    file  does not exist and the user has write-access to  the directory
    of the specified location.


eexxiissttss__ffiillee((_+_F_i_l_e))
    True if _F_i_l_e exists  and is a regular file.  This does not imply the
    user has read and/or write permission for the file.


ffiillee__ddiirreeccttoorryy__nnaammee((_+_F_i_l_e_, _-_D_i_r_e_c_t_o_r_y))
    Extracts  the  directory-part  of  _F_i_l_e.    The  returned  _D_i_r_e_c_t_o_r_y
    name  does  not end  in  /.   There  are  two special  cases.    The
    directory-name of /  is / itself and the directory-name if _F_i_l_e does
    not contain any / characters is ..


ffiillee__bbaassee__nnaammee((_+_F_i_l_e_, _-_B_a_s_e_N_a_m_e))
    Extracts the filename part  from a path specification.  If _F_i_l_e does
    not contain any directory separators, _F_i_l_e is returned.


ssaammee__ffiillee((_+_F_i_l_e_1_, _+_F_i_l_e_2))
    True  if both  filenames  refer to  the same  physical file.    That
    is,  if _F_i_l_e_1  and _F_i_l_e_2  are the  same string or  both names  exist
    and  point to the  same file (due to  hard or symbolic links  and/or
    relative vs.  absolute paths).


eexxiissttss__ddiirreeccttoorryy((_+_D_i_r_e_c_t_o_r_y))
    True  if  _D_i_r_e_c_t_o_r_y exists  and  is  a directory.    This  does  not
    imply  the user  has read, search  and or  write permission for  the
    directory.


ddeelleettee__ffiillee((_+_F_i_l_e))
    Remove _F_i_l_e from the file system.


rreennaammee__ffiillee((_+_F_i_l_e_1_, _+_F_i_l_e_2))
    Rename  _F_i_l_e_1 into _F_i_l_e_2.   Currently  files cannot be moved  across
    devices.


ssiizzee__ffiillee((_+_F_i_l_e_, _-_S_i_z_e))
    Unify _S_i_z_e with the size of _F_i_l_e in characters.


ttiimmee__ffiillee((_+_F_i_l_e_, _-_T_i_m_e))
    Unify  the last  modification time of  _F_i_l_e with  _T_i_m_e.   _T_i_m_e is  a
    floating  point number expressing the  seconds elapsed since Jan  1,
    1970.  See also convert_time/[2,8] and get_time/1.


aabbssoolluuttee__ffiillee__nnaammee((_+_F_i_l_e_, _-_A_b_s_o_l_u_t_e))
    Expand  a local file-name into an absolute path.  The  absolute path
    is  canonised:  references to . and .. are deleted.   This predicate
    ensures  that expanding  a file-name  it returns  the same  absolute
    path  regardless of  how the  file is  addressed.   SWI-Prolog  uses
    absolute  file names  to register  source files  independent of  the
    current  working directory.    See also  absolute_file_name/3.    See
    also absolute_file_name/3 and expand_file_name/2.


aabbssoolluuttee__ffiillee__nnaammee((_+_S_p_e_c_, _+_O_p_t_i_o_n_s_, _-_A_b_s_o_l_u_t_e))
    Converts  the  given  file  specification  into  an  absolute  path.
    _O_p_t_i_o_n is a list of options to guide the conversion:

    eexxtteennssiioonnss((_L_i_s_t_O_f_E_x_t_e_n_s_i_o_n_s))
         List of  file-extensions to  try.   Default is  ''.   For  each
         extension,  absolute_file_name/3 will first  add the  extension
         and then verify  the conditions imposed  by the other  options.
         If the  condition  fails, the  next extension  of  the list  is
         tried.   Extensions  may be specified  both as  ..ext or  plain
         ext.

    rreellaattiivvee__ttoo((_+_F_i_l_e_O_r_D_i_r))
         Resolve  the   path  relative   to  the   given  directory   or
         directory  the  holding   the  given  file.      Without   this
         option, paths  are resolved relative  to the working  directory
         (see   working_directory/2)   or,   if   _S_p_e_c  is   atomic   and
         absolute_file_name/[2,3] is executed  in a  directive, it  uses
         the current source-file as reference.

    aacccceessss((_M_o_d_e))
         Imposes the condition  access_file(_F_i_l_e, _M_o_d_e).   _M_o_d_e is on  of
         read, write, append, exist or none.  See also access_file/2.

    ffiillee__ttyyppee((_T_y_p_e))
         Defines  extensions.    Current  mapping:   txt  implies  [''],
         prolog  implies ['.pl', ''],  executable  implies  ['.so', ''],
         qlf implies  ['.qlf', '']  and  directory implies  [''].    The
         file-type source is  an alias for  prolog for compatibility  to
         SICStus Prolog.  See also prolog_file_type/2.

    ffiillee__eerrrroorrss((_f_a_i_l_/_e_r_r_o_r))
         If error (default), throw and  existence_error exception  if the
         file cannot be found.  If fail, stay silent.

    ssoolluuttiioonnss((_f_i_r_s_t_/_a_l_l))
         If  first (default),  the  predicates leaves  no  choice-point.
         Otherwise a  choice-point  will be  left and  backtracking  may
         yield more solutions.

    eexxppaanndd((_t_r_u_e_/_f_a_l_s_e))
         If  true  (default   is  false)  and   _S_p_e_c  is  atomic,   call
         expand_file_name/2  followed  by   member/2   on  _S_p_e_c   before
         proceeding.  This is a SWI-Prolog extension.

    The  prolog-flag  verbose_file_search  can be  set  to true  to  help
    debugging Prolog's search for files.

    Compatibility  considerations  to common  argument-order in  ISO  as
    well  as SICStus absolute_file_name/3forced us to  be flexible here.
    If  the last argument is a list  and the 2nd not, the  arguments are
    swapped,  making the call absolute_file_name(_+_S_p_e_c_,  _-_P_a_t_h_, _+_O_p_t_i_o_n_s)
    valid as well.


iiss__aabbssoolluuttee__ffiillee__nnaammee((_+_F_i_l_e))
    True  if _F_i_l_e specifies  and absolute path-name.   On Unix  systems,
    this   implies  the  path  starts  with  a  `/'.      For  Microsoft
    based   systems  this  implies  the   path  starts  with  <_l_e_t_t_e_r>:.
    This   predicate   is  intended   to  provide   platform-independent
    checking  for  absolute paths.    See also  absolute_file_name/2 and
    prolog_to_os_filename/2.


ffiillee__nnaammee__eexxtteennssiioonn((_?_B_a_s_e_, _?_E_x_t_e_n_s_i_o_n_, _?_N_a_m_e))
    This  predicate is used to add, remove or test  filename extensions.
    The  main reason  for  its introduction  is to  deal with  different
    filename  properties  in a  portable manner.    If  the file  system
    is   case-insensitive,  testing  for  an  extension  will   be  done
    case-insensitive too.   _E_x_t_e_n_s_i_o_n may be specified with or without a
    leading  dot (.).  If an _E_x_t_e_n_s_i_o_n is generated, it will  not have a
    leading dot.


eexxppaanndd__ffiillee__nnaammee((_+_W_i_l_d_C_a_r_d_, _-_L_i_s_t))
    Unify  _L_i_s_t with  a  sorted list  of files  or directories  matching
    _W_i_l_d_C_a_r_d.   The  normal Unix wildcard  constructs `?', `*',  `[...]'
    and  `{...}'  are recognised.    The  interpretation  of `{...}'  is
    interpreted  slightly different  from  the C  shell (csh(1)).    The
    comma  separated  argument  can  be  arbitrary  patterns,  including
    `{...}'  patterns.  The empty pattern is legal as  well:  `\{.pl,\}'
    matches either `.pl' or the empty string.

    If  the pattern  does contains  wildcard  characters, only  existing
    files  and directories are returned.  Expanding a  `pattern' without
    wildcard  characters returns the argument, regardless on  whether or
    not it exists.

    Before  expanding wildcards, the construct  $var is expanded to  the
    value  of the  environment  variable _v_a_r  and a  possible leading  ~
    character is expanded to the user's home directory..


pprroolloogg__ttoo__ooss__ffiilleennaammee((_?_P_r_o_l_o_g_P_a_t_h_, _?_O_s_P_a_t_h))
    Converts  between the internal  Prolog pathname conventions and  the
    operating-system  pathname conventions.    The internal  conventions
    are  Unix and this predicates is  equivalent to =/2 (unify) on  Unix
    systems.   On  DOS systems it  will change the  directory-separator,
    limit  the filename length map dots,  except for the last one,  onto
    underscores.


rreeaadd__lliinnkk((_+_F_i_l_e_, _-_L_i_n_k_, _-_T_a_r_g_e_t))
    If _F_i_l_e points to  a symbolic link, unify _L_i_n_k with the value of the
    link and _T_a_r_g_e_t to  the file the link is pointing to.  _T_a_r_g_e_t points
    to  a file, directory or non-existing entry in the file  system, but
    never  to a link.   Fails if _F_i_l_e  is not a link.   Fails always  on
    systems that do not support symbolic links.


ttmmpp__ffiillee((_+_B_a_s_e_, _-_T_m_p_N_a_m_e))
    Create  a name for a temporary file.  _B_a_s_e is an  identifier for the
    category  of file.  The _T_m_p_N_a_m_e is guaranteed to be unique.   If the
    system  halts, it  will automatically  remove all created  temporary
    files.


mmaakkee__ddiirreeccttoorryy((_+_D_i_r_e_c_t_o_r_y))
    Create  a  new directory  (folder) on  the filesystem.    Raises  an
    exception  on failure.   On Unix systems,  the directory is  created
    with default permissions (defined by the process _u_m_a_s_k setting).


ddeelleettee__ddiirreeccttoorryy((_+_D_i_r_e_c_t_o_r_y))
    Delete directory (folder)  from the filesystem.  Raises an exception
    on failure.   Please note that in general it will not be possible to
    delete a non-empty directory.


wwoorrkkiinngg__ddiirreeccttoorryy((_-_O_l_d_, _+_N_e_w))
    Unify  _O_l_d with an  absolute path to  the current working  directory
    and   change  working   directory  to   _N_e_w.      Use  the   pattern
    working_directory(_C_W_D_, _C_W_D) to get the current directory.   See also
    absolute_file_name/2 and chdir/1.   Note that the working  directory
    is shared between all threads.


cchhddiirr((_+_P_a_t_h))
    Compatibility predicate.  New code should use working_directory/2.


44..3366 UUsseerr TToopp--lleevveell MMaanniippuullaattiioonn


bbrreeaakk
    Recursively  start a new  Prolog top level.   This Prolog top  level
    has its own  stacks, but shares the heap with all break environments
    and  the top level.  Debugging  is switched off on entering  a break
    and  restored on leaving one.   The break environment is  terminated
    by  typing the  system's end-of-file character  (control-D). If  the
    -t toplevel  command  line  option is  given  this goal  is  started
    instead of entering the default interactive top level (prolog/0).


aabboorrtt
    Abort  the Prolog  execution  and restart  the top  level.   If  the
    -t toplevel  command  line options  is given  this  goal is  started
    instead of entering the default interactive top level.

    There  are two  implementations of abort/0.    The default one  uses
    the  exception  mechanism  (see  throw/1),  throwing  the  exception
    $aborted.   The other one uses the C-construct longjmp()  to discard
    the  entire environment  and rebuild a  new one.   Using  exceptions
    allows  for  proper recovery  of predicates  exploiting  exceptions.
    Rebuilding  the  environment  is  safer if  the  Prolog  stacks  are
    corrupt.    Therefore the  system will use  the rebuild-strategy  if
    the  abort was generated  by an internal  consistency check and  the
    exception  mechanism otherwise.   Prolog  can be  forced to use  the
    rebuild-strategy  setting  the prolog  flag  abort_with_exception  to
    false.


hhaalltt
    Terminate  Prolog  execution.   Open  files are  closed  and if  the
    command  line option  -tty is  not active the  terminal status  (see
    Unix  stty(1)) is restored.  Hooks may be registered both  in Prolog
    and  in foreign code.  Prolog hooks  are registered using at_halt/1.
    halt/0 is equivalent to halt(0).


hhaalltt((_+_S_t_a_t_u_s))
    Terminate  Prolog  execution  with  given  status.    Status  is  an
    integer.  See also halt/0.


pprroolloogg
    This  goal starts the  default interactive top  level.  Queries  are
    read  from the stream user_input.   See also the history prolog_flag
    (current_prolog_flag/2).    The  prolog/0  predicate  is  terminated
    (succeeds)  by typing  the  end-of-file character  (On most  systems
    control-D).

The following  two hooks allow  for expanding  queries and handling  the
result of  a query.    These hooks are  used by  the top-level  variable
expansion mechanism described in section 2.8.


eexxppaanndd__qquueerryy((_+_Q_u_e_r_y_, _-_E_x_p_a_n_d_e_d_, _+_B_i_n_d_i_n_g_s_, _-_E_x_p_a_n_d_e_d_B_i_n_d_i_n_g_s))
    Hook  in module  user, normally  not defined.    _Q_u_e_r_y and  _B_i_n_d_i_n_g_s
    represents  the  query read  from  the user  and  the names  of  the
    free  variables as obtained  using read_term/3.   If this  predicate
    succeeds, it should  bind _E_x_p_a_n_d_e_d and _E_x_p_a_n_d_e_d_B_i_n_d_i_n_g_s to the query
    and  bindings to be  executed by the top-level.   This predicate  is
    used  by the  top-level (prolog/0).    See also  expand_answer/2 and
    term_expansion/2.


eexxppaanndd__aannsswweerr((_+_B_i_n_d_i_n_g_s_, _-_E_x_p_a_n_d_e_d_B_i_n_d_i_n_g_s))
    Hook  in module user,  normally not defined.   Expand the result  of
    a  successfully executed  top-level query.   _B_i_n_d_i_n_g_s  is the  query
    <_N_a_m_e>= <_V_a_l_u_e>binding list  from the query.  _E_x_p_a_n_d_e_d_B_i_n_d_i_n_g_s must
    be unified with the bindings the top-level should print.


44..3377 CCrreeaattiinngg aa PPrroottooccooll ooff tthhee UUsseerr IInntteerraaccttiioonn

SWI-Prolog offers the  possibility to log the interaction with  the user
on  a file.    All  Prolog interaction,  including warnings  and  tracer
output, are written on the protocol file.


pprroottooccooll((_+_F_i_l_e))
    Start  protocolling on file  _F_i_l_e.  If  there is already a  protocol
    file open then close it first.  If _F_i_l_e exists it is truncated.


pprroottooccoollaa((_+_F_i_l_e))
    Equivalent  to protocol/1,  but  does not  truncate the  _F_i_l_e if  it
    exists.


nnoopprroottooccooll
    Stop  making a protocol of the user interaction.  Pending  output is
    flushed on the file.


pprroottooccoolllliinngg((_-_F_i_l_e))
    True  if a protocol was  started with protocol/1 or protocola/1  and
    unifies _F_i_l_e with the current protocol output file.


44..3388 DDeebbuuggggiinngg aanndd TTrraacciinngg PPrrooggrraammss

This section is a  reference to the debugger interaction predicates.   A
more use-oriented overview of the debugger is in section 2.9.

If you have installed  XPCE, you can use the graphical front-end  of the
tracer.  This front-end is installed using the predicate guitracer/0.


ttrraaccee
    Start  the tracer.   trace/0  itself cannot be  seen in the  tracer.
    Note  that the  Prolog top-level  treats trace/0  special; it  means
    `trace the next goal'.


ttrraacciinngg
    True  if the tracer is currently switched on.  tracing/0  itself can
    not be seen in the tracer.


nnoottrraaccee
    Stop the tracer.  notrace/0 itself cannot be seen in the tracer.


gguuiittrraacceerr
    Installs  hooks  (see prolog_trace_interception/4) into  the  system
    that  redirects tracing  information  to a  GUI front-end  providing
    structured  access to variable-bindings,  graphical overview of  the
    stack and highlighting of relevant source-code.


nnoogguuiittrraacceerr
    Reverts back to the textual tracer.


ttrraaccee((_+_P_r_e_d))
    Equivalent to trace(Pred, +all).


ttrraaccee((_+_P_r_e_d_, _+_P_o_r_t_s))
    Put  a  trace-point  on  all  predicates  satisfying  the  predicate
    specification  _P_r_e_d.   _P_o_r_t_s is  a list of  port names (call,  redo,
    exit,  fail).   The atom all refers  to all ports.   If the port  is
    preceded  by a - sign the trace-point  is cleared for the port.   If
    it is preceded by a + the trace-point is set.

    The  predicate trace/2  activates debug  mode (see debug/0).    Each
    time  a port (of the 4-port model) is passed that has  a trace-point
    set  the goal is printed as  with trace/0.  Unlike trace/0  however,
    the  execution is continued without asking for  further information.
    Examples:

        ?- trace(hello).         Trace all  ports of hello  with any
                                 arity in any module.
        ?- trace(foo/2, +fail).  Trace  failures  of  foo/2  in  any
                                 module.
        ?- trace(bar/1, -all).   Stop tracing bar/1.

    The predicate debugging/0 shows all currently defined trace-points.


nnoottrraaccee((_+_G_o_a_l))
    Call  _G_o_a_l,  but  suspend  the  debugger while  _G_o_a_l  is  executing.
    The  current implementation  cuts  the choice-points  of _G_o_a_l  after
    successful completion.   See once/1.  Later implementations may have
    the same semantics as call/1.


ddeebbuugg
    Start  debugger.     In  debug  mode,   Prolog  stops  at  spy-  and
    trace-points,  disables tail-recursion  optimisation and  aggressive
    destruction   of   choice-points  to   make  debugging   information
    accessible.  Implemented by the Prolog flag debug.


nnooddeebbuugg
    Stop  debugger.   Implemented by the  prolog flag debug.   See  also
    debug/0.


ddeebbuuggggiinngg
    Print  debug status and  spy points on current  output stream.   See
    also the prolog flag debug.


ssppyy((_+_P_r_e_d))
    Put   a  spy   point  on  all   predicates  meeting  the   predicate
    specification _P_r_e_d.  See section 4.4.


nnoossppyy((_+_P_r_e_d))
    Remove   spy  point  from  all  predicates  meeting   the  predicate
    specification _P_r_e_d.


nnoossppyyaallll
    Remove all spy points from the entire program.


lleeaasshh((_?_P_o_r_t_s))
    Set/query leashing (ports  which allow for user interaction).  _P_o_r_t_s
    is  one of _+_N_a_m_e, _-_N_a_m_e,  _?_N_a_m_e or a list  of these.  _+_N_a_m_e  enables
    leashing  on that  port,  _-_N_a_m_e disables  it and  _?_N_a_m_e succeeds  or
    fails  according to  the  current setting.    Recognised ports  are:
    call, redo, exit,  fail and unify.  The special shorthand all refers
    to  all ports, full  refers to all ports  except for the unify  port
    (default).  half refers to the call, redo and fail port.


vviissiibbllee((_+_P_o_r_t_s))
    Set the ports shown  by the debugger.  See leash/1 for a description
    of the port specification.  Default is full.


uunnkknnoowwnn((_-_O_l_d_, _+_N_e_w))
    Edinburgh-prolog  compatibility predicate,  interfacing  to the  ISO
    prolog  flag unknown.   Values are  trace (meaning error) and  fail.
    If  the unknown flag is set to warning, unknown/2 reports  the value
    as trace.


ssttyyllee__cchheecckk((_+_S_p_e_c))
    Set  style checking options.   _S_p_e_c  is either  +<_o_p_t_i_o_n>, -<_o_p_t_i_o_n>,
    ?(<_o_p_t_i_o_n>) or  a list  of  such options.    +<_o_p_t_i_o_n> sets a  style
    checking  option,  -<_o_p_t_i_o_n> clears it  and ?(<_o_p_t_i_o_n>) succeeds  or
    fails  according to the current setting.  consult/1  and derivatives
    resets the style  checking options to their value before loading the
    file.   If---for  example---a file containing  long atoms should  be
    loaded the user can start the file with:

    :- style_check(-atom).

    Currently available options are:

    ____________________________________________________________________
    |_Name__________|Default_|Description_______________________________||singletonon|  ||

    |                         read_clause/1 (used by  consult/1)  warns |               ||
    |                         on variables  only  appearing once  in  a |               ||
    |                         term  (clause)  which  have  a  name  not |               ||
    ||atom          |  on    |starting with an underscore.              ||              ||

    |               |        |read/1 and  derivatives will  produce  an |
    |               |        |error message on quoted atoms or  strings |
    |               |        |longer than 5 lines.                      |

    | dollar        |  off   |Accept dollar as a lower case  character, |
    |               |        |thus avoiding the need for quoting  atoms |
    |               |        |with dollar  signs.   System  maintenance |
    |               |        |use only.                                 |
    | discontiguous |  on    |Warn if the clauses  for a predicate  are |
    |               |        |not together in the same source file.     |
    | string        |  off   |Backward    compatibility.            See |
    |               |        |the       prolog-flag       double_quotes |
    |               |        |(current_prolog_flag/2).                  |

    | charset       |  off   |Warn on atoms and variables holding  non- |
    |               |        |ASCII characters  that  are  not  quoted. |
    |_______________|________|See_also_section_2.15.1.1.________________|


44..3399 OObbttaaiinniinngg RRuunnttiimmee SSttaattiissttiiccss


ssttaattiissttiiccss((_+_K_e_y_, _-_V_a_l_u_e))
    Unify system statistics  determined by _K_e_y with _V_a_l_u_e.  The possible
    keys  are given  in  the table  4.2.   The  last part  of the  table
    contains  keys  for compatibility  to other  Prolog  implementations
    (Quintus)  for improved  portability.   Note that  the ISO  standard
    does not define methods to collect system statistics.
_________________________________________________________________________
| agc                    |Number of atom garbage-collections performed   |
| agc_gained             N|umber of atoms removed                        |
| agc_time               T|ime spent in atom garbage-collections         |
| cputime                |(User) cpu  time since  Prolog  was started  in|

|                        |seconds                                        |
| inferences             |Total number  of passes via  the call and  redo|
|                        |ports since Prolog was started.                |
| heap                   |Estimated  total   size   of  the   heap   (see|
|                        |section 2.18.1.1)                              |
| heapused               |Bytes heap in use by Prolog.                   |
| heaplimit              |Maximum   size   of   the   heap    (see   sec-|
|                        |tion 2.18.1.1)                                 |

| local                  |Allocated size of the local stack in bytes.    |
| localused              |Number of bytes in use on the local stack.     |
| locallimit             |Size to  which the  local stack  is allowed  to|
|                        |grow                                           |
| global                 |Allocated size of the global stack in bytes.   |
| globalused             |Number of bytes in use on the global stack.    |
| globallimit            |Size to  which the global  stack is allowed  to|

|                        |grow                                           |
| trail                  |Allocated size of the trail stack in bytes.    |
| trailused              |Number of bytes in use on the trail stack.     |
| traillimit             |Size to  which the  trail stack  is allowed  to|
|                        |grow                                           |
| atoms                  |Total number of defined atoms.                 |
| functors               |Total number of defined name/arity pairs.      |
| predicates             |Total number of predicate definitions.         |

| modules                |Total number of module definitions.            |
| codes                  |Total amount of byte codes in all clauses.     |
| threads                |MT-version:  number of active threads          |
| threads_created        M|T-version:  number of created threads         |
| thread_cputime         M|T-version:  seconds CPU time used  by finished|
|                        t|hreads.   Supported on  Windows-NT and  later,|
|                        L|inux and  possibly  a few  more.    Verify  it|

|________________________g|ives_plausible_results_before_using.__________|
|___________________________Compatibility_keys___________________________ |
| runtime                |[   CPU   time,   CPU   time   since   last   ]|
|                        |(milliseconds)                                 |
| system_time            [|System CPU  time, System CPU time since  last |
|                        ]|(milliseconds)                                |
| real_time              [|Wall  time, Wall time  since last ]  (seconds |

|                        s|ince 1970)                                    |
| memory                 |[  Total unshared  data,  free memory  ]  (Uses|
|                        |getrusage() if available, otherwise  incomplete|
|                        |own statistics.                                |
| stacks                 |[ global use, local use ]                      |
| program                |[ heap, 0 ]                                    |
| global_stack           [|global use, global free ]                     |
| local_stack            [|local use, local free ]                       |

| trail                  |[ trail use, 0 ]                               |
| garbage_collection     [|number of GC, bytes gained, time spent ]      |
| stack_shifts           [|global  shifts,  local shifts,  time spent  ] |
|                        (|fails if no shifter in this version)          |
| atoms                  |[ number, memory use, 0 ]                      |
| atom_garbage_collection[|number of AGC, bytes gained, time spent ]     |
|_core___________________|Same_as_memory_________________________________|

                   Table 4.2:  Keys for statistics/2


ssttaattiissttiiccss
    Display a table of system statistics on the current output stream.


ttiimmee((_+_G_o_a_l))
    Execute  _G_o_a_l just  like once/1  (i.e., leaving  no choice  points),
    but  print used time, number  of logical inferences and the  average
    number  of  _l_i_p_s  (logical  inferences  per  second).     Note  that
    SWI-Prolog  counts the actual  executed number of inferences  rather
    than  the number of passes through  the call- and redo ports of  the
    theoretical 4-port model.


44..4400 EExxeeccuuttiioonn pprrooffiilliinngg

This section  describes the  hierarchical execution profiler  introduced
in  SWI-Prolog 5.1.10.    This profiler  is based  on  ideas from  gprof
described  in [Graham _e_t _a_l_., 1982].     The profiler  consists  of  two
parts:   the  information-gathering  is built  into  the kernel,  and  a
presentation component which is defined in the statistics library.   The
latter can be  hooked, which is used by the  XPCE module swi/pce_profile
to provide an interactive graphical representation of results.


44..4400..11 PPrrooffiilliinngg pprreeddiiccaatteess

Currently, the interface is  kept compatible with the old profiler.   As
experience grows, it is  likely that the old interface is  replaced with
one that  better reflects the  new capabilities.   Feel free to  examine
the internal interfaces and report useful application thereof.


pprrooffiillee((_:_G_o_a_l))
    Execute  _G_o_a_l just like time/1, collecting profiling  statistics and
    call  show_profile(_p_l_a_i_n_, _2_5).   With  XPCE installed  this opens  a
    graphical interface to the collected profiling data.


pprrooffiillee((_:_G_o_a_l_, _+_S_t_y_l_e_, _+_N_u_m_b_e_r))
    Execute  _G_o_a_l just like  time/1.   Collect profiling statistics  and
    show  the top _N_u_m_b_e_r  procedures on the  current output stream  (see
    show_profile/1) using _S_t_y_l_e.   The results are kept in  the database
    until  reset_profiler/0or  profile/3 is called and can  be displayed
    again  with show_profile/1.  The  profile/1 predicate is a  backward
    compatibility interface to  profile/1.  The other predicates in this
    section are low-level predicates for special cases.


sshhooww__pprrooffiillee((_+_S_t_y_l_e_, _+_N_u_m_b_e_r))
    Show  the collected  results  of the  profiler.   It  shows the  top
    _N_u_m_b_e_r predicates according  the percentage cpu-time used.  If _S_t_y_l_e
    is  plain the time spent in the predicates itself is displayed.   If
    _S_t_y_l_e  is cumulative  the time  spent in its  siblings (callees)  is
    added to the predicate.

    This  predicate first calls  prolog:show_profile_hook/2.   If XPCE  is
    loaded  this hook is used to  activate a GUI interface to  visualise
    the profile results.


sshhooww__pprrooffiillee((_+_N_u_m_b_e_r))
    Compatibility.  Same as show_profile(_p_l_a_i_n_, _N_u_m_b_e_r).


pprrooffiilleerr((_-_O_l_d_, _+_N_e_w))
    Query  or  change the  status of  the profiler.    The  status is  a
    boolean  (true or  false)  stating whether  or not  the profiler  is
    collecting  data.   It can  be used to  enable or disable  profiling
    certain parts of the program.


rreesseett__pprrooffiilleerr
    Switches the profiler to false and clears all collected statistics.


nnoopprrooffiillee((_+_N_a_m_e_/_+_A_r_i_t_y_, _._._.))
    Declares  the predicate _N_a_m_e/_A_r_i_t_y to be invisible to  the profiler.
    The  time  spend in  the  named predicate  is  added to  the  caller
    and  the  callees are  linked  directly  to the  caller.    This  is
    particularly  useful  for  simple meta-predicates  such  as  call/1,
    ignore/1, catch/3, etc.


44..4400..22 VViissuuaalliizziinngg pprrooffiilliinngg ddaattaa

Browsing the annotated  call-tree as described in section 4.40.3  itself
is  not very  attractive.    Therefore,  the  results are  combined  per
predicate,  collecting  all _c_a_l_l_e_r_s  and  and  _c_a_l_l_e_e_s as  well  as  the
propagation  of time  and activations  in both  directions.   Figure  ????
illustrates  this.     The  central  yellowish  line  is  the  `current'
predicate with  counts for time  spent in  the predicate (`Self'),  time
spent in  its children  (`Siblings'), activations through  the call  and
redo ports.   Above  that are the _c_a_l_l_e_r_s.   Here,  the two time  fields
indicate  how much  time is  spent serving  each of  the callers.    The
columns sum to the time in the yellowish  line.  The caller <_r_e_c_u_r_s_i_v_e>
are  the number  of recursive  calls.   Below  the  yellowish lines  are
the callees, with  the time spent in  the callee itself for serving  the
current  predicate and  the time  spent  in the  callees of  the  callee
('Siblings'), so  the whole time-block adds  up to the `Siblings'  field
of the current predicate.   The `Access' fields show how many  times the
current predicate accesses each of the callees.

The predicates have a  menu that allows changing the view of  the detail
window to the given  caller or callee, showing the documentation  (if it
is a built-in) and/or jumping to the source.

The  statistics  shown  in  the  report-field  of  figure  ????  show  the
following information:

  o _s_a_m_p_l_e_s
    Number  of  times  the call-tree  was  sampled for  collecting  time
    statistics.   On  most hardware the  resolution of SIGPROF is  1/100
    second.    This number must  be sufficiently  large to get  reliable
    timing  figures.   The  Time menu  allows viewing  time as  samples,
    relative time or absolute time.

  o _s_e_c
    Total user CPU time with the profiler active.

  o _p_r_e_d_i_c_a_t_e_s
    Total  count of predicates that have  been called at least one  time
    during the profile.

  o _n_o_d_e_s
    Number of nodes in the call-tree.

  o _d_i_s_t_o_r_t_i_o_n
    How  much  of  the  time  is  spend  building  the  call-tree  as  a
    percentage  of the total execution time.   Timing samples while  the
    profiler is building the call-tree are not added to the call-tree.


44..4400..33 IInnffoorrmmaattiioonn ggaatthheerriinngg

While  the program  executes under  the profiler,  the  system builds  a
_d_y_n_a_m_i_c call-tree.    It does this  using three  hooks from the  kernel:
one that starts  a new goal (_p_r_o_f_C_a_l_l),  one the tells the system  which
goal  is  resumed after  an  _e_x_i_t  (_p_r_o_f_E_x_i_t)  and one  that  tells  the
system which  goal is  resumed after  a _f_a_i_l  (i.e. which  goal is  used
to _r_e_t_r_y  (_p_r_o_f_R_e_d_o)).   The  profCall() function finds  or creates  the
subnode for  the argument predicate below  the current node,  increments
the call-count of this  link and returns the sub-node which  is recorded
in the Prolog  stack-frame.  Choice-points  are marked with the  current
profiling  node.   profExit()  and profRedo()  pass  the profiling  node
where execution resumes.

Just using the above  algorithm would create a much too big tree  due to
recursion.  For this reason the system performs  detection of recursion.
In the  simplest case,  recursive procedures  increment the  `recursive'
count on  the current  node.   Mutual  recursion however  is not  easily
detected.   For example,  call/1 can call a  predicate that uses  call/1
itself.   This  can be  viewed as a  recursive invocation,  but this  is
generally not  desirable.   Recursion is currently  assumed if the  same
predicate _w_i_t_h _t_h_e _s_a_m_e _p_a_r_e_n_t appears higher in the  call-graph.  Early
experience with a some arbitrary non-trivial programs are promising.

The last part of  the profiler collects statistics on the  CPU-time used
in  each node.    On  systems  providing  setitimer() with  SIGPROF,  it
`ticks' the  current node of  the call-tree each  time the timer  fires.
On Windows a MM-timer  in a separate thread checks 100 times  per second
how much  time is  spent in  the profiled thread  and adds  this to  the
current node.  See section 4.40.3.1 for details.


44..4400..33..11 PPrrooffiilliinngg iinn tthhee WWiinnddoowwss IImmpplleemmeennttaattiioonn

Profiling  in the  Windows version  is  similar but  as profiling  is  a
statistical process  it is good  to be aware  of the implementation  for
proper interpretation of the results.

Windows  does not  provide  timers that  fire  asynchronously,  frequent
and proportional  to the CPU  time used  by the process.   Windows  does
provide multi-media timers that can run at high frequency.   Such timers
however run  in a  separate thread of  execution and  they are fired  on
the wall-clock rather  than the amount of CPU  time used.  The  profiler
installs such a timer running, for saving CPU  time, rather inaccurately
at about 100 Hz.  Each time it is fired,  it determines the milliseconds
CPU time  used by  Prolog since the  last time it  was fired.   If  this
value is non-zero, active predicates are incremented with this value.


44..4411 MMeemmoorryy MMaannaaggeemmeenntt

Note:    limit_stack/2 and  trim_stacks/0 have  no  effect  on  machines
that do  not offer  dynamic stack expansion.    On these machines  these
predicates simply succeed to improve portability.


ggaarrbbaaggee__ccoolllleecctt
    Invoke  the global-  and trail  stack garbage collector.    Normally
    the  garbage   collector  is  invoked  automatically  if  necessary.
    Explicit  invocation  might   be  useful  to  reduce  the  need  for
    garbage  collections in time critical segments  of the code.   After
    the  garbage  collection  trim_stacks/0 is  invoked to  release  the
    collected memory resources.


ggaarrbbaaggee__ccoolllleecctt__aattoommss
    Reclaim  unused  atoms.     Normally  invoked  after  agc_margin  (a
    prolog  flag) atoms have been  created.  On multi-threaded  versions
    the   actual  collection  is  delayed  until  there  there   are  no
    threads  performing  normal  garbage  collection.     In  this  case
    garbage_collect_atoms/0  returns immediately.    Note  this  implies
    there  is no guarantee  it will _e_v_e_r happen  as there may always  be
    threads performing garbage collection.


lliimmiitt__ssttaacckk((_+_K_e_y_, _+_K_b_y_t_e_s))
    Limit one of the  stack areas to the specified value.  _K_e_y is one of
    local,  global or trail.   The limit  is an integer, expressing  the
    desired  stack limit in K  bytes.  If  the desired limit is  smaller
    than  the currently  used value,  the limit  is set  to the  nearest
    legal  value above the currently used  value.  If the desired  value
    is  larger than the maximum, the maximum is taken.  Finally,  if the
    desired value is either  0 or the atom unlimited the limit is set to
    its  maximum.   The maximum and initial  limit is determined by  the
    command line options -L, -G and -T.


ttrriimm__ssttaacckkss
    Release  stack memory resources that are not in use at  this moment,
    returning them to the  operating system.  Trim stack is a relatively
    cheap  call.    It can  be used  to release  memory  resources in  a
    backtracking  loop, where the  iterations require typically  seconds
    of execution time  and very different, potentially large, amounts of
    stack space.  Such a loop should be written as follows:

    loop :-
            generator,
                trim_stacks,
                potentially_expensive_operation,
            stop_condition, !.

    The  prolog top level  loop is written  this way, reclaiming  memory
    resources after every user query.


ssttaacckk__ppaarraammeetteerr((_+_S_t_a_c_k_, _+_K_e_y_, _-_O_l_d_, _+_N_e_w))
    Query/set  a parameter  for the  runtime stacks.   _S_t_a_c_k  is one  of
    local,  global, trail or  argument.   The table below describes  the
    _K_e_y/_V_a_l_u_e pairs.  Old is first unified with the current value.

        ____________________________________________________________
        | limit    |Maximum size of the stack in bytes               |
        |_min_free_|Minimum_free_space_at_entry_of_foreign_predicate_|

    This predicate is  currently only available on versions that use the
    stack-shifter  to enlarge the runtime  stacks when necessary.   It's
    definition is subject to change.


44..4422 WWiinnddoowwss DDDDEE iinntteerrffaaccee

The  predicates in  this  section  deal with  MS-Windows  `Dynamic  Data
Exchange'  or DDE  protocol.    A Windows  DDE  conversation is  a  form
of interprocess  communication based  on sending reserved  window-events
between the communicating processes.

See also section 9.4 for loading Windows DLL's into SWI-Prolog.


44..4422..11 DDDDEE cclliieenntt iinntteerrffaaccee

The DDE client interface  allows Prolog to talk to DDE  server programs.
We  will demonstrate  the use  of the  DDE interface  using the  Windows
PROGMAN (Program Manager) application:

1 ?- open_dde_conversation(progman, progman, C).

C = 0
2 ?- dde_request(0, groups, X)

--> Unifies X with description of groups

3 ?- dde_execute(0, '[CreateGroup("DDE Demo")]').

Yes

4 ?- close_dde_conversation(0).

Yes

For  details   on  interacting  with   progman,   use  the  SDK   online
manual  section on  the  Shell  DDE interface.     See also  the  Prolog
library(progman),  which  may be  used  to  write simple  Windows  setup
scripts in Prolog.


ooppeenn__ddddee__ccoonnvveerrssaattiioonn((_+_S_e_r_v_i_c_e_, _+_T_o_p_i_c_, _-_H_a_n_d_l_e))
    Open a conversation  with a server supporting the given service name
    and  topic (atoms).    If successful,  _H_a_n_d_l_e  may be  used to  send
    transactions  to the server.    If no willing  server is found  this
    predicate fails silently.


cclloossee__ddddee__ccoonnvveerrssaattiioonn((_+_H_a_n_d_l_e))
    Close  the  conversation   associated  with  _H_a_n_d_l_e.     All  opened
    conversations  should  be  closed  when they're  no  longer  needed,
    although  the system  will  close any  that remain  open on  process
    termination.


ddddee__rreeqquueesstt((_+_H_a_n_d_l_e_, _+_I_t_e_m_, _-_V_a_l_u_e))
    Request  a value from the server.   _I_t_e_m is an atom  that identifies
    the  requested  data,  and  _V_a_l_u_e will  be  a string  (CF_TEXT  data
    in  DDE  parlance)  representing  that  data,   if  the  request  is
    successful.    If unsuccessful, _V_a_l_u_e  will be  unified with a  term
    of  form error(<_R_e_a_s_o_n>),  identifying the problem.   This call  uses
    SWI-Prolog  string objects  to return  the value  rather then  atoms
    to  reduce the  load on  the atom-space.    See section  4.23 for  a
    discussion on this data type.


ddddee__eexxeeccuuttee((_+_H_a_n_d_l_e_, _+_C_o_m_m_a_n_d))
    Request  the  DDE   server  to  execute  the  given  command-string.
    Succeeds  if the  command  could be  executed and  fails with  error
    message otherwise.


ddddee__ppookkee((_+_H_a_n_d_l_e_, _+_I_t_e_m_, _+_C_o_m_m_a_n_d))
    Issue  a POKE command to the server on the specified _I_t_e_m.   Command
    is passed as data of type CF_TEXT.


44..4422..22 DDDDEE sseerrvveerr mmooddee

The (autoload)  library(dde) defines  primitives to  realise simple  DDE
server applications in  SWI-Prolog.  These  features are provided as  of
version 2.0.6 and should be regarded prototypes.  The  C-part of the DDE
server can  handle some  more primitives,  so if you  need features  not
provided by this interface, please study library(dde).


ddddee__rreeggiisstteerr__sseerrvviiccee((_+_T_e_m_p_l_a_t_e_, _+_G_o_a_l))
    Register  a server  to handle  DDE request or  DDE execute  requests
    from  other applications.  To register a service for a  DDE request,
    _T_e_m_p_l_a_t_e is of the form:

         +Service(+Topic, +Item, +Value)

    _S_e_r_v_i_c_e  is the name  of the DDE  service provided (like progman  in
    the  client example  above).   _T_o_p_i_c is either  an atom,  indicating
    _G_o_a_l  only handles requests  on this topic  or a variable that  also
    appears  in _G_o_a_l.  _I_t_e_m and _V_a_l_u_e are variables that also  appear in
    _G_o_a_l.  _I_t_e_m represents the request data as a Prolog atom.

    The   example  below  registers   the  Prolog  current_prolog_flag/2
    predicate  to be accessible  from other applications.   The  request
    may  be  given  from  the  same  Prolog  as  well  as  from  another
    application.

    ?- dde_register_service(prolog(current_prolog_flag, F, V),
                            current_prolog_flag(F, V)).

    ?- open_dde_conversation(prolog, current_prolog_flag, Handle),
       dde_request(Handle, home, Home),
       close_dde_conversation(Handle).

    Home = '/usr/local/lib/pl-2.0.6/'

    Handling  DDE execute requests  is very similar.   In this case  the
    template is of the form:

         +Service(+Topic, +Item)

    Passing  a _V_a_l_u_e argument is  not needed as execute requests  either
    succeed  or fail.  If _G_o_a_l  fails, a `not processed' is  passed back
    to the caller of the DDE request.


ddddee__uunnrreeggiisstteerr__sseerrvviiccee((_+_S_e_r_v_i_c_e))
    Stop  responding  to  _S_e_r_v_i_c_e.     If  Prolog  is  halted,  it  will
    automatically call this on all open services.


ddddee__ccuurrrreenntt__sseerrvviiccee((_-_S_e_r_v_i_c_e_, _-_T_o_p_i_c))
    Find currently registered services and the topics served on them.


ddddee__ccuurrrreenntt__ccoonnnneeccttiioonn((_-_S_e_r_v_i_c_e_, _-_T_o_p_i_c))
    Find currently open conversations.


44..4433 MMiisscceellllaanneeoouuss


ddwwiimm__mmaattcchh((_+_A_t_o_m_1_, _+_A_t_o_m_2))
    True  if _A_t_o_m_1 matches _A_t_o_m_2 in `Do What I Mean' sense.   Both _A_t_o_m_1
    and _A_t_o_m_2 may also be integers or floats.  The two atoms match if:

      o  They are identical

      o  They differ by one character (spy  spu)

      o  One character is inserted/deleted (debug  deug)

      o  Two characters are transposed (trace  tarce)

      o  `Sub-words' are glued  differently (existsfile   existsFile
         exists_file)

      o  Two   adjacent  sub   words  are   transposed   (existsFile
         fileExists)


ddwwiimm__mmaattcchh((_+_A_t_o_m_1_, _+_A_t_o_m_2_, _-_D_i_f_f_e_r_e_n_c_e))
    Equivalent  to  dwim_match/2,  but unifies  _D_i_f_f_e_r_e_n_c_e  with an  atom
    identifying the the  difference between _A_t_o_m_1 and _A_t_o_m_2.  The return
    values  are (in the same  order as above):   equal, mismatched_char,
    inserted_char, transposed_char, separated and transposed_word.


wwiillddccaarrdd__mmaattcchh((_+_P_a_t_t_e_r_n_, _+_S_t_r_i_n_g))
    True  if _S_t_r_i_n_g matches  the wildcard pattern  _P_a_t_t_e_r_n.  _P_a_t_t_e_r_n  is
    very  similar the the  Unix csh pattern matcher.   The patterns  are
    given below:

     ?      Matches one arbitrary character.
     *      Matches any number of arbitrary characters.
     [...]  Matches one of the characters specified between the brackets.
            <_c_h_a_r_1>-<_c_h_a_r_2>indicates a range.
     {...}  Matches any of the patterns of the comma separated list between the braces.

    Example:

    ?- wildcard_match('[a-z]*.{pro,pl}[%~]', 'a_hello.pl%').

    Yes


sslleeeepp((_+_T_i_m_e))
    Suspend  execution _T_i_m_e seconds.   _T_i_m_e  is either a floating  point
    number  or an  integer.   Granularity is  dependent on the  system's
    timer  granularity.   A  negative time  causes the  timer to  return
    immediately.   On  most non-realtime operating  systems we can  only
    ensure execution is suspended for aatt lleeaasstt _T_i_m_e seconds.

    On  Unix systems the  sleep/1 predicate is  realised ---in order  of
    preference---  by nanosleep(),  usleep(),  select() if  the time  is
    below 1 minute or sleep().  On Windows systems Sleep() is used.


CChhaapptteerr 55..  UUSSIINNGG MMOODDUULLEESS


55..11 WWhhyy UUssiinngg MMoodduulleess??

In  traditional Prolog  systems the  predicate  space was  flat.    This
approach   is  not   very  suitable   for  the   development  of   large
applications, certainly not if these applications are  developed by more
than  one programmer.     In many  cases,  the  definition of  a  Prolog
predicate requires  sub-predicates that  are intended  only to  complete
the definition of the main predicate.  With a  flat and global predicate
space these support predicates will be visible from the entire program.

For this  reason, it is  desirable that each source  module has its  own
predicate space.    A module  consists of  a declaration  for its  name,
its _p_u_b_l_i_c  _p_r_e_d_i_c_a_t_e_s and  the predicates  themselves.   This  approach
allow the programmer  to use short (local) names for  support predicates
without worrying  about name  conflicts with the  support predicates  of
other  modules.    The  module  declaration also  makes  explicit  which
predicates are  meant for public usage  and which for private  purposes.
Finally,  using the  module information,  cross  reference programs  can
indicate possible problems much better.


55..22 NNaammee--bbaasseedd vveerrssuuss PPrreeddiiccaattee--bbaasseedd MMoodduulleess

Two approaches to  realize a module system  are commonly used in  Prolog
and other  languages.  The  first one is  the _n_a_m_e _b_a_s_e_d module  system.
In these  systems, each  atom read  is tagged  (normally prefixed)  with
the module  name, with  the exception  of those atoms  that are  defined
_p_u_b_l_i_c.   In the  second approach, each  module actually implements  its
own predicate space.

A  critical  problem with  using  modules  in Prolog  is  introduced  by
the  meta-predicates  that  transform between  Prolog  data  and  Prolog
predicates.  Consider the case where we write:

:- module(extend, [add_extension/3]).

add_extension(Extension, Plain, Extended) :-
        maplist(extend_atom(Extension), Plain, Extended).

extend_atom(Extension, Plain, Extended) :-
        atom_concat(Plain, Extension, Extended).

In this case  we would like maplist to  call extend_atom/3 in the  module
extend.   A name based module  system will do this  correctly.  It  will
tag the atom  extend_atom  with the module and  maplist will use this  to
construct the tagged  term extend_atom/3.   A name based module  however,
will not only tag the  atoms that will eventually be used to refer  to a
predicate, but aallll atoms that are not declared public.   So, with a name
based module system also  data is local to the module.   This introduces
another serious problem:

:- module(action, [action/3]).

action(Object, sleep, Arg) :- ....
action(Object, awake, Arg) :- ....

:- module(process, [awake_process/2]).

awake_process(Process, Arg) :-
        action(Process, awake, Arg).

This code  uses a simple  object-oriented implementation technique  were
atoms are used as  method selectors.  Using a name based  module system,
this code will  not work, unless we  declare the selectors public  atoms
in all  modules that use them.   Predicate  based module systems do  not
require particular precautions for handling this case.

It  appears  we  have to  choose  either  to  have  local  data,  or  to
have  trouble with  meta-predicates.    Probably it  is best  to  choose
for  the  predicate  based approach  as  novice  users  will  not  often
write  generic meta-predicates  that  have to  be used  across  multiple
modules, but are likely  to write programs that pass data  around across
modules.   Experienced Prolog  programmers should be  able to deal  with
the complexities of meta-predicates in a predicate based module system.


55..33 DDeeffiinniinngg aa MMoodduullee

Modules normally are  created by loading a _m_o_d_u_l_e  _f_i_l_e.  A module  file
is a file holding a module/2 directive as its first term.   The module/2
directive declares  the name and the  public (i.e., externally  visible)
predicates of  the module.   The  rest of  the file is  loaded into  the
module.  Below is an example of a module file, defining reverse/2.

:- module(reverse, [reverse/2]).

reverse(List1, List2) :-
        rev(List1, [], List2).

rev([], List, List).
rev([Head|List1], List2, List3) :-
        rev(List1, [Head|List2], List3).


55..44 IImmppoorrttiinngg PPrreeddiiccaatteess iinnttoo aa MMoodduullee

As  explained  before,  in  the  predicate  based  approach  adapted  by
SWI-Prolog, each  module has its  own predicate space.   In  SWI-Prolog,
a module  initially is  completely empty.   Predicates  can be added  to
a  module by  loading a  module  file as  demonstrated in  the  previous
section, using assert or by _i_m_p_o_r_t_i_n_g them from another module.

Two mechanisms for  importing predicates explicitly from another  module
exist.   The use_module/[1,2] predicates load  a module file and  import
(part of  the) public predicates  of the file.   The import/1  predicate
imports any predicate from any module.


uussee__mmoodduullee((_+_F_i_l_e))
    Load  the  file(s) specified  with _F_i_l_e  just like  ensure_loaded/1.
    The  files should  all be  module files.    All exported  predicates
    from  the loaded files are imported  into the context module.   This
    predicate  is equivalent to  ensure_loaded/1,  except that it  raises
    an error if _F_i_l_e is not a module file.


uussee__mmoodduullee((_+_F_i_l_e_, _+_I_m_p_o_r_t_L_i_s_t))
    Load  the file  specified  with _F_i_l_e  (only one  file is  accepted).
    _F_i_l_e  should be a module file.   _I_m_p_o_r_t_L_i_s_t is a list  of name/arity
    pairs  specifying the  predicates that should  be imported from  the
    loaded  module.   If a predicate is  specified that is not  exported
    from  the loaded module  a warning will be  printed.  The  predicate
    will nevertheless be imported to simplify debugging.


iimmppoorrtt((_+_H_e_a_d))
    Import predicate _H_e_a_d  into the current context module.  _H_e_a_d should
    specify the  source module using the <_m_o_d_u_l_e>:<_t_e_r_m>construct.  Note
    that  predicates are normally imported  using one of the  directives
    use_module/[1,2].    import/1  is meant  for handling  imports  into
    dynamically created modules.

It  would  be rather  inconvenient  to  have to  import  each  predicate
referred to by  the module, including the  system predicates.  For  this
reason each  module is  assigned a _d_e_f_a_u_l_t  _m_o_d_u_l_e.   All predicates  in
the default  module are  available without  extra declarations.    Their
definition however can be overruled in the local module.   This schedule
is implemented  by the exception handling  mechanism of SWI-Prolog:   if
an  undefined predicate  exception is  raised for  a  predicate in  some
module, the exception  handler first tries to import the  predicate from
one of  the module's _i_m_p_o_r_t  _m_o_d_u_l_e_s.  On  success, normal execution  is
resumed.


55..44..11 RReesseerrvveedd MMoodduulleess

SWI-Prolog contains two  special modules.   The first one is the  module
system.    This module  contains all  built-in  predicates described  in
this  manual.   Module  system has  no default  module  assigned to  it.
The second  special module is the  module user.   This module forms  the
initial  working space  of the  user.    Initially  it is  empty.    The
import module  of module user is  system, making all built-in  predicate
definitions available  as defaults.    Built-in predicates  thus can  be
overruled by defining them in module user before they are used.

All  other modules  import from  the module  user.    This implies  they
can use all  predicates imported into user without explicitly  importing
them.


55..55 UUssiinngg tthhee MMoodduullee SSyysstteemm

The  current structure  of  the module  system  has been  designed  with
some specific  organisations for  large programs  in mind.   Many  large
programs  define a  basic  library layer  on  top  of which  the  actual
program itself  is defined.    The module  user, acting  as the  default
module for all  other modules of the  program can be used to  distribute
these definitions over  all program module without introducing the  need
to  import this  common layer  each time  explicitly.   It  can also  be
used to  redefine built-in predicates  if this  is required to  maintain
compatibility  to some  other Prolog  implementation.    Typically,  the
loadfile of a large application looks like this:

:- use_module(compatibility).   % load XYZ prolog compatibility

:- use_module(                  % load generic parts
        [ error                 % errors and warnings
        , goodies               % general goodies (library extensions)
        , debug                 % application specific debugging
        , virtual_machine       % virtual machine of application
        , ...                   % more generic stuff
        ]).

:- ensure_loaded(
        [ ...                   % the application itself
        ]).

The  `use_module' declarations  will  import the  public predicates  from
the  generic  modules  into  the  user  module.     The  `ensure_loaded'
directive  loads the  modules that  constitute  the actual  application.
It is  assumed these  modules import  predicates from  each other  using
use_module/[1,2] as far as necessary.

In combination  with the  object-oriented schema described  below it  is
possible  to define  a neat  modular  architecture.   The  generic  code
defines general utilities  and the message passing predicates  (invoke/3
in the  example below).    The application modules  define classes  that
communicate using the message passing predicates.


55..55..11 OObbjjeecctt OOrriieenntteedd PPrrooggrraammmmiinngg

Another typical  way to use  the module system  is for defining  classes
within  an object  oriented  paradigm.    The  class structure  and  the
methods  of  a class  can  be  defined  in a  module  and  the  explicit
module-boundary overruling  describes in  section 5.6.2 can  by used  by
the message passing  code to invoke the behaviour.   An outline of  this
mechanism is given below.

%       Define class point

:- module(point, []).           % class point, no exports

%        name           type,           default access
%                                       value

variable(x,             integer,        0,      both).
variable(y,             integer,        0,      both).

%         method name   predicate name  arguments

behaviour(mirror,       mirror,         []).

mirror(P) :-
        fetch(P, x, X),
        fetch(P, y, Y),
        store(P, y, X),
        store(P, x, Y).

The predicates fetch/3  and store/3 are predicates that change  instance
variables of instances.  The figure below indicates  how message passing
can easily be implemented:

%       invoke(+Instance, +Selector, ?ArgumentList)
%       send a message to an instance

invoke(I, S, Args) :-
        class_of_instance(I, Class),
        Class:behaviour(S, P, ArgCheck), !,
        convert_arguments(ArgCheck, Args, ConvArgs),
        Goal =.. [P|ConvArgs],
        Class:Goal.

The construct  <_M_o_d_u_l_e>:<_G_o_a_l> explicitly  calls _G_o_a_l  in module  _M_o_d_u_l_e.
It is discussed in more detail in section 5.6.


55..66 MMeettaa--PPrreeddiiccaatteess iinn MMoodduulleess

As indicated  in the introduction,  the problem  with a predicate  based
module system lies in the difficulty to find the  correct predicate from
a Prolog  term.   The predicate `solution(Solution)'  can exist in  more
than one  module, but `assert(solution(4))'  in some module is  supposed
to refer to the correct version of solution/1.

Various  approaches are  possible to  solve this  problem.    One is  to
add an extra  argument to all predicates (e.g. `assert(Module,  Term)').
Another is to tag  the term somehow to indicate which module  is desired
(e.g. `assert(Module:Term)').   Both approaches are not very  attractive
as  they make  the user  responsible for  choosing  the correct  module,
inviting  unclear  programming by  asserting  in  other modules.     The
predicate assert/1  is supposed  to assert  in the module  it is  called
from and should do  so without being told explicitly.  For  this reason,
the notion _c_o_n_t_e_x_t _m_o_d_u_l_e has been introduced.


55..66..11 DDeeffiinniittiioonn aanndd CCoonntteexxtt MMoodduullee

Each  predicate  of  the  program  is  assigned  a  module,  called  its
_d_e_f_i_n_i_t_i_o_n _m_o_d_u_l_e.   The definition module of a predicate is  always the
module in which the predicate was originally defined.   Each active goal
in the Prolog system has a _c_o_n_t_e_x_t _m_o_d_u_l_e assigned to it.

The  context module  is used  to  find predicates  from a  Prolog  term.
By  default, this  module  is the  definition  module of  the  predicate
running the  goal.   For meta-predicates  however, this  is the  context
module of the  goal that invoked them.   We call this _m_o_d_u_l_e___t_r_a_n_s_p_a_r_e_n_t
in SWI-Prolog.    In the `using  maplist' example  above, the  predicate
maplist/3  is declared  module_transparent.    This  implies the  context
module remains extend, the  context module of add_extension/3.   This way
maplist/3 can decide to call extend_atom in module extend rather than in
its own definition module.

All built-in predicates that  refer to predicates via a Prolog  term are
declared module_transparent.  Below is the code defining maplist.

:- module(maplist, maplist/3).

:- module_transparent maplist/3.

%       maplist(+Goal, +List1, ?List2)
%       True if Goal can successfully be applied to all succes-
sive pairs
%       of elements of List1 and List2.

maplist(_, [], []).
maplist(Goal, [Elem1|Tail1], [Elem2|Tail2]) :-
        apply(Goal, [Elem1, Elem2]),
        maplist(Goal, Tail1, Tail2).


55..66..22 OOvveerrrruulliinngg MMoodduullee BBoouunnddaarriieess

The  mechanism  above  is sufficient  to  create  an  acceptable  module
system.   There are however cases in which  we would like to be  able to
overrule this  schema and  explicitly call  a predicate  in some  module
or assert  explicitly in some  module.   The first  is useful to  invoke
goals  in some  module  from the  user's  top-level  or to  implement  a
object-oriented system (see above).  The latter is  useful to create and
modify _d_y_n_a_m_i_c _m_o_d_u_l_e_s (see section 5.7).

For  this purpose,  the reserved  term :/2  has  been introduced.    All
built-in predicates  that transform  a term into  a predicate  reference
will check whether  this term is of  the form `<_M_o_d_u_l_e>:<_T_e_r_m>'.  If  so,
the predicate is  searched for in _M_o_d_u_l_e  instead of the goal's  context
module.   The :  operator may be  nested, in  which case the  inner-most
module is used.

The special  calling construct  <_M_o_d_u_l_e>:<_G_o_a_l>pretends  _G_o_a_l is  called
from _M_o_d_u_l_e instead of the context module.  Examples:

?- assert(world:done).  % asserts done/0 into module world
?- world:assert(done).  % the same
?- world:done.          % calls done/0 in module world


55..77 DDyynnaammiicc MMoodduulleess

So  far,   we  discussed  modules  that   were  created  by  loading   a
module-file.     These  modules  have  been   introduced  on  facilitate
the  development  of  large  applications.     The   modules  are  fully
defined at  load-time of the  application and  normally will not  change
during  execution.   Having  the  notion of  a set  of  predicates as  a
self-contained world can be attractive for other purposes as  well.  For
example, assume  an application that  can reason about multiple  worlds.
It is attractive  to store the data of  a particular world in a  module,
so we extract information from a world simply by  invoking goals in this
world.

Dynamic modules  can easily  be created.   Any  built-in predicate  that
tries  to locate  a predicate  in  a specific  module will  create  this
module as a side-effect if it did not yet exist.  Example:

?- assert(world_a:consistent),
   world_a:unknown(_, fail).

These  calls  create  a  module  called  `world_a'  and  make  the  call
`world_a:consistent'  succeed.  Undefined  predicates will not start  the
tracer or autoloader for this module (see unknown/2).

Import and  export from dynamically  created world  is arranged via  the
predicates import/1 and export/1:

?- world_b:export(solve(_,_)).          % exports solve/2 from world_b
?- world_c:import(world_b:solve(_,_)).  % and import it to world_c


55..88 MMoodduullee HHaannddlliinngg PPrreeddiiccaatteess

This section gives the predicate definitions for  the remaining built-in
predicates that handle modules.


::-- mmoodduullee((_+_M_o_d_u_l_e_, _+_P_u_b_l_i_c_L_i_s_t))
    This  directive can  only  be used  as the  first term  of a  source
    file.   It declares  the file to be  a _m_o_d_u_l_e _f_i_l_e, defining  _M_o_d_u_l_e
    and  exporting the predicates of _P_u_b_l_i_c_L_i_s_t.   _P_u_b_l_i_c_L_i_s_t is a  list
    of predicate  indicators (name/arity pairs) or operator declarations
    using  the format  op(_P_r_e_c_e_d_e_n_c_e_, _T_y_p_e_,  _N_a_m_e).   Operators  defined
    in  the export list  are available inside the  module as well as  to
    modules importing this module.  See also section 4.24.


mmoodduullee__ttrraannssppaarreenntt _+_P_r_e_d_s
    _P_r_e_d_s   is  a  comma  separated  list  of  name/arity   pairs  (like
    dynamic/1).     Each goal  associated  with a  transparent  declared
    predicate will inherit the _c_o_n_t_e_x_t _m_o_d_u_l_e from its parent goal.


mmeettaa__pprreeddiiccaattee _+_H_e_a_d_s
    This  predicate  is  defined  in  quintus  and  provides  a  partial
    emulation of the Quintus predicate.  See section 5.9.1 for details.


ccuurrrreenntt__mmoodduullee((_-_M_o_d_u_l_e))
    Generates all currently known modules.


ccuurrrreenntt__mmoodduullee((_?_M_o_d_u_l_e_, _?_F_i_l_e))
    Is  true if _F_i_l_e is the file from which _M_o_d_u_l_e was loaded.   _F_i_l_e is
    the internal canonical filename.  See also source_file/[1,2].


ccoonntteexxtt__mmoodduullee((_-_M_o_d_u_l_e))
    Unify   _M_o_d_u_l_e  with  the  context  module  of  the   current  goal.
    context_module/1 itself is transparent.


ssttrriipp__mmoodduullee((_+_T_e_r_m_, _-_M_o_d_u_l_e_, _-_P_l_a_i_n))
    Used  in  module  transparent  or  meta-predicates  to  extract  the
    referenced  module and plain  term.   If _T_e_r_m is a  module-qualified
    term, i.e. of  the format _M_o_d_u_l_e:_P_l_a_i_n, _M_o_d_u_l_e and _P_l_a_i_n are unified
    to  these values.  Otherwise _P_l_a_i_n is unified to _T_e_r_m and  _M_o_d_u_l_e to
    the context module.


eexxppoorrtt((_+_H_e_a_d))
    Add  a predicate to  the public list  of the context  module.   This
    implies  the predicate will be imported into another module  if this
    module is  imported with use_module/[1,2].   Note that predicates are
    normally  exported using the directive module/2.  export/1  is meant
    to handle export from dynamically created modules.


eexxppoorrtt__lliisstt((_+_M_o_d_u_l_e_, _?_E_x_p_o_r_t_s))
    Unifies  _E_x_p_o_r_t_s with a list of terms.   Each term has the  name and
    arity  of a public predicate of _M_o_d_u_l_e.   The order of the  terms in
    _E_x_p_o_r_t_s is not defined.  See also predicate_property/2.


iimmppoorrtt__mmoodduullee((_+_M_o_d_u_l_e_, _-_I_m_p_o_r_t))
    True  if _I_m_p_o_r_t  is defined as  an import  module for _M_o_d_u_l_e.    All
    normal  modules only import  from user,  which imports from  system.
    The predicates  add_import_module/3and delete_import_module/2 can be
    used to manipulate the import list.


aadddd__iimmppoorrtt__mmoodduullee((_+_M_o_d_u_l_e_, _+_I_m_p_o_r_t_, _+_S_t_a_r_t_O_r_E_n_d))
    If  _I_m_p_o_r_t is not  already an  import module for  _M_o_d_u_l_e, add it  to
    this  list at the start  or end depending on  _S_t_a_r_t_O_r_E_n_d.  See  also
    import_module/2 and delete_import_module/2.


ddeelleettee__iimmppoorrtt__mmoodduullee((_+_M_o_d_u_l_e_, _+_I_m_p_o_r_t))
    Delete  _I_m_p_o_r_t from the  list of import modules  for _M_o_d_u_l_e.   Fails
    silently if _I_m_p_o_r_t is not in the list.


ddeeffaauulltt__mmoodduullee((_+_M_o_d_u_l_e_, _-_D_e_f_a_u_l_t))
    Successively  unifies _D_e_f_a_u_l_t  with the  module names  from which  a
    call  in _M_o_d_u_l_e  attempts to  use the definition.    For the  module
    user,  this will generate  user and system.   For any other  module,
    this will generate the module itself, followed by user and system.

    Backward compatibility.  New code should use import_module/2.


mmoodduullee((_+_M_o_d_u_l_e))
    The  call module(Module) may be  used to switch the default  working
    module  for the interactive top-level (see  prolog/0).  This may  be
    used  to when  debugging  a module.    The example  below lists  the
    clauses of file_of_label/2 in the module tex.

    1 ?- module(tex).

    Yes
    tex: 2 ?- listing(file_of_label/2).
    ...


55..99 CCoommppaattiibbiilliittyy ooff tthhee MMoodduullee SSyysstteemm

The  principles behind  the  module system  of  SWI-Prolog differ  in  a
number of aspects from the Quintus Prolog module system.

  o The  SWI-Prolog module  system allows  the user  to redefine  system
    predicates.

  o All  predicates that are  available in the  system and user  modules
    are visible in all other modules as well.

  o Quintus  has the `meta_predicate/1' declaration were SWI-Prolog  has
    the module_transparent/1 declaration.

  o Operator declarations are local to a module and may be exported.

The meta_predicate/1 declaration  causes the  compiler to tag  arguments
that pass  module sensitive information  with the  module using the  :/2
operator.  This approach has some disadvantages:

  o Changing   a  meta_predicate   declaration  implies  all   predicates
    ccaalllliinngg  the predicate need to be reloaded.  This can  cause serious
    consistency problems.

  o It  does not help for dynamically defined predicates  calling module
    sensitive predicates.

  o It   slows  down   the  compiler   (at  least   in  the   SWI-Prolog
    architecture).

  o At  least within the  SWI-Prolog architecture the run-time  overhead
    is   larger  than  the   overhead  introduced  by  the   transparent
    mechanism.

Unfortunately  the   transparent  predicate   approach  also  has   some
disadvantages.   If a  predicate A passes  module sensitive information
to a  predicate B, passing  the same information to  a module sensitive
system predicate both  A and B  should be declared transparent.   Using
the Quintus approach only A needs to be treated special (i.e., declared
with  meta_predicate/1).    A  second problem  arises if  the  body of  a
transparent  predicate uses  module sensitive  predicates  for which  it
wants to refer to its  own module.  Suppose we want to  define findall/3
using assert/1  and retract/1.    The example  in figure  5.1 gives  the
solution.
:- module(findall, [findall/3]).

:- dynamic

        solution/1.

:- module_transparent
        findall/3,
        store/2.

findall(Var, Goal, Bag) :-

        assert(findall:solution('$mark')),
        store(Var, Goal),
        collect(Bag).

store(Var, Goal) :-
        Goal,                   % refers to context module of
                                % caller of findall/3
        assert(findall:solution(Var)),

        fail.
store(_, _).

collect(Bag) :-
        ...,

                  Figure 5.1:  findall/3 using modules


55..99..11 EEmmuullaattiinngg mmeettaa__pprreeddiiccaattee

The Quintus  meta_predicate/1 directive can  in many  cases be  replaced
by  the   transparent  declaration.      Below  is  the  definition   of
meta_predicate/1 as available from quintus.

:- op(1150, fx, (meta_predicate)).

meta_predicate((Head, More)) :- !,
        meta_predicate1(Head),
        meta_predicate(More).
meta_predicate(Head) :-
        meta_predicate1(Head).

meta_predicate1(Head) :-
        Head =.. [Name|Arguments],
        member(Arg, Arguments),
        module_expansion_argument(Arg), !,
        functor(Head, Name, Arity),
        module_transparent(Name/Arity).
meta_predicate1(_).             % just a mode declaration

module_expansion_argument(:).
module_expansion_argument(N) :- integer(N).

The discussion above  about the problems with the transparent  mechanism
show the two cases in which this simple transformation does not work.


CChhaapptteerr 66..  SSPPEECCIIAALL VVAARRIIAABBLLEESS AANNDD CCOORROOUUTTIINNIINNGG

This  chapter  deals  with  extensions  primarily  designed  to  support
constraint logic programming (CLP).


66..11 AAttttrriibbuutteedd vvaarriiaabblleess

_A_t_t_r_i_b_u_t_e_d  _v_a_r_i_a_b_l_e_s  provide  a technique  for  extending  the  Prolog
unification  algorithm  [Holzbaur, 1990]  by  hooking  the   binding  of
attributed  variables.     There  is  little  consensus  in  the  Prolog
community  on   the  exact  definition   and  interface  to   attributed
variables.   The SWI-Prolog interface is  identical to the one  realised
by Bart Demoen for hProlog [Demoen, 2002].

Binding  an attributed  variable  schedules a  goal  to be  executed  at
the  first possible  opportunity.   In  the  current implementation  the
hooks are  executed immediately  after a successful  unification of  the
clause-head or  successful completion of  a foreign language  (built-in)
predicate.    Each attribute  is associated  to a  module  and the  hook
(attr_unify_hook/2)  is  executed in  this  module.   The  example  below
realises a very simple and incomplete finite domain reasoner.

:- module(domain,
          [ domain/2                    % Var, ?Domain
          ]).
:- use_module(library(ordsets)).

domain(X, Dom) :-
        var(Dom), !,
        get_attr(X, domain, Dom).
domain(X, List) :-
        list_to_ordset(List, Domain),
        put_attr(Y, domain, Domain),
        X = Y.

%       An attributed variable with attribute value Domain has been
%       assigned the value Y

attr_unify_hook(Domain, Y) :-
        (   get_attr(Y, domain, Dom2)
        ->  ordset_intersection(Domain, Dom2, NewDomain),
            (   NewDomain == []
            ->  fail
            ;   NewDomain = [Value]
            ->  Y = Value
            ;   put_attr(Y, domain, NewDomain)
            )
        ;   var(Y)
        ->  put_attr( Y, domain, Domain )
        ;   ordset_memberchk(Y, Domain)
        ).

Before explaining the code we give some example queries:

 ?- domain(X, [a,b]), X = c                no
 ?- domain(X, [a,b]), domain(X, [a,c]).    X = a
 ?- domain(X, [a,b,c]), domain(X, [a,c]).  X = _G492att(domain, [a, c], [])

The  predicate  domain/2  fetches  (first  clause)  or  assigns  (second
clause) the variable a  _d_o_m_a_i_n, a set of values it can be  unified with.
In the second clause  first associates the domain with a  fresh variable
and then unifies X to this variable to deal with  the possibility that X
already has a domain.   The predicate attr_unify_hook/2is a  hook called
after a variable with a domain is assigned a value.   In the simple case
where the variable is bound to a concrete value  we simply check whether
this value is in the domain.  Otherwise we take  the intersection of the
domains and  either fail if the  intersection is empty (first  example),
simply assign the value  if there is only one value in  the intersection
(second example)  or assign the  intersection as the  new domain of  the
variable (third example).


aattttvvaarr((_@_T_e_r_m))
    Succeeds  if _T_e_r_m is an attributed  variable.  Note that var/1  also
    succeeds on attributed  variables.  Attributed variables are created
    with put_attr/3.


ppuutt__aattttrr((_+_V_a_r_, _+_M_o_d_u_l_e_, _+_V_a_l_u_e))
    If  _V_a_r is  a variable  or attributed  variable, set  the value  for
    the  attribute named _M_o_d_u_l_e  to _V_a_l_u_e.   If  an attribute with  this
    name  is already  associated with  _V_a_r, the old  value is  replaced.
    Backtracking  will restore  the old  value (i.e. an  attribute is  a
    mutable  term.  See  also setarg/3).   This predicate raises a  type
    error if _V_a_r is not a variable or _M_o_d_u_l_e is not an atom.


ggeett__aattttrr((_+_V_a_r_, _+_M_o_d_u_l_e_, _-_V_a_l_u_e))
    Request  the  current _v_a_l_u_e  for the  attribute named  _M_o_d_u_l_e.    If
    _V_a_r  is not  an attributed variable  or the  named attribute is  not
    associated  to _V_a_r this predicate fails silently.  If _M_o_d_u_l_e  is not
    an atom, a type error is raised.


ddeell__aattttrr((_+_V_a_r_, _+_M_o_d_u_l_e))
    Delete  the named attribute.  If _V_a_r loses its last attribute  it is
    transformed  back into a traditional Prolog variable.  If  _M_o_d_u_l_e is
    not  an atom,  a type  error is  raised.   In all  other cases  this
    predicate succeeds regardless  whether or not the named attribute is
    present.


aattttrr__uunniiffyy__hhooookk((_+_A_t_t_V_a_l_u_e_, _+_V_a_r_V_a_l_u_e))
    Hook  that must  be  defined in  the module  an attributed  variable
    refers  to.   Is is  called _a_f_t_e_r the  attributed variable has  been
    unified  with a non-var term, possibly another  attributed variable.
    _A_t_t_V_a_l_u_e  is  the  attribute that  was  associated to  the  variable
    in  this  module and  _V_a_r_V_a_l_u_e is  the new  value  of the  variable.
    Normally  this  predicate  fails to  veto  binding the  variable  to
    _V_a_r_V_a_l_u_e,  forcing backtracking to  undo the binding.   If  _V_a_r_V_a_l_u_e
    is  another  attributed variable  the hook  often  combines the  two
    attribute and associates  the combined attribute with _V_a_r_V_a_l_u_e using
    put_attr/3.


aattttrr__ppoorrttrraayy__hhooookk((_+_A_t_t_V_a_l_u_e_, _+_V_a_r))
    Called by  write_term/2and friends for each  attribute if the option
    attributes(_p_o_r_t_r_a_y)  is  in  effect.    If  the  hook  succeeds  the
    attribute is considered  printed.  Otherwise Module = ... is printed
    to indicate the existence of a variable.


66..11..11 SSppeecciiaall ppuurrppoossee pprreeddiiccaatteess ffoorr aattttrriibbuutteess

Normal user code should deal with put_attr/3, get_attr/3 and del_attr/2.
The routines in this  section fetch or set the entire attribute  list of
a variables.   Use of these  predicates is anticipated to be  restricted
to printing and other special purpose operations.


ggeett__aattttrrss((_+_V_a_r_, _-_A_t_t_r_i_b_u_t_e_s))
    Get  all  attributes of  _V_a_r.   _A_t_t_r_i_b_u_t_e_s  is a  term  of the  form
    att(_M_o_d_u_l_e_,  _V_a_l_u_e_, _M_o_r_e_A_t_t_r_i_b_u_t_e_s), where _M_o_r_e_A_t_t_r_i_b_u_t_e_s is  [] for
    the last attribute.


ppuutt__aattttrrss((_+_V_a_r_, _-_A_t_t_r_i_b_u_t_e_s))
    Set  all attributes of  _V_a_r.  See  get_attrs/2 for a description  of
    _A_t_t_r_i_b_u_t_e_s.


ccooppyy__tteerrmm__nnaatt((_+_T_e_r_m_, _-_C_o_p_y))
    As copy_term/2.  Attributes however, are _n_o_t  copied but replaced by
    fresh variables.


66..22 CCoorroouuttiinniinngg

Coroutining deals  with having Prolog goals  scheduled for execution  as
soon as some conditions is fulfilled.  In Prolog  the most commonly used
conditions is the instantiation  (binding) of a variable.   Scheduling a
goal to  execute immediately  after a  variable is bound  allows may  be
used to  avoid instantiation errors for  some built-in predicates  (e.g.
arithmetic),  do work  _l_a_z_y,  prevent the  binding of  a  variable to  a
particular value,  etc.   Using  freeze/2 for  example we  can define  a
variable can only be assigned an even number:

?- freeze(X, X mod 2 =:= 0), X = 3

No


ffrreeeezzee((_+_V_a_r_, _:_G_o_a_l))
    Delay  the execution  of  _G_o_a_l until  _V_a_r is  bound (i.e.  is not  a
    variable  or  attributed  variable).    If  _V_a_r is  bound  on  entry
    freeze/2  is  equivalent  to call/1.     The freeze/2  predicate  is
    realised  using an  attributed variable associated  with the  module
    freeze,  so get_attr(Var, freeze, AttVal)  can be used  to find  out
    whether and which goals are delayed on _V_a_r.


ffrroozzeenn((_@_V_a_r_, _-_G_o_a_l))
    Unify  _G_o_a_l with the  goal or conjunction  of goals delayed on  _V_a_r.
    If no goals are frozen on _V_a_r, _G_o_a_l is unified to true.


wwhheenn((_@_C_o_n_d_i_t_i_o_n_, _:_G_o_a_l))
    Execute  _G_o_a_l when  _C_o_n_d_i_t_i_o_n becomes  true.   _C_o_n_d_i_t_i_o_n  is one  of
    ?=(_X_, _Y), nonvar(_X), ground(_X),  ,(_C_o_n_d_1_, _C_o_n_d_2) or ;(_C_o_n_d_1_, _C_o_n_d_2).
    See  also freeze/2  and dif/2.    The implementation  can deal  with
    cyclic terms.

    The   when/2  predicate  is   realised  using  attributed   variable
    associated  with the module  when.   It is  defined in the  autoload
    library when.


ddiiff((_@_A_, _@_B))
    The  dif/2 predicate  provides  a constraint  stating that  _A and  _B
    are  different terms.   If _A  and _B can  never unify dif/2  succeeds
    deterministically.   If _A and  _B are identical it fails  immediately
    and  finally,  if  _A  and  _B  can  unify,  goals  are  delayed  that
    prevent  _A and _B to  become equal.   The dif/2 predicate behaves  as
    if  defined by dif(X, Y) :- when(?=(X, Y), X \== Y). See also  ?=/2.
    The implementation can deal with cyclic terms.

    The   dif/2  predicate   is  realised   using  attributed   variable
    associated  with the  module dif.   It  is defined  in the  autoload
    library dif.


66..33 GGlloobbaall vvaarriiaabblleess

Global  variables are  associations  between  names (atoms)  and  terms.
They differ in  various ways from storing information using  assert/1 or
recorda/3.

  o The  value lives on  the Prolog (global) stack.   This implies  that
    lookup  time is  independent from the  size of  the term.   This  is
    particularly  interesting for large  data structures such as  parsed
    XML documents or the CHR global constraint store.

  o They   support  both   global  assignment   using  nb_setval/2   and
    backtrackable assignment using b_setval/2.

  o Only  one value (which can be an arbitrary complex Prolog  term) can
    be associated to a variable at a time.

  o Their  value cannot be  shared among threads.   Each thread has  its
    own namespace and values for global variables.

  o Currently  global variables are  scoped globally.   We may  consider
    module scoping in future versions.

Both  b_setval/2 and nb_setval/2  implicitly create  a  variable if  the
referenced name does not already refer to a variable.

Global  variables  may  be initialised  from  directives  to  make  them
available  during the  program  lifetime,  but some  considerations  are
necessary  for saved-states  and threads.    Saved-states  to not  store
global  variables,   which  implies  they  have  to  be   declared  with
initialization/1 to recreate them  after loading the saved state.   Each
thread  has its  own set  of global  variables, starting  with an  empty
set.    Using thread_initialization/1  to define  a global  variable  it
will be  defined, restored  after reloading  a saved  state and  created
in  all threads  that are  created  _a_f_t_e_r the  registration.    Finally,
global  variables can  be initialised  using the  exception hook  called
exception/3.  The latter technique is by CHR (see chapter 7.


bb__sseettvvaall((_+_N_a_m_e_, _+_V_a_l_u_e))
    Associate  the  term  _V_a_l_u_e  with  the atom  _N_a_m_e  or  replaces  the
    currently  associated value  with _V_a_l_u_e.    If _N_a_m_e  does not  refer
    to  an existing  global variable  a variable with  initial value  []
    is  created (the  empty list).   On  backtracking the assignment  is
    reversed.


bb__ggeettvvaall((_+_N_a_m_e_, _-_V_a_l_u_e))
    Get the value  associated with the global variable _N_a_m_e and unify it
    with _V_a_l_u_e.   Note that this unification may further instantiate the
    value  of the global  variable.  If  this is undesirable the  normal
    precautions  (double negation or  copy_term/2) must be  taken.   The
    b_getval/2 predicate generates errors if _N_a_m_e is not  an atom or the
    requested variable does not exist.


nnbb__sseettvvaall((_+_N_a_m_e_, _+_V_a_l_u_e))
    Associates  a copy of  _V_a_l_u_e created with duplicate_term/2 with  the
    atom  _N_a_m_e.   Note that  this can be  used to  set an initial  value
    other than [] prior to backtrackable assignment.


nnbb__ggeettvvaall((_+_N_a_m_e_, _-_V_a_l_u_e))
    The  nb_getval/2 predicate is  a synonym for  b_getval/2,  introduced
    for  compatibility  and  symmetry.    As  most  scenarios  will  use
    a  particular  global  variable  either  using  non-backtracable  or
    backtrackable assignment,  using nb_getval/2can be  used to document
    that the variable is used non-backtracable.


nnbb__lliinnkkvvaall((_+_N_a_m_e_, _+_V_a_l_u_e))
    Associates  the term _V_a_l_u_e  with the atom  _N_a_m_e without copying  it.
    This  is a  fast special-purpose  variation of  nb_setval/2 intended
    for  expert users only  because the semantics  on backtracking to  a
    point  before  creating the  link are  poorly  defined for  compound
    terms.      The  principal  term  is  always  left  untouched,   but
    backtracking  behaviour  on  arguments  is undone  if  the  original
    assignment  was  _t_r_a_i_l_e_d and  left  alone otherwise,  which  implies
    that  the history  that created  the term affects  the behaviour  on
    backtracking.  Please consider the following example:

    demo_nb_linkval :-
            T = nice(N),
            (   N = world,
                nb_linkval(myvar, T),
                fail
            ;   nb_getval(myvar, V),
                writeln(V)
            ).


nnbb__ccuurrrreenntt((_?_N_a_m_e_, _?_V_a_l_u_e))
    Enumerate  all defined  variables with their  value.   The order  of
    enumeration is undefined.


nnbb__ddeelleettee((_+_N_a_m_e))
    Delete the named global variable.


66..33..11 CCoommppaattiibbiilliittyy ooff SSWWII--PPrroolloogg GGlloobbaall VVaarriiaabblleess

Global variables have been introduced by  various Prolog implementations
recently.  The implementation of them in SWI-Prolog  is based on hProlog
by  Bart Demoen.    In  discussion with  Bart it  was  decided that  the
semantics if hProlog nb_setval/2, which is equivalent to nb_linkval/2 is
not acceptable  for normal Prolog users  as the behaviour is  influenced
by  how built-in  predicates constructing  terms (read/1,  =../2,  etc.)
are implemented.

GNU-Prolog provides  a rich set of  global variables, including  arrays.
Arrays  can be  implemented  easily in  SWI-Prolog using  functor/3  and
setarg/3 due to the unrestricted arity of compound terms.


CChhaapptteerr 77..  CCHHRR:: CCOONNSSTTRRAAIINNTT HHAANNDDLLIINNGG RRUULLEESS

This chapter is written by Tom Schrijvers, K.U.  Leuven, and adjustments
by Jan Wielemaker.

The CHR system of SWI-Prolog is the _K_._U_._L_e_u_v_e_n _C_H_R _s_y_s_t_e_m.   The runtime
environment is  written by Christian Holzbaur  and Tom Schrijvers  while
the compiler  is written by  Tom Schrijvers.   Both are integrated  with
SWI-Prolog  and licensed  under  compatible conditions  with  permission
from the authors.

The main reference for the K.U.Leuven CHR system is:

  o T.   Schrijvers,  and   B.  Demoen,   _T_h_e  _K_._U_._L_e_u_v_e_n  _C_H_R   _S_y_s_t_e_m_:
    _I_m_p_l_e_m_e_n_t_a_t_i_o_n   _a_n_d  _A_p_p_l_i_c_a_t_i_o_n,  First  Workshop   on  Constraint
    Handling Rules:   Selected Contributions (Fr"uhwirth, T. and Meister,
    M., eds.), pp.  1--5, 2004.

On  the K.U.Leuven  CHR  website  (http://www.cs.kuleuven.be/~toms/CHR/)
you can find more related papers, references and example programs.


77..11 IInnttrroodduuccttiioonn

Constraint  Handling  Rules  (CHR)  is  a   committed-choice  rule-based
language embedded  in Prolog.    It is designed  for writing  constraint
solvers and  is particularly  useful for providing  application-specific
constraints.   It  has been  used in  many kinds  of applications,  like
scheduling, model checking, abduction, type checking among many others.

CHR has  previously been implemented in  other Prolog systems  (SICStus,
Eclipse,  Yap), Haskell  and Java.    This CHR  system is  based on  the
compilation scheme and runtime environment of CHR in SICStus.

In this documentation  we restrict ourselves to giving a  short overview
of  CHR  in general  and  mainly  focus  on elements  specific  to  this
implementation.    For  a  more thorough  review  of  CHR we  refer  the
reader to  [Fr"uhwirth, 1998].   More background on CHR  can be found  at
[Fr"uhwirth,].

In  section 7.2  we present  the syntax  of CHR  in  Prolog and  explain
informally its  operational semantics.    Next, section  7.3 deals  with
practical issues  of writing  and compiling  Prolog programs  containing
CHR.  Section  7.4  explains  the  currently   primitive  CHR  debugging
facilities.  Section  7.4.3 provides a few useful predicates  to inspect
the constraint  store and section 7.5  illustrates CHR with two  example
programs.  In section 7.6 some compatibility issues  with older versions
of this system and SICStus' CHR system.   Finally, section 7.7 concludes
with a few practical guidelines for using CHR.


77..22 SSyynnttaaxx aanndd SSeemmaannttiiccss


77..22..11 SSyynnttaaxx

The syntax of CHR rules is the following:

rules --> rule, rules.
rules --> [].

rule --> name, actual_rule, pragma, [atom('.')].

name --> atom, [atom('@')].
name --> [].

actual_rule --> simplification_rule.
actual_rule --> propagation_rule.
actual_rule --> simpagation_rule.

simplification_rule --> head, [atom('<=>')], guard, body.
propagation_rule --> head, [atom('==>')], guard, body.
simpagation_rule --> head, [atom('\')], head, [atom('<=>')],
                     guard, body.

head --> constraints.

constraints --> constraint, constraint_id.
constraints --> constraint, constraint_id, [atom(',')], constraints.

constraint --> compound_term.

constraint_id --> [].
constraint_id --> [atom('#')], variable.
constraint_id --> [atom('#')], [atom('passive')] .

guard --> [].
guard --> goal, [atom('|')].

body --> goal.

pragma --> [].
pragma --> [atom('pragma')], actual_pragmas.

actual_pragmas --> actual_pragma.
actual_pragmas --> actual_pragma, [atom(',')], actual_pragmas.

actual_pragma --> [atom('passive(')], variable, [atom(')')].

Note that  the guard of  a rule may  not contain any  goal that binds  a
variable in  the head of  the rule with a  non-variable or with  another
variable in the  head of the rule.   It may however bind variables  that
do  not appear  in the  head of  the rule,  e.g.  an auxiliary  variable
introduced in the guard.


77..22..22 SSeemmaannttiiccss

In  this subsection  the  operational semantics  of  CHR in  Prolog  are
presented informally.   They  do not differ  essentially from other  CHR
systems.

When a constraint is  called, it is considered an active  constraint and
the system  will try  to apply the  rules to it.    Rules are tried  and
executed sequentially in the order they are written.

A rule is conceptually  tried for an active constraint in  the following
way.  The active constraint is matched with a constraint  in the head of
the rule.   If more constraints appear  in the head they are looked  for
among the  suspended constraints, which  are called passive  constraints
in this context.  If the necessary passive constraints  can be found and
all match with the head of the rule and the guard  of the rule succeeds,
then the rule  is committed and the body of  the rule executed.   If not
all the  necessary passive constraint can  be found, the matching  fails
or the  guard fails, then the  body is not  executed and the process  of
trying and  executing simply  continues with  the following rules.    If
for a  rule,  there are  multiple constraints  in the  head, the  active
constraint  will try  the rule  sequentially multiple  times, each  time
trying to match with another constraint.

This process ends either when the active constraint  disappears, i.e. it
is removed by some rule, or after the last rule has  been processed.  In
the latter case the active constraint becomes suspended.

A  suspended constraint  is  eligible as  a  passive constraint  for  an
active constraint.  The other way it may interact  again with the rules,
is when a variable  appearing in the constraint becomes bound  to either
a non-variable or another variable involved in one  or more constraints.
In that  case the  constraint is triggered,  i.e. it  becomes an  active
constraint and all the rules are tried.

RRuullee TTyyppeess There  are three different  kinds of  rules, each with  their
specific semantics:

  o _s_i_m_p_l_i_f_i_c_a_t_i_o_n
    The  simplification rule  removes the  constraints in  its head  and
    calls its body.

  o _p_r_o_p_a_g_a_t_i_o_n
    The   propagation  rule  calls  its   body  exactly  once  for   the
    constraints in its head.

  o _s_i_m_p_a_g_a_t_i_o_n
    The  simpagation rule removes the constraints in its head  after the
    \ and then  calls its body.  It is an optimization of simplification
    rules of the form:

                  constraints1;constraints2<=> constraints1;body

    Namely, in the simpagation form:

                       constraints1\constraints2<=> body
    The constraints1constraints are not called in the body.

RRuullee NNaammeess

Naming  a rule  is optional  and has  no semantical  meaning.   It  only
functions as documentation for the programmer.

PPrraaggmmaass The semantics of the pragmas are:

ppaassssiivvee((_I_d_e_n_t_i_f_i_e_r))
    The  constraint in the  head of a rule  _I_d_e_n_t_i_f_i_e_r can only match  a
    passive  constraint in that  rule.   There is an abbreviated  syntax
    for this pragma.  Instead of:

                    ..., c # Id, ... <=> ... pragma passive(Id)

    you can also write

                    ..., c # passive, ... <=> ...

Additional pragmas may be released in the future.


::-- cchhrr__ooppttiioonn((_+_O_p_t_i_o_n_, _+_V_a_l_u_e))
    It  is  possible  to specify  options  that  apply to  all  the  CHR
    rules  in the module.   Options are specified  with the chr_option/2
    declaration:

    :- chr_option(Option,Value).

    and  may appear  in the  file anywhere after  the first  constraints
    declaration.

    Available options are:

    cchheecckk__gguuaarrdd__bbiinnddiinnggss
         This  option controls  whether  guards  should be  checked  for
         (illegal) variable bindings or  not.  Possible values  for this
         option are on,  to enable the checks,  and off, to disable  the
         checks.   If this option is on,  any guard fails when it  binds
         a variable  that appears in  the head of  the rule.   When  the
         option is  off  (default), the  behavior of  a  binding in  the
         guard is undefined.

    ooppttiimmiizzee
         This option  controls the  degree  of optimization.    Possible
         values are  full, to  enable all  available optimizations,  and
         off (default),  to  disable  all optimizations.    The  default
         is derived  from the  SWI-Prolog flag optimise,  where true  is
         mapped to full.  Therefore the command-line  option -O provides
         full CHR optimization.   If optimization is enabled,  debugging
         must be disabled.

    ddeebbuugg
         This options enables or  disables the possibility to debug  the
         CHR code.    Possible values  are on (default)  and off.    See
         section 7.4  for more  details on debugging.    The default  is
         derived  from the  prolog  flag  generate_debug_info,  which  is
         true by  default.   See  -nodebug.   If  debugging is  enabled,
         optimization must be disabled.


77..33 CCHHRR iinn SSWWII--PPrroolloogg PPrrooggrraammss


77..33..11 EEmmbbeeddddiinngg iinn PPrroolloogg PPrrooggrraammss

The CHR constraints defined in a .pl file are  associated with a module.
The default module is  user.  One should never load different  .pl files
with the same CHR module name.


77..33..22 CCoonnssttrraaiinntt ddeeccllaarraattiioonn


::-- cchhrr__ccoonnssttrraaiinntt((_+_S_p_e_c_i_f_i_e_r))
    Every  constraint  used in  CHR  rules has  to  be declared  with  a
    chr_constraint/1  declaration  by the  _c_o_n_s_t_r_a_i_n_t  _s_p_e_c_i_f_i_e_r.    For
    convenience  multiple constraints may be  declared at once with  the
    same  chr_constraint/1  declaration followed  by  a  comma-separated
    list of constraint specifiers.

    A  constraint specifier is,  in its compact form,  F/A where F  and
    A  are respectively the  functor name and arity  of the constraint,
    e.g.:

    :- chr_constraint foo/1.
    :- chr_constraint bar/2, baz/3.

    In  its extended form, a constraint specifier is c(A1,...,An) where
    c  is the constraint's functor, n its arity and  the Aiare  argument
    specifiers.   An argument  specifier is a mode, optionally  followed
    by a type.  E.g.

    :- chr_constraint get_value(+,?).
    :- chr_constraint domain(?int, +list(int)),
                      alldifferent(?list(int)).

MMooddeess

A mode is one of:

-
    The corresponding argument  of every occurrence of the constraint is
    always unbound.

+
    The corresponding argument  of every occurrence of the constraint is
    always ground.

?
    The  corresponding argument  of every occurrence  of the  constraint
    can  have any instantiation,  which may change over  time.  This  is
    the default value.

TTyyppeess

A type can be a user-defined type or one of the built-in  types.  A type
comprises a  (possibly infinite) set  of values.   The type  declaration
for  a  constraint  argument means  that  for  every  instance  of  that
constraint the  corresponding argument is only  ever bound to values  in
that set.   It does  not state that the  argument necessarily has to  be
bound to a value.

The built-in types are:

iinntt
    The corresponding argument  of every occurrence of the constraint is
    an integer number.

ddeennssee__iinntt
    The corresponding argument  of every occurrence of the constraint is
    an  integer that can be used as an  array index.  Note that  if this
    argument takes values in [0; n], the array takes O(n) space.

ffllooaatt
    ...a floating point number.

nnuummbbeerr
    ...a number.

nnaattuurraall
    ...a positive integer.

aannyy
    The  corresponding argument  of every occurrence  of the  constraint
    can have any type.  This is the default value.


::-- cchhrr__ttyyppee((_+_T_y_p_e_D_e_c_l_a_r_a_t_i_o_n))
    User-defined  types are algebraic  data types,  similar to those  in
    Haskell  or the discriminated unions in Mercury.  An  algebraic data
    type is defined using chr_type/1:

    :- chr_type type ---> body.

    If  the type term is a functor  of arity zero (i.e. one  having zero
    arguments),  it names  a monomorphic type.   Otherwise,  it names  a
    polymorphic  type;  the arguments of  the functor  must be  distinct
    type  variables.     The body  term  is  defined as  a  sequence  of
    constructor definitions separated by semi-colons.

    Each  constructor  definition  must  be a  functor  whose  arguments
    (if  any)  are  types.    Discriminated  union definitions  must  be
    transparent:   all type  variables occurring in  the body must  also
    occur in the type.

    Here are some examples of algebraic data type definitions:

    :- chr_type color ---> red ; blue ; yellow ; green.

    :- chr_type tree --->  empty ; leaf(int) ; branch(tree, tree).

    :- chr_type list(T) ---> [] ; [T | list(T)].

    :- chr_type pair(T1, T2) ---> (T1 - T2).

    Each  algebraic data  type  definition introduces  a distinct  type.
    Two  algebraic data types that  have the same bodies are  considered
    to be distinct types (name equivalence).

    Constructors may be  overloaded among different types:  there may be
    any  number of constructors with a given name and arity, so  long as
    they all have different types.

    Aliases can be defined  using ==.  For example, if your program uses
    lists of lists of integers, you can define an alias as follows:

    :- chr_type lli == list(list(int)).

TTyyppee CChheecckkiinngg

Currently two complementary forms of type checking are performed:

 1. Static  type checking is  always performed by the  compiler.  It  is
    limited to CHR rule heads and CHR constraint calls in rule bodies.

    Two  kinds  of type  error  are detected.    The  first is  where  a
    variable has to belong to two types.  For example, in the program:

    :-chr_type foo ---> foo.
    :-chr_type bar ---> bar.

    :-chr_constraint abc(?foo).
    :-chr_constraint def(?bar).

    foobar @ abc(X) <=> def(X).

    the  variable  X has  to be  of both  type foo  and bar.    This  is
    reported by the type clash error:

    CHR compiler ERROR:
        `--> Type clash for variable _G5398 in rule foobar:
                    expected type foo in body goal def(_G5398, _G5448)
                    expected type bar in head def(_G5448, _G5398)

    The  second kind of error is where  a functor is used that  does not
    belong to the declared type.  For example in:

    :-chr_type foo ---> foo.
    :-chr_type bar ---> bar.

    :-chr_constraint abc(?foo).

    foo @ abc(bar) <=> true.

    in  the head of the rule bar appears where something of type  foo is
    expected.  This is reported as:

    CHR compiler ERROR:
        `--> Invalid functor in head abc(bar) of rule foo:
                    found `bar',
                    expected type `foo'!

    No runtime overhead is incurred in static type checking.

 2. Dynamic  type checking checks at runtime, during  program execution,
    whether  the arguments  of  CHR constraints  respect their  declared
    types.   The  when/2 co-routining library  is used to delay  dynamic
    type checks until variables are instantiated.

    The  kind of  error detected  by dynamic  type checking  is where  a
    functor  is used that does  not belong to the  declared type.   E.g.
    for the program:

    :-chr_type foo ---> foo.

    :-chr_constraint abc(?foo).

    we get the following error in an erroneous query:

    ?- abc(bar).
    ERROR: Type error: `foo' expected, found `bar' (CHR Run-
    time Type Error)

    Dynamic  type checking is  weaker than static  type checking in  the
    sense  that it only checks the particular program execution  at hand
    rather  than all possible executions.   It is stronger in the  sense
    that it tracks types throughout the whole program.

    Note  that it  is enabled  only in  debug  mode, as  it incurs  some
    (minor) runtime overhead.


77..33..33 CCoommppiillaattiioonn

The  SWI-Prolog   CHR  compiler   exploits  term_expansion/2  rules   to
translate the constraint  handling rules to plain  Prolog.  These  rules
are loaded  from the library chr.   They  are activated if the  compiled
file  has the  .chr extension  or  after finding  a declaration  of  the
format below.

:- chr_constraint ...

It is  advised to define CHR  rules in a  module file, where the  module
declaration  is  immediately  followed  by  including  the  library(chr)
library as exemplified below:

:- module(zebra, [ zebra/0 ]).
:- use_module(library(chr)).

:- chr_constraint ...

Using this style CHR  rules can be defined in ordinary Prolog  .pl files
and the operator  definitions required by CHR  do not leak into  modules
where they might cause conflicts.


77..44 DDeebbuuggggiinngg

The  CHR  debugging facilities  are  currently  rather limited.     Only
tracing is  currently available.   To use  the CHR debugging  facilities
for a  CHR file it  must be  compiled for debugging.   Generating  debug
info is  controlled by the  CHR option debug,  whose default is  derived
from the SWI-Prolog  flag generate_debug_info.   Therefore debug info  is
provided unless the -nodebug is used.


77..44..11 PPoorrttss

For CHR constraints the four standard ports are defined:

ccaallll
    A new constraint is called and becomes active.

eexxiitt
    An  active  constraint  exits:    it has  either  been  inserted  in
    the  store  after trying  all rules  or has  been  removed from  the
    constraint store.

ffaaiill
    An active constraint fails.

rreeddoo
    An active constraint starts looking for an alternative solution.

In addition  to the above  ports, CHR  constraints have five  additional
ports:

wwaakkee
    A suspended constraint is woken and becomes active.

iinnsseerrtt
    An  active constraint has  tried all rules  and is suspended in  the
    constraint store.

rreemmoovvee
    An  active  or passive  constraint is  removed  from the  constraint
    store.

ttrryy
    An  active  constraints  tries a  rule  with possibly  some  passive
    constraints.  The  try port is entered just before committing to the
    rule.

aappppllyy
    An  active constraints commits to a rule with possibly  some passive
    constraints.   The  apply port is  entered just after committing  to
    the rule.


77..44..22 TTrraacciinngg

Tracing is enabled with the chr_trace/0 predicate and disabled  with the
chr_notrace/0 predicate.

When enabled  the tracer will  step through the  call, exit, fail,  wake
and apply  ports, accepting  debug commands,  and simply  write out  the
other ports.

The following debug commands are currently supported:

        CHR debug options:

                <cr>    creep           c       creep
                s       skip
                g       ancestors
                n       nodebug
                b       break
                a       abort
                f       fail
                ?       help            h       help

Their meaning is:

ccrreeeepp
    Step to the next port.

sskkiipp
    Skip to exit port of this call or wake port.

aanncceessttoorrss
    Print list of ancestor call and wake ports.

nnooddeebbuugg
    Disable the tracer.

bbrreeaakk
    Enter a recursive Prolog top-level.  See break/0.

aabboorrtt
    Exit to the top-level.  See abort/0.

ffaaiill
    Insert failure in execution.

hheellpp
    Print the above available debug options.


77..44..33 CCHHRR DDeebbuuggggiinngg PPrreeddiiccaatteess

The chr  module contains  several predicates that  allow inspecting  and
printing the content of the constraint store.


cchhrr__ttrraaccee
    Activate  the CHR tracer.   By default  the CHR tracer is  activated
    and  deactivated automatically by the Prolog predicates  trace/0 and
    notrace/0.


cchhrr__nnoottrraaccee
    De-activate the CHR tracer.   By default the CHR tracer is activated
    and  deactivated automatically by the Prolog predicates  trace/0 and
    notrace/0.


cchhrr__lleeaasshh((_+_S_p_e_c))
    Define  the set of CHR ports on  which the CHR tracer asks  for user
    intervention  (i.e. stops).    _S_p_e_c  is either  a list  of ports  as
    defined  in section 7.4.1 or a predefined `alias'.   Defined aliases
    are:   full to  stop at all ports,  none or off  to never stop,  and
    default to stop at  the call, exit, fail, wake and apply ports.  See
    also leash/1.


cchhrr__sshhooww__ssttoorree((_+_M_o_d))
    Prints  all  suspended constraints  of module  _M_o_d  to the  standard
    output.   This predicate  is automatically called by the  SWI-Prolog
    top-level  at the end of each  query for every CHR module  currently
    loaded.    The prolog-flag  chr_toplevel_show_store controls  whether
    the  top-level shows the constraint stores.  The value  true enables
    it.  Any other value disables it.


77..55 EExxaammpplleess

Here are two example constraint solvers written in CHR.

  o The  program below  defines a  solver with  one constraint,  leq/2/,
    which  is a less-than-or-equal constraint,  also known as a  partial
    order constraint.

    :- module(leq,[leq/2]).
    :- use_module(library(chr)).

    :- chr_constraint leq/2.
    reflexivity  @ leq(X,X) <=> true.
    antisymmetry @ leq(X,Y), leq(Y,X) <=> X = Y.
    idempotence  @ leq(X,Y) \ leq(X,Y) <=> true.
    transitivity @ leq(X,Y), leq(Y,Z) ==> leq(X,Z).

    When the above program  is saved in a file and loaded in SWI-Prolog,
    you can call the leq/2 constraints in a query, e.g.:

    ?- leq(X,Y), leq(Y,Z).
    leq(_G23837, _G23841)
    leq(_G23838, _G23841)
    leq(_G23837, _G23838)

    X = _G23837{leq = ...}
    Y = _G23838{leq = ...}
    Z = _G23841{leq = ...}

    Yes

    When  the  query  succeeds,  the  SWI-Prolog  top-level  prints  the
    content  of  the  CHR constraint  store  and displays  the  bindings
    generate  during  the  query.    Some  of the  query  variables  may
    have  been bound to  attributed variables, as  you see in the  above
    example.

  o The  program  below implements  a  simple finite  domain  constraint
    solver.

    :- module(dom,[dom/2]).
    :- use_module(library(chr)).

    :- chr_constraint dom(?int,+list(int)).
    :- chr_type list(T) ---> [] ; [T|list(T)].

    dom(X,[]) <=> fail.
    dom(X,[Y]) <=> X = Y.
    dom(X,L) <=> nonvar(X) | memberchk(X,L).
    dom(X,L1), dom(X,L2) <=> intersection(L1,L2,L3), dom(X,L3).

    When the above program  is saved in a file and loaded in SWI-Prolog,
    you can call the dom/2 constraints in a query, e.g.:

    ?- dom(A,[1,2,3]), dom(A,[3,4,5]).

    A = 3

    Yes


77..66 BBaacckkwwaarrddss CCoommppaattiibbiilliittyy


77..66..11 TThhee OOlldd SSIICCSSttuuss CCHHRR iimmpplleemmeennaattiioonn

There are  small differences between the  current K.U.Leuven CHR  system
in  SWI-Prolog, older  versions  of the  same  system and  SICStus'  CHR
system.

The  current  system maps  old  syntactic  elements onto  new  ones  and
ignores a number  of no longer required elements.   However, for each  a
_d_e_p_r_e_c_a_t_e_d warning  is issued.   You  are strongly  urged to replace  or
remove deprecated features.

Besides  differences in  available options  and pragmas,  the  following
differences should be noted:

  o _T_h_e constraints/1 _d_e_c_l_a_r_a_t_i_o_n
    This  declaration is  deprecated.   It  has been  replaced with  the
    chr_constraint/1 declaration.

  o _T_h_e option/2 _d_e_c_l_a_r_a_t_i_o_n
    This  declaration is  deprecated.   It  has been  replaced with  the
    chr_option/2 declaration.

  o _T_h_e handler/1 _d_e_c_l_a_r_a_t_i_o_n
    In  SICStus  every  CHR  module  requires  a  handler/1  declaration
    declaring  a unique handler name.  This declaration is  valid syntax
    in  SWI-Prolog, but will have  no effect.   A warning will be  given
    during compilation.

  o _T_h_e rules/1 _d_e_c_l_a_r_a_t_i_o_n
    In  SICStus, for every  CHR module it is  possible to only enable  a
    subset of the  available rules through the rules/1 declaration.  The
    declaration  is valid syntax in  SWI-Prolog, but has  no effect.   A
    warning is given during compilation.

  o _G_u_a_r_d _b_i_n_d_i_n_g_s
    The   check_guard_bindings  option   only  turns  invalid  calls   to
    unification  into failure.   In SICStus this  option does more:   it
    intercepts  instantiation errors from Prolog built-ins such  as is/2
    and turns them into  failure.  In SWI-Prolog, we do not go this far,
    as  we like to separate  concerns more.   The CHR compiler is  aware
    of  the CHR code, the Prolog  system and programmer should be  aware
    of  the appropriate meaning of the  Prolog goals used in guards  and
    bodies of CHR rules.


77..66..22 TThhee OOlldd EECCLLiiPPSSee CCHHRR iimmpplleemmeennaattiioonn

The old  ECLiPSe CHR implementations  features a  label_with/1 construct
for  labeling variables  in CHR  constraints.    This feature  has  long
since been abandoned.   However, a simple transformation is all  that is
required to port the functionality.

label_with Constraint1 if Condition1.
...
label_with ConstraintN if ConditionN.
Constraint1 :- Body1.
...
ConstraintN :- BodyN.

is transformed into

:- chr_constraint my_labeling/0.

my_labeling \ Constraint1 <=> Condition1 | Body1.
...
my_labeling \ ConstraintN <=> ConditionN | BodyN.
my_labeling <=> true.

Be sure to  put this code after all other  rules in your program!   With
my_labeling/0 (or another predicate name of your choosing)  the labeling
is initiated, rather than ECLiPSe's chr_labeling/0.


77..77 PPrrooggrraammmmiinngg TTiippss aanndd TTrriicckkss

In this section we  cover several guidelines on how to use CHR  to write
constraint solvers and how to do so efficiently.

  o _C_h_e_c_k _g_u_a_r_d _b_i_n_d_i_n_g_s _y_o_u_r_s_e_l_f
    It  is considered bad practice  to write guards that bind  variables
    of  the head and to  rely on the system  to detect this at  runtime.
    It is inefficient and obscures the working of the program.

  o _S_e_t _s_e_m_a_n_t_i_c_s
    The  CHR system allows the  presence of identical constraints,  i.e.
    multiple  constraints with  the same functor,  arity and  arguments.
    For  most constraint  solvers, this  is not desirable:   it  affects
    efficiency and possibly  termination.  Hence appropriate simpagation
    rules should be added of the form:

                         constraint\constraint <=>true

  o _M_u_l_t_i_-_h_e_a_d_e_d _r_u_l_e_s
    Multi-headed   rules  are   executed  more   efficiently  when   the
    constraints share one or more variables.

  o _M_o_d_e _a_n_d _t_y_p_e _d_e_c_l_a_r_a_t_i_o_n_s
    Provide  mode and  type declarations to  get more efficient  program
    execution.     Make sure  to  disable  debug (-nodebug)  and  enable
    optimization (-O).

  o _C_o_m_p_i_l_e _o_n_c_e_, _r_u_n _m_a_n_y _t_i_m_e_s
    Does  consulting your CHR  program take a  long time in  SWI-Prolog?
    Probably  it takes the CHR compiler  a long time to compile the  CHR
    rules  into Prolog  code.   When you  disable optimizations the  CHR
    compiler  will be  a lot  quicker,  but you  may loose  performance.
    Alternatively, you  can just use SWI-Prolog's qcompile/1 to generate
    a  .qlf  file once  from your  .pl file.    This  .qlf contains  the
    generated  code of  the CHR  compiler (be  it in  a binary  format).
    When you consult the  .qlf file, the CHR compiler is not invoked and
    consultation is much faster.


77..88 CCoommppiilleerr EErrrroorrss aanndd WWaarrnniinnggss

In  this section  we  summarize the  most  important error  and  warning
messages of the CHR compiler.


77..88..11 CCHHRR CCoommppiilleerr EErrrroorrss

TTyyppee ccllaasshh  for variable ...  in rule ...

    This  error indicates  an inconsistency  between declared  types;  a
    variable should belong to two types.  See static type checking.

IInnvvaalliidd ffuunnccttoorr  in head ...  of rule ...

    This  error indicates an inconsistency  between a declared type  and
    the use of a functor in a rule.  See static type checking.

CCyycclliicc aalliiaass  definition:  ...  == ...

    You  have defined a type alias  in terms of itself, either  directly
    or indirectly.

AAmmbbiigguuoouuss ttyyppee aalliiaasseess  You have defined two overlapping type aliases.

MMuullttiippllee ddeeffiinniittiioonnss  for type

    You have defined the same type multiple times.

NNoonn--ggrroouunndd ttyyppee  in constraint definition:  ...

    You have declared a non-ground type for a constraint argument.

CCoouulldd nnoott ffiinndd ttyyppee ddeeffiinniittiioonn  for ...

    You have used an undefined type in a type declaration.

IIlllleeggaall mmooddee//ttyyppee ddeeccllaarraattiioonn  You  have  used   invalid  syntax  in   a
    constraint declaration.

CCoonnssttrraaiinntt mmuullttiippllyy ddeeffiinneedd  There is more than one declaration  for the
    same constraint.

UUnnddeeccllaarreedd ccoonnssttrraaiinntt  ...  in head of ...

    You have used an  undeclared constraint in the head of a rule.  This
    often  indicates a  misspelled constrained name  or wrong number  of
    arguments.

IInnvvaalliidd pprraaggmmaa  ...  in ...  Pragma should not be a variable.

    You  have used  a variable  as a  pragma in  a rule.    This is  not
    allowed.

IInnvvaalliidd iiddeennttiiffiieerr  ...  in pragma passive in ...

    You  have  used an  identifier in  a passive  pragma  that does  not
    correspond  to an identifier in  the head of the  rule.  Likely  the
    identifier name is misspelled.

UUnnkknnoowwnn pprraaggmmaa  ...  in ...

    You  have used an unknown  pragma in a rule.   Likely the pragma  is
    misspelled or not supported.

SSoommeetthhiinngg uunneexxppeecctteedd  happened in the CHR compiler

    You have most likely  bumped into a bug in the CHR compiler.  Please
    contact Tom Schrijvers to notify him of this error.


CChhaapptteerr 88..  MMUULLTTII--TTHHRREEAADDEEDD AAPPPPLLIICCAATTIIOONNSS

SWI-Prolog multithreading is  based on standard C-language  multithread-
ing support.   It is not  like _P_a_r_L_o_g or other parallel  implementations
of the Prolog language.   Prolog threads have their own stacks  and only
share the  Prolog _h_e_a_p:   predicates,  records, flags  and other  global
non-backtrackable data.  SWI-Prolog thread support is  designed with the
following goals in mind.

  o _M_u_l_t_i_-_t_h_r_e_a_d_e_d _s_e_r_v_e_r _a_p_p_l_i_c_a_t_i_o_n_s
    Todays   computing  services  often   focus  on  (internet)   server
    applications.   Such applications often have need  for communication
    between  services  and/or  fast  non-blocking  service  to  multiple
    concurrent  clients.   The shared  heap provides fast  communication
    and thread creation is relatively cheap.

  o _I_n_t_e_r_a_c_t_i_v_e _a_p_p_l_i_c_a_t_i_o_n_s
    Interactive   applications   often   need   to   perform   extensive
    computation.   If  such computations are  executed in a new  thread,
    the  main thread  can process events  and allow  the user to  cancel
    the  ongoing computation.    User interfaces can  also use  multiple
    threads,  each thread dealing  with input from  a distinct group  of
    windows.  See also section 8.7.

  o _N_a_t_u_r_a_l _i_n_t_e_g_r_a_t_i_o_n _w_i_t_h _f_o_r_e_i_g_n _c_o_d_e
    Each Prolog thread  runs in a native thread of the operating system,
    automatically  making them cooperate with _M_T_-_s_a_f_e foreign-code.   In
    addition,  any foreign thread can  create its own Prolog engine  for
    dealing with calling Prolog from C-code.

SWI-Prolog  multi-threading  is  based  on  the  POSIX  thread  standard
[Butenhof, 1997] used  on most  popular systems  except for  MS-Windows.
On Windows  it uses the pthread-win32  emulation of POSIX threads  mixed
with the Windows native API for smoother and faster operation.


88..11 CCrreeaattiinngg aanndd ddeessttrrooyyiinngg PPrroolloogg tthhrreeaaddss


tthhrreeaadd__ccrreeaattee((_:_G_o_a_l_, _-_I_d_, _+_O_p_t_i_o_n_s))
    Create  a new Prolog thread  (and underlying C-thread) and start  it
    by  executing _G_o_a_l.    If the  thread is  created successfully,  the
    thread-identifier  of the created thread is unified to _I_d.   _O_p_t_i_o_n_s
    is  a list of  options.   The currently  defined options are  below.
    Stack  size options can also take  the value inf or infinite,  which
    is mapped to the maximum stack size supported by the platform.

    llooccaall((_K_-_B_y_t_e_s))
         Set the  limit to  which  the local  stack of  this thread  may
         grow.   If omitted,  the limit of the  calling thread is  used.
         See also the -L command-line option.

    gglloobbaall((_K_-_B_y_t_e_s))
         Set the  limit to  which the global  stack of  this thread  may
         grow.   If omitted,  the limit of the  calling thread is  used.
         See also the -G command-line option.

    ttrraaiill((_K_-_B_y_t_e_s))
         Set the  limit to  which  the trail  stack of  this thread  may
         grow.   If omitted,  the limit of the  calling thread is  used.
         See also the -T command-line option.

    aarrgguummeenntt((_K_-_B_y_t_e_s))
         Set the limit  to which the argument  stack of this thread  may
         grow.   If omitted,  the limit of the  calling thread is  used.
         See also the -A command-line option.

    ssttaacckk((_K_-_B_y_t_e_s))
         Set  the  limit  to which  the  system  stack  of  this  thread
         may  grow.    The  default,  minimum  and  maximum  values  are
         system-dependant.

    aalliiaass((_A_l_i_a_s_N_a_m_e))
         Associate an `alias-name' with  the thread.  This named  may be
         used to refer to the thread and remains valid  until the thread
         is joined (see thread_join/2).

    ddeettaacchheedd((_B_o_o_l))
         If  false  (default),  the  thread  can  be  waited  for  using
         thread_join/2.   thread_join/2 must  be called  on this  thread
         to  reclaim  the  all  resources  associated  to   the  thread.
         If  true, the  system  will  reclaim all  associated  resources
         automatically after  the thread  finishes.    Please note  that
         thread identifiers are freed for reuse after  a detached thread
         finishes  or a  normal  thread  has  been joined.     See  also
         thread_join/2 and thread_detach/1.

         If a detached  thread dies due to  failure or exception of  the
         initial goal the thread prints a message using print_message/2.
         If  such termination  is considered  normal  the code  must  be
         wrapped  using ignore/1  and/or  catch/3 to  ensure  successful
         completion.

    The _G_o_a_l argument is  _c_o_p_i_e_d to the new Prolog engine.  This implies
    further  instantiation of this term  in either thread does not  have
    consequences  for the  other thread:   Prolog threads  do not  share
    data from their stacks.


tthhrreeaadd__sseellff((_-_I_d))
    Get  the Prolog thread  identifier of  the running thread.   If  the
    thread has an alias, the alias-name is returned.


tthhrreeaadd__jjooiinn((_+_I_d_, _-_S_t_a_t_u_s))
    Wait  for the termination of thread with  given _I_d.  Then  unify the
    result-status  of  the thread  with _S_t_a_t_u_s.    After this  call,  _I_d
    becomes  invalid and  all resources associated  with the thread  are
    reclaimed.    Note that  threads with  the attribute  detached(_t_r_u_e)
    cannot be joined.  See also current_thread/2.

    A thread  that has been completed without thread_join/2 being called
    on  it is partly reclaimed:  the Prolog stacks are released  and the
    C-thread  is destroyed.    A small  data-structure representing  the
    exit-status of  the thread is retained until thread_join/2 is called
    on the thread.  Defined values for _S_t_a_t_u_s are:

    ttrruuee
         The goal has been proven successfully.

    ffaallssee
         The goal has failed.

    eexxcceeppttiioonn((_T_e_r_m))
         The thread is terminated on an  exception.  See print_message/2
         to turn system exceptions into readable messages.

    eexxiitteedd((_T_e_r_m))
         The thread  is terminated on  thread_exit/1 using the  argument
         _T_e_r_m.


tthhrreeaadd__ddeettaacchh((_+_I_d))
    Switch  thread  into detached-state  (see detached(_B_o_o_l)  option  at
    thread_create/3)  at  runtime.     _I_d  is   the  identifier  of  the
    thread  placed  in detached  state.    This  may  be the  result  of
    PL_thread_self/1.

    One   of  the  possible  applications  is  to   simplify  debugging.
    Threads  that  are  created as  _d_e_t_a_c_h_e_d  leave  no traces  if  they
    crash.   For not-detached threads the status can be  inspected using
    current_thread/2.    Threads nobody is  waiting for  may be  created
    normally  and detach themselves  just before completion.   This  way
    they  leave no  traces  on normal  completion and  their reason  for
    failure can be inspected.


tthhrreeaadd__eexxiitt((_+_T_e_r_m))
    Terminates   the  thread   immediately,   leaving  exited(_T_e_r_m)   as
    result-state  for thread_join/2.   If the  thread has the  attribute
    detached(_t_r_u_e)  it  terminates,   but  its  exit  status  cannot  be
    retrieved  using thread_join/2 making the value of _T_e_r_m  irrelevant.
    The Prolog stacks and C-thread are reclaimed.


tthhrreeaadd__iinniittiiaalliizzaattiioonn((_:_G_o_a_l))
    Run  _G_o_a_l  when   thread  is  started.     This  predicate  must  be
    compared  with initialization/1, but is intended  for initialization
    operations  of the runtime stacks, such as setting  global variables
    as  described  in section  6.3.    _G_o_a_l is  run on  four  occasions:
    at  the call  to this  predicate, after  loading a  saved state,  on
    starting  a new thread and on  creating a Prolog engine through  the
    C  interface.   On  loading a saved  state, _G_o_a_l  is executed  _a_f_t_e_r
    running the initialization/1 hooks.


tthhrreeaadd__aatt__eexxiitt((_:_G_o_a_l))
    Run  _G_o_a_l  just before  releasing the  thread resources.    This  is
    to  be  compared to  at_halt/1, but  only  for the  current  thread.
    These  hooks are ran regardless of  why the execution of the  thread
    has  been  completed.    As these  hooks  are run,  the  return-code
    is  already available through  current_thread/2 using the result  of
    thread_self/1 as thread-identifier.


tthhrreeaadd__sseettccoonnccuurrrreennccyy((_-_O_l_d_, _+_N_e_w))
    Determine  the concurrency of the  process, which is defined as  the
    maximum  number  of  concurrently active  threads.    `Active'  here
    means  they are  using CPU time.    This option is  provided if  the
    thread-implementation  provides  pthread_setconcurrency().    Solaris
    is  a  typical example  of  this  family.    On other  systems  this
    predicate unifies _O_l_d to 0 (zero) and succeeds silently.


88..22 MMoonniittoorriinngg tthhrreeaaddss

Normal multi-threaded applications should not need  these the predicates
from  this section  because  almost any  usage  of these  predicates  is
unsafe.     For  example checking  the  existence  of  a  thread  before
signalling it  is of  no use  as it may  vanish between  the two  calls.
Catching exceptions  using catch/3  is the  only safe way  to deal  with
thread-existence errors.

These predicates are provided  for diagnosis and monitoring tasks.   See
also section 8.5, describing more high-level primitives.


ccuurrrreenntt__tthhrreeaadd((_?_I_d_, _?_S_t_a_t_u_s))
    Enumerates  identifiers and status  of all currently known  threads.
    Calling  current_thread/2 does not influence any  thread.  See  also
    thread_join/2.   For threads that  have an alias-name, this name  is
    returned  in _I_d instead of the numerical thread identifier.   _S_t_a_t_u_s
    is one of:

    rruunnnniinngg
         The  thread is  running.    This is  the  initial status  of  a
         thread.   Please note  that threads  waiting for something  are
         considered running too.

    ffaallssee
         The _G_o_a_l of the thread has been completed and failed.

    ttrruuee
         The _G_o_a_l of the thread has been completed and succeeded.

    eexxiitteedd((_T_e_r_m))
         The _G_o_a_l of the thread has  been terminated using thread_exit/1
         with _T_e_r_m  as argument.   If the  underlying native thread  has
         exited (using pthread_exit()) _T_e_r_m is unbound.

    eexxcceeppttiioonn((_T_e_r_m))
         The _G_o_a_l of the  thread has been terminated due to  an uncaught
         exception (see throw/1 and catch/3).


tthhrreeaadd__ssttaattiissttiiccss((_+_I_d_, _+_K_e_y_, _-_V_a_l_u_e))
    Obtains  statistical information on  thread _I_d as statistics/2  does
    in  single-threaded applications.    This call returns  all keys  of
    statistics/2, although  only information statistics about the stacks
    and CPU time yield different values for each thread.


mmuutteexx__ssttaattiissttiiccss
    Print  usage statistics on  internal mutexes and mutexes  associated
    with  dynamic predicates.  For  each mutex two numbers are  printed:
    the  number  of times  the  mutex was  acquired  and the  number  of
    _c_o_l_l_i_s_i_o_n_s:   the number  times the calling  thread has to wait  for
    the  mutex.   The  collision-count is  not available  on Windows  as
    this  would break portability  to Windows-95/98/ME or  significantly
    harm  performance.   Generally collision count  is close to zero  on
    single-CPU hardware.


88..22..11 LLiinnuuxx::  lliinnuuxxtthhrreeaaddss vvss.. NNPPTTLL

Linux has  introduces POSIX  threads (pthread)  using an  implementation
called _l_i_n_u_x_t_h_r_e_a_d_s,  where each thread  was `almost' a  process.   This
approach  has  various  disadvantages,  such  as  poor  performance  and
non-compliance  with several  aspects of  POSIX. However,  there is  one
advantage.   Where  pthread does  not provide  a way  to get  statistics
per thread,  we could  get this info  from the process-oriented  times()
function.    Since the  2.6.x kernels,  Linux  by default  now uses  the
_N_P_T_L implementation  which is POSIX compliant.   Unfortunately,  getting
per-thread CPU  statistics involves reading /proc  and is therefore  too
slow for some applications.

SWI-Prolog   is  setup   to   run  with   the  default   thread   model.
Unfortunately  there is  no way  to modify  this at  runtime, but  there
is a  way to  select the  old thread model  on modern  machines at  _l_i_n_k
_t_i_m_e.  This is  achieved using the environment variable LD_ASSUME_KERNEL
which must  be set  to a pre-nptl  kernel version  for linking the  main
executable.    The value  2.4.21 is  appropriate.    When building  from
source, this  flag can be set  during the build process.   When using  a
binary distribution one could create a minimal C program  and relink the
system using the plld utility.


88..33 TThhrreeaadd ccoommmmuunniiccaattiioonn


88..33..11 MMeessssaaggee qquueeuueess

Prolog  threads can  exchange data  using dynamic  predicates,  database
records,  and other globally  shared data.   These  provide no  suitable
means to wait for data or a condition as they can  only be checked in an
expensive polling loop.   _M_e_s_s_a_g_e _q_u_e_u_e_s provide a means for  threads to
wait for data or conditions without using the CPU.

Each thread  has a message-queue  attached to it  that is identified  by
the thread.  Additional queues are created using message_queue_create/1.


tthhrreeaadd__sseenndd__mmeessssaaggee((_+_Q_u_e_u_e_O_r_T_h_r_e_a_d_I_d_, _+_T_e_r_m))
    Place  _T_e_r_m in  the given queue  or default  queue of the  indicated
    thread  (which  can  even  be  the  message  queue  of  itself  (see
    thread_self/1).    Any  term  can  be  placed in  a  message  queue,
    but  note  that the  term  is copied  to  the receiving  thread  and
    variable-bindings are thus lost.  This call returns immediately.

    If  more than one thread is waiting for messages on the  given queue
    and  at least one of these is waiting with a  partially instantiated
    _T_e_r_m, the waiting  threads are _a_l_l sent a wake-up signal, starting a
    rush  for the available messages in  the queue.  This behaviour  can
    seriously  harm performance  with many threads  waiting on the  same
    queue  as all-but-the-winner  perform a useless  scan of the  queue.
    If  there is  only one waiting  thread or  all waiting threads  wait
    with  an unbound variable an  arbitrary thread is restarted to  scan
    the queue.


tthhrreeaadd__ggeett__mmeessssaaggee((_?_T_e_r_m))
    Examines the thread  message-queue and if necessary blocks execution
    until  a term that unifies  to _T_e_r_m arrives in  the queue.  After  a
    term  from the queue has been  unified unified to _T_e_r_m, the  term is
    deleted from the queue and this predicate returns.

    Please  note that not-unifying messages remain in the queue.   After
    the  following has been  executed, thread 1  has the term b(_g_n_u)  in
    its queue and continues execution using _A is gnat.

       <thread 1>
       thread_get_message(a(A)),

       <thread 2>
       thread_send_message(Thread_1, b(gnu)),
       thread_send_message(Thread_1, a(gnat)),

    See also thread_peek_message/1.


tthhrreeaadd__ppeeeekk__mmeessssaaggee((_?_T_e_r_m))
    Examines  the thread  message-queue  and compares  the queued  terms
    with   _T_e_r_m  until  one  unifies  or  the  end  of  the   queue  has
    been  reached.    In  the first  case  the call  succeeds  (possibly
    instantiating  _T_e_r_m.   If no term from  the queue unifies this  call
    fails.


mmeessssaaggee__qquueeuuee__ccrreeaattee((_?_Q_u_e_u_e))
    If  _Q_u_e_u_e is an atom, create a  named queue.  To avoid  ambiguity of
    thread_send_message/2, the name  of a queue may  not be in use as  a
    thread-name.  If  _Q_u_e_u_e is unbound an anonymous queue is created and
    _Q_u_e_u_e is unified to its identifier.


mmeessssaaggee__qquueeuuee__ddeessttrrooyy((_+_Q_u_e_u_e))
    Destroy  a message queue created with message_queue_create/1.   It is
    _n_o_t allows to destroy  the queue of a thread.  Neither is it allowed
    to  destroy a queue other threads are waiting for or,  for anonymous
    message queues, may try to wait for later.


tthhrreeaadd__ggeett__mmeessssaaggee((_+_Q_u_e_u_e_, _?_T_e_r_m))
    As  thread_get_message/1, operating on a given queue.   It is allowed
    (but not advised) to get messages from the queue of other threads.


tthhrreeaadd__ppeeeekk__mmeessssaaggee((_+_Q_u_e_u_e_, _?_T_e_r_m))
    As  thread_peek_message/1,  operating on  a  given  queue.    It  is
    allowed  to peek into another  thread's message queue, an  operation
    that  can be used to check whether a thread has swallowed  a message
    sent to it.


mmeessssaaggee__qquueeuuee__ssiizzee((_+_Q_u_e_u_e_, _-_S_i_z_e))
    Unify  _S_i_z_e with the number  of terms waiting in  _Q_u_e_u_e.  Note  that
    due  to concurrent access the returned value may be  outdated before
    it  is returned.  It can  be used for debugging purposes as  well as
    work distribution purposes.

Explicit  message queues  are  designed with  the _w_o_r_k_e_r_-_p_o_o_l  model  in
mind,  where  multiple  threads wait  on  a  single queue  and  pick  up
the first  goal to  execute.   Below  is a  simple implementation  where
the workers  execute arbitrary  Prolog goals.   Note  that this  example
provides no means to tell when all work is done.   This must be realised
using additional synchronisation.

%       create_workers(+Id, +N)
%
%       Create a pool with given Id and number of workers.

create_workers(Id, N) :-
        message_queue_create(Id),
        forall(between(1, N, _),
               thread_create(do_work(Id), _, [])).

do_work(Id) :-
        repeat,
          thread_get_message(Id, Goal),
          (   catch(Goal, E, print_message(error, E))
          ->  true
          ;   print_message(error, goal_failed(Goal, worker(Id)))
          ),
        fail.

%       work(+Id, +Goal)
%
%       Post work to be done by the pool

work(Id, Goal) :-
        thread_send_message(Id, Goal).


88..33..22 SSiiggnnaalllliinngg tthhrreeaaddss

These  predicates provide  a mechanism  to make  another thread  execute
some  goal as  an  _i_n_t_e_r_r_u_p_t.    Signalling  threads  is safe  as  these
interrupts  are only  checked at  safe points  in  the virtual  machine.
Nevertheless,  signalling  in  multi-threaded   environments  should  be
handled  with  care as  the  receiving  thread  may hold  a  _m_u_t_e_x  (see
with_mutex).   Signalling  probably only makes  sense to start  debugging
threads and to  cancel no-longer-needed threads with throw/1,  where the
receiving thread  should be designed carefully  do handle exceptions  at
any point.


tthhrreeaadd__ssiiggnnaall((_+_T_h_r_e_a_d_I_d_, _:_G_o_a_l))
    Make thread _T_h_r_e_a_d_I_d execute  _G_o_a_l at the first opportunity.  In the
    current  implementation, this implies at the first pass  through the
    _C_a_l_l_-_p_o_r_t.   The  predicate thread_signal/2 itself places _G_o_a_l  into
    the signalled-thread's signal queue and returns immediately.

    Signals  (interrupts)  do  not  cooperate well  with  the  world  of
    multi-threading,  mainly  because the  status of  mutexes cannot  be
    guaranteed  easily.   At the call-port,  the Prolog virtual  machine
    holds no locks and therefore the asynchronous execution is safe.

    _G_o_a_l  can be any  valid Prolog goal,  including throw/1 to make  the
    receiving thread generate  an exception and trace/0 to start tracing
    the receiving thread.

    In  the Windows version,  the receiving thread immediately  executes
    the  signal  if  it  reaches  a  Windows  GetMessage()  call,  which
    generally happens of the thread is waiting for (user-)input.


88..33..33 TThhrreeaaddss aanndd ddyynnaammiicc pprreeddiiccaatteess

Besides  queues (section  8.3.1)  threads can  share and  exchange  data
using dynamic  predicates.  The  multi-threaded version knows about  two
types of dynamic predicates.   By default, a predicate  declared _d_y_n_a_m_i_c
(see dynamic/1)  is shared  by all  threads.   Each  thread may  assert,
retract and  run the dynamic predicate.   Synchronisation inside  Prolog
guarantees  the consistency  of the  predicate.    Updates are  _l_o_g_i_c_a_l:
visible  clauses  are  not affected  by  assert/retract  after  a  query
started on  the predicate.   In  many cases  primitive from section  8.4
should be  used to ensure  application invariants  on the predicate  are
maintained.

Besides shared predicates,  dynamic predicates can be declared  with the
thread_local/1 directive.   Such predicates share their attributes,  but
the clause-list is different in each thread.


tthhrreeaadd__llooccaall _+_F_u_n_c_t_o_r_/_+_A_r_i_t_y_, _._._.
    This  directive is  related to the  dynamic/1 directive.   It  tells
    the  system  that  the predicate  may  be modified  using  assert/1,
    retract/1,  etc. during  execution of  the program.   Unlike  normal
    shared dynamic data  however each thread has its own clause-list for
    the  predicate.   As  a thread starts,  this clause  list is  empty.
    If  there  are still  clauses  as the  thread terminates  these  are
    automatically  reclaimed by the system  (see also volatile/1).   The
    thread_local property implies the properties dynamic and volatile.

    Thread-local   dynamic  predicates  are  intended  for   maintaining
    thread-specific state or intermediate results of a computation.

    It  is not recommended to  put clauses for a thread-local  predicate
    into  a file as in the example  below as the clause is  only visible
    from  the thread  that loaded the  source-file.   All other  threads
    start with an empty clause-list.

    :- thread_local
            foo/1.

    foo(gnat).

    DDIISSCCLLAAIIMMEERR  Whether or  not this declaration  is appropriate in  the
    sense  of the proper mechanism to  reach the goal is still  debated.
    If  you have strong feeling in favour or against, please  share them
    in the SWI-Prolog mailing list.


88..44 TThhrreeaadd ssyynncchhrroonniissaattiioonn

All  internal Prolog  operations  are thread-safe.    This  implies  two
Prolog  threads  can  operate on  the  same  dynamic  predicate  without
corrupting the consistency  of the predicate.   This section deals  with
user-level  _m_u_t_e_x_e_s (called  _m_o_n_i_t_o_r_s  in  ADA or  _c_r_i_t_i_c_a_l_-_s_e_c_t_i_o_n_s  by
Microsoft).   A  mutex is a  MMUUTTual EEXXclusive device,  which implies  at
most one thread can _h_o_l_d a mutex.

Mutexes are  used to  realise related  updates to  the Prolog  database.
With `related', we refer to the situation where  a `transaction' implies
two  or more  changes to  the Prolog  database.   For  example, we  have
a  predicate address/2,  representing the  address of  a  person and  we
want  to change  the address  by retracting  the old  and asserting  the
new address.    Between these  two operations the  database is  invalid:
this person  has either no  address or two  addresses, depending on  the
assert/retract order.

Here is how to realise a correct update:

:- initialization
        mutex_create(addressbook).

change_address(Id, Address) :-
        mutex_lock(addressbook),
        retractall(address(Id, _)),
        asserta(address(Id, Address)),
        mutex_unlock(addressbook).


mmuutteexx__ccrreeaattee((_?_M_u_t_e_x_I_d))
    Create  a mutex.  if _M_u_t_e_x_I_d  is an atom, a _n_a_m_e_d mutex  is created.
    If  it is  a variable,  an  anonymous mutex  reference is  returned.
    There is no limit to the number of mutexes that can be created.


mmuutteexx__ddeessttrrooyy((_+_M_u_t_e_x_I_d))
    Destroy  a mutex.    After this  call, _M_u_t_e_x_I_d  becomes invalid  and
    further references yield an existence_error exception.


wwiitthh__mmuutteexx((_+_M_u_t_e_x_I_d_, _:_G_o_a_l))
    Execute  _G_o_a_l while holding _M_u_t_e_x_I_d.  If _G_o_a_l  leaves choice-points,
    these  are  destroyed  (as  in  once/1).    The  mutex  is  unlocked
    regardless  of whether _G_o_a_l succeeds, fails or raises  an exception.
    An  exception thrown by _G_o_a_l is  re-thrown after the mutex has  been
    successfully unlocked.  See also mutex_create/1 and call_cleanup/3.

    Although  described in  the thread-section,  this predicate is  also
    available  in the single-threaded  version, where it behaves  simply
    as once/1.


mmuutteexx__lloocckk((_+_M_u_t_e_x_I_d))
    Lock  the mutex.   Prolog mutexes are  _r_e_c_u_r_s_i_v_e mutexes:  they  can
    be  locked multiple times by the same thread.  Only  after unlocking
    it  as many times as it  is locked, the mutex becomes  available for
    locking  by other threads.   If another thread has locked the  mutex
    the calling thread is suspended until to mutex is unlocked.

    If  _M_u_t_e_x_I_d is  an atom,  and there  is no current  mutex with  that
    name,  the  mutex  is  created automatically  using  mutex_create/1.
    This implies named mutexes need not be declared explicitly.

    Please  note that  locking and  unlocking mutexes  should be  paired
    carefully.    Especially make  sure to  unlock mutexes  even if  the
    protected  code  fails or  raises an  exception.    For most  common
    cases  use with_mutex/2,  which provides  a safer  way for  handling
    Prolog-level  mutexes.  The predicate  call_cleanup/[2-3]is  another
    way  to  guarantee  that  the  mutex  is  unlocked  while  retaining
    non-determinism.


mmuutteexx__ttrryylloocckk((_+_M_u_t_e_x_I_d))
    As  mutex_lock/1,  but if the mutex  is held by another thread,  this
    predicates fails immediately.


mmuutteexx__uunnlloocckk((_+_M_u_t_e_x_I_d))
    Unlock  the mutex.  This can only be called if the mutex  is held by
    the  calling thread.   If this is  not the case,  a permission_error
    exception is raised.


mmuutteexx__uunnlloocckk__aallll
    Unlock  all  mutexes held  by the  current  thread.   This  call  is
    especially  useful  to handle  thread-termination using  abort/0  or
    exceptions.  See also thread_signal/2.


ccuurrrreenntt__mmuutteexx((_?_M_u_t_e_x_I_d_, _?_T_h_r_e_a_d_I_d_, _?_C_o_u_n_t))
    Enumerates  all existing  mutexes.   If the  mutex is  held by  some
    thread,  _T_h_r_e_a_d_I_d  is unified  with the  identifier  of the  holding
    thread and _C_o_u_n_t with  the recursive count of the mutex.  Otherwise,
    _T_h_r_e_a_d_I_d is [] and _C_o_u_n_t is 0.


88..55 TThhrreeaadd--ssuuppppoorrtt lliibbrraarryy((tthhrreeaadduuttiill))

This library  defines a  couple of useful  predicates for  demonstrating
and debugging  multi-threaded applications.   This library is  certainly
not complete.


tthhrreeaaddss
    Lists all current threads and their status.


jjooiinn__tthhrreeaaddss
    Join all terminated  threads.  For normal applications, dealing with
    terminated  threads must be  part of  the application logic,  either
    detaching  the thread before termination  or making sure it will  be
    joined.   The  predicate join_threads/0 is intended for  interactive
    sessions  to reclaim resources  from threads that died  unexpectedly
    during development.


iinntteerraaccttoorr
    Create  a  new console  and run  the Prolog  top-level  in this  new
    console.   See also attach_console/0.  In the Windows version  a new
    interactor can also be created from the Run/New thread menu.


88..55..11 DDeebbuuggggiinngg tthhrreeaaddss

Support  for  debugging threads  is  still  very  limited.    Debug  and
trace  mode are  flags  that  are local  to  each  thread.    Individual
threads can  be debugged either using  the graphical debugger  described
in  section 3.5  (see tspy/1  and  friends) or  by attaching  a  console
to the  thread and  running the traditional  command-line debugger  (see
attach_console/0).   When  using the  graphical debugger,  the  debugger
must  be _l_o_a_d_e_d  from  the main  thread  (for example  using  guitracer)
before gtrace/0 can be called from a thread.


aattttaacchh__ccoonnssoollee
    If  the  current thread  has no  console  attached yet,  attach  one
    and  redirect the  user streams (input,  output, and  error) to  the
    new  console  window.   On  Unix  systems the  console is  an  xterm
    application.    On  Windows systems  this requires  the GUI  version
    plwin.exe rather than the console based plcon.exe.

    This  predicate has  a couple  of useful applications.    One is  to
    separate (debugging) I/O  of different threads.  Another is to start
    debugging  a thread that  is running in the  background.  If  thread
    10  is running,  the following  sequence starts the  tracer on  this
    thread:

    ?- thread_signal(10, (attach_console, trace)).


ttddeebbuugg((_+_T_h_r_e_a_d_I_d))
    Prepare  _T_h_r_e_a_d_I_d for debugging  using the graphical  tracer.   This
    implies installing the  tracer hooks in the thread and switching the
    thread  to debug-mode  using debug/0.    The call  is injected  into
    the  thread using thread_signal/2.   We  refer to the  documentation
    of  this predicate for asynchronous  interaction with threads.   New
    threads  created  inherit  their  debug-mode from  the  thread  that
    created them.


ttddeebbuugg
    Call tdebug/1 in all running threads.


ttnnooddeebbuugg((_+_T_h_r_e_a_d_I_d))
    Disable debugging thread _T_h_r_e_a_d_I_d.


ttnnooddeebbuugg
    Disable debugging in all threads.


ttssppyy((_:_S_p_e_c_, _+_T_h_r_e_a_d_I_d))
    Set  a spy-point as spy/1 and enable the thread for  debugging using
    tdebug/1.   Note that  a spy-point is a  global flag on a  predicate
    that  is visible from all threads.   Spy points are honoured  in all
    threads  that are in debug-mode and  ignored in threads that are  in
    nodebug mode.


ttssppyy((_:_S_p_e_c))
    Set  a  spy-point  as spy/1  and  enable  debugging in  all  threads
    using  tdebug/0.   Note that removing  spy-points can be done  using
    nospy/1.   Disabling spy-points in a specific thread is  achieved by
    tnodebug/1.


88..55..22 PPrrooffiilliinngg tthhrreeaaddss

In the  current implementation, at  most one thread  can be profiled  at
any moment.   Any thread can call profile/1 to profile the  execution of
some part of  its code.   The predicate tprofile/1 allows for  profiling
the execution of another thread until the user  stops collecting profile
data.


ttpprrooffiillee((_+_T_h_r_e_a_d_I_d))
    Start  collecting profile data in _T_h_r_e_a_d_I_d  and ask the user to  hit
    <_r_e_t_u_r_n>  to stop the profiler.  See section 4.40 for details on the
    execution profiler.


88..66 MMuullttii--tthhrreeaaddeedd mmiixxeedd CC aanndd PPrroolloogg aapppplliiccaattiioonnss

All foreign-code  linked to  the multi-threading  version of  SWI-Prolog
should  be   thread-safe  (_r_e_e_n_t_r_a_n_t)   or  guarded   in  Prolog   using
with_mutex/2 from  simultaneous  access  from multiple  Prolog  threads.
If  you want  to write  mixed multi-threaded  C  and Prolog  application
you  should  first  familiarise  yourself  with  writing  multi-threaded
applications in C (C++).

If you are  using SWI-Prolog as an  embedded engine in a  multi-threaded
application you  can access the Prolog  engine from multiple threads  by
creating an _e_n_g_i_n_e in  each thread from which you call Prolog.   Without
creating an engine, a thread can only use functions that  do _n_o_t use the
term_t type (for example PL_new_atom()).

The system supports  two models.   Section 8.6.1 describes the  original
one-to-one mapping.   In this schema  a native thread attaches a  Prolog
thread if  it needs  to call Prolog  and detaches is  when finished,  as
opposed to the  model from section 8.6.2  where threads temporary use  a
Prolog engine.

PPlleeaassee nnoottee  tthhaatt tthhee  iinntteerrffaaccee bbeellooww  wwiillll oonnllyy wwoorrkk  iiff tthhrreeaaddiinngg  iinn
yyoouurr aapppplliiccaattiioonn iiss bbaasseedd oonn tthhee ssaammee tthhrreeaadd--lliibbrraarryy aass  uusseedd ttoo ccoommppiillee
SSWWII--PPrroolloogg..


88..66..11 AA PPrroolloogg tthhrreeaadd ffoorr eeaacchh nnaattiivvee tthhrreeaadd ((oonnee--ttoo--oonnee))

In  the one-to-one  model, the  thread  that called  PL_initialise()  has
a  Prolog  engine  attached.     If  another  C-thread   in  the  system
wishes  to   call  Prolog  it   must  first   attach  an  engine   using
PL_thread_attach_engine()  and  call  PL_thread_destroy_engine()  after  all
Prolog  work is  finished.    This  model  is especially  suitable  with
long  running threads  that  need to  do Prolog  work  regularly.    See
section 8.6.2 for the alternative many-to-many model.


int PPLL__tthhrreeaadd__sseellff()
    Returns  the integer Prolog  identifier of the  engine or -1 if  the
    calling  thread  has  no Prolog  engine.    This  function  is  also
    provided  in the  single-threaded version  of  SWI-Prolog, where  it
    returns -2.


int PPLL__tthhrreeaadd__aattttaacchh__eennggiinnee(_c_o_n_s_t _P_L___t_h_r_e_a_d___a_t_t_r___t _*_a_t_t_r)
    Creates  a new Prolog engine in the calling thread.  If  the calling
    thread  already has an engine the  reference count of the engine  is
    incremented.  The  _a_t_t_r argument can be NULL to create a thread with
    default  attributes.  Otherwise it is a pointer to a  structure with
    the  definition below.   For any field  with value `0', the  default
    is  used.    The cancel  field may  be filled  with a  pointer to  a
    function  that is  called when  PL_cleanup()  terminates the  running
    Prolog  engines.  If this  function is not present or returns  FALSE
    pthread_cancel() is used.

    typedef struct
    { unsigned long     local_size;    /* Stack sizes (K-bytes) */
      unsigned long     global_size;
      unsigned long     trail_size;
      unsigned long     argument_size;
      char *            alias;         /* alias name */
      int              (*cancel)(int thread);
    } PL_thread_attr_t;

    The  structure may  be destroyed  after PL_thread_attach_engine()  has
    returned.    On success  it returns  the Prolog  identifier for  the
    thread (as  returned by PL_thread_self()).  If  an error occurs, -1 is
    returned.   If this Prolog  is not compiled for multi-threading,  -2
    is returned.


int PPLL__tthhrreeaadd__ddeessttrrooyy__eennggiinnee()
    Destroy  the  Prolog engine  in  the calling  thread.    Only  takes
    effect  if  PL_thread_destroy_engine()  is  called as  many  times  as
    PL_thread_attach_engine() in this  thread.   Returns TRUE on  success
    and  FALSE if the calling thread  has no engine or this Prolog  does
    not support threads.

    Please  note  that  construction  and  destruction  of  engines  are
    relatively  expensive  operations.     Only  destroy  an  engine  if
    performance is not critical and memory is a critical resource.


int PPLL__tthhrreeaadd__aatt__eexxiitt(_v_o_i_d _(_*_f_u_n_c_t_i_o_n_)_(_v_o_i_d _*_)_, _v_o_i_d _*_c_l_o_s_u_r_e_, _i_n_t _g_l_o_b_a_l)

    Register   a  handle  to   be  called  as   the  Prolog  engine   is
    destroyed.   The handler function is called with one void * argument
    holding  _c_l_o_s_u_r_e.  If _g_l_o_b_a_l  is TRUE, the handler is installed  _f_o_r
    _a_l_l  _t_h_r_e_a_d_s.   Globally installed handlers  are executed after  the
    thread-local  handlers.  If the  handler is installed local for  the
    current thread only (_g_l_o_b_a_l  == FALSE) it is stored in the same FIFO
    queue as used by thread_at_exit/1.


88..66..22 PPoooolliinngg PPrroolloogg eennggiinneess ((mmaannyy--ttoo--mmaannyy))

In this model Prolog engines live as entities that  are independent from
threads.  If a  thread needs to call Prolog it takes one of  the engines
from the pool and returns the engine when done.   This model is suitable
in the following identified cases:

  o _C_o_m_p_a_t_i_b_i_l_i_t_y _w_i_t_h _t_h_e _s_i_n_g_l_e_-_t_h_r_e_a_d_e_d _v_e_r_s_i_o_n
    In  the  single-threaded  version,  foreign threads  must  serialise
    access  the the one  and only  thread engine.   Functions from  this
    section allow sharing one engine among multiple threads.

  o _M_a_n_y _n_a_t_i_v_e _t_h_r_e_a_d_s _w_i_t_h _i_n_f_r_e_q_u_e_n_t _P_r_o_l_o_g _w_o_r_k
    Prolog  threads are expensive in terms of memory and time  to create
    and  destroy them.  Systems that use a large number of  threads that
    only  infrequently need  to call  Prolog are better  take an  engine
    from a pool and return it there.

  o _P_r_o_l_o_g _s_t_a_t_u_s _m_u_s_t _b_e _h_a_n_d_e_d _t_o _a_n_o_t_h_e_r _t_h_r_e_a_d
    This  situation has  been identified  by Uwe Lesta  when creating  a
    .NET  interface for SWI-Prolog.   .NET  distributes work for  active
    internet  connection over a  pool of  threads.   If a Prolog  engine
    contains  state for a connection, it must be possible to  detach the
    engine  from a thread  and re-attach it  to another thread  handling
    the same connection.


PL_engine_t PPLL__ccrreeaattee__eennggiinnee(_P_L___t_h_r_e_a_d___a_t_t_r___t _*_a_t_t_r_i_b_u_t_e_s)
    Create  a  new   Prolog  engine.     _a_t_t_r_i_b_u_t_e_s  is  described  with
    PL_thread_attach_engine().    Any thread  can  make this  call  after
    PL_initialise()  returned success.     The returned  engine  is  not
    attached  to any thread  and lives until PL_destroy_engine() is  used
    on the returned handle.

    In  the  single-threaded  version  this call  always  returns  NULL,
    indicating failure.


int PPLL__ddeessttrrooyy__eennggiinnee(_P_L___e_n_g_i_n_e___t _e)
    Destroy  the given engine.  Destroying an engine is only  allowed if
    the engine is not  attached to any thread or attached to the calling
    thread.    On success  this function  returns TRUE,  on failure  the
    return value is FALSE.


int PPLL__sseett__eennggiinnee(_P_L___e_n_g_i_n_e___t _e_n_g_i_n_e_, _P_L___e_n_g_i_n_e___t _*_o_l_d)
    Make  the calling thread ready  to use _e_n_g_i_n_e.   If _o_l_d is  non-NULL
    the  current engine  associated with  the calling  thread is  stored
    at  the  given  location.     If  _e_n_g_i_n_e equals  PL_ENGINE_MAIN  the
    initial  engine is attached  to the  calling thread.   If _e_n_g_i_n_e  is
    PL_ENGINE_CURRENT the  engine is  not  changed.   This  can be  used
    to  query the current  engine.   This call  returns PL_ENGINE_SET  if
    the  engine was switched successfully,  PL_ENGINE_INVAL if _e_n_g_i_n_e  is
    not  a  valid engine  handle and  PL_ENGINE_INUSE  if  the engine  is
    currently in use by another thread.

    Engines  can be changed  at any time.   For  example, it is  allowed
    to  select an engine to initiate a  Prolog goal, detach it and  at a
    later  moment execute the  goal from another  thread.  Note  however
    that  the term_t, qid_t and fid_t types are interpreted relative  to
    the  engine for which they are created.  Behaviour when  passing one
    of these types from one engine to another is undefined.

    In  the single-threaded  version this call  only succeeds if  _e_n_g_i_n_e
    refers to the main engine.


88..66..22..11 EEnnggiinneess iinn ssiinnggllee--tthhrreeaaddeedd SSWWII--PPrroolloogg

In  theory it  is possible  to  port the  API of  section  8.6.2 to  the
single-threaded  version of  SWI-Prolog.    This  allows  C-programs  to
control multiple  Prolog engines concurrently.   This  has not yet  been
realised.


88..77 MMuullttiitthhrreeaaddiinngg aanndd tthhee XXPPCCEE ggrraapphhiiccss ssyysstteemm

GUI applications  written in  XPCE can benefit  from the  multi-threaded
version of  XPCE/SWI-Prolog if  they need to  do expensive  computations
that block to UI in the single-threaded version.

Due  to  various  technical  problems  on  both   Windows  and  Unix/X11
threading is  best exploited by handing  long computations to their  own
thread.

The XPCE message  passing system is guarded  with a single _m_u_t_e_x,  which
synchronises both  access from  Prolog and activation  through the  GUI.
In  MS-Windows, GUI  events are  processed by  the  thread that  created
the window  in which the  event occurred, whereas  in Unix/X11 they  are
processed by the thread that dispatches messages.

Some  tentative work  is  underway to  improve the  integration  between
XPCE  and multi-threaded  SWI-Prolog.   There  are two  sets of  support
predicates.    The  first model  assumes that  XPCE  is running  in  the
main thread  and background threads  are used for computation.   In  the
second model, XPCE  event dispatching runs in the background,  while the
foreground thread is used for Prolog.

XXPPCCEE  iinn  tthhee   ffoorreeggrroouunndd Using  XPCE  in  the  foreground   simplifies
debugging  of  the  UI  and  generally  provides  the  most  comfortable
development  environment.       The  GUI  creates   new  threads   using
thread_create/3  and,  after  work  in  the  thread  is  completed,  the
sub-thread  signals  the  main  thread  of  the   the  completion  using
in_pce_thread/1.


iinn__ppccee__tthhrreeaadd((_:_G_o_a_l))
    Assuming  XPCE is running in the foreground thread, this  call gives
    background  threads  the  opportunity  to make  calls  to  the  XPCE
    thread.    A call to  in_pce_thread/1 succeeds immediately,  copying
    _G_o_a_l  to the XPCE  thread.   _G_o_a_l is added  to the XPCE  event-queue
    and  executed  synchronous to  normal user  events  like typing  and
    clicking.

XXPPCCEE iinn tthhee bbaacckkggrroouunndd

In this model a thread for  running XPCE is created using pce_dispatch/1
and actions are sent to this thread using pce_call/1.


ppccee__ddiissppaattcchh((_+_O_p_t_i_o_n_s))
    Create   a  Prolog   thread  with  the   alias-name  pce  for   XPCE
    event-handling.  In  the X11 version this call creates a thread that
    executes  the X11  event-dispatch loop.   In  MS-Windows it  creates
    a  thread that  executes a windows  event-dispatch loop.   The  XPCE
    event-handling  thread has  the alias  pce.   _O_p_t_i_o_n_s specifies  the
    thread-attributes as thread_create/3.


ppccee__ccaallll((_:_G_o_a_l))
    Post  _G_o_a_l to  the pce  thread,  executing it  synchronous with  the
    thread's  event-loop.  The pce_call/1 predicate returns  immediately
    without waiting.  Note that _G_o_a_l is _c_o_p_i_e_d to the pce thread.

For          further           information          about           XPCE
in         threaded         applications,           please         visit
http://gollem.science.uva.nl/twiki/pl/bin/view/Development/MultiThreadsXPCE


<


CChhaapptteerr 99..  FFOORREEIIGGNN LLAANNGGUUAAGGEE IINNTTEERRFFAACCEE

SWI-Prolog      offers      a     powerful      interface      to      C
[Kernighan & Ritchie, 1978].     The  main  design   objectives  of  the
foreign language interface  are flexibility and performance.   A foreign
predicate  is a  C-function that  has the  same number  of arguments  as
the predicate  represented.   C-functions  are provided  to analyse  the
passed terms,  convert them to basic C-types  as well as to  instantiate
arguments using unification.   Non-deterministic foreign predicates  are
supported,  providing the  foreign  function with  a handle  to  control
backtracking.

C can  call Prolog  predicates, providing  both an  query interface  and
an interface  to extract  multiple solutions  from an  non-deterministic
Prolog predicate.   There is no limit  to the nesting of Prolog  calling
C, calling Prolog,  etc.  It is also  possible to write the `main'  in C
and use Prolog as an embedded logical engine.


99..11 OOvveerrvviieeww ooff tthhee IInntteerrffaaccee

A special include file called SWI-Prolog.h should be included  with each
C-source file  that is  to be  loaded via the  foreign interface.    The
installation process installs this file in the directory  include in the
SWI-Prolog home  directory (?- current_prolog_flag(home, Home).).   This
C-header file defines various data types, macros and  functions that can
be used  to communicate with  SWI-Prolog.  Functions  and macros can  be
divided into the following categories:

  o Analysing Prolog terms

  o Constructing new terms

  o Unifying terms

  o Returning control information to Prolog

  o Registering foreign predicates with Prolog

  o Calling Prolog from C

  o Recorded database interactions

  o Global actions on Prolog (halt, break, abort, etc.)


99..22 LLiinnkkiinngg FFoorreeiiggnn MMoodduulleess

Foreign modules  may be  linked to  Prolog in two  ways.   Using  _s_t_a_t_i_c
_l_i_n_k_i_n_g, the extensions,  a (short) file defining main()  which attaches
the extensions calls  Prolog and the SWI-Prolog kernel distributed  as a
C-library are linked together  to form a new executable.   Using _d_y_n_a_m_i_c
_l_i_n_k_i_n_g,  the extensions are  linked to  a shared library  (.so file  on
most  Unix systems)  or  dynamic-link library  (.DLL file  on  Microsoft
platforms) and loaded into the the running Prolog process..


99..22..11 WWhhaatt lliinnkkiinngg iiss pprroovviiddeedd??

The _s_t_a_t_i_c  _l_i_n_k_i_n_g schema can  be used on  all versions of  SWI-Prolog.
Whether or  not dynamic  linking is  supported can be  deduced from  the
prolog-flag  open_shared_object  (see current_prolog_flag/2).    If  this
prolog-flag yields true, open_shared_object/2 and related predicates are
defined.  See  section 9.4 for a suitable high-level interface  to these
predicates.


99..22..22 WWhhaatt kkiinndd ooff llooaaddiinngg sshhoouulldd II bbee uussiinngg??

All  described  approaches  have  their  advantages  and  disadvantages.
Static linking  is portable and allows  for debugging on all  platforms.
It is relatively  cumbersome and the libraries  you need to pass to  the
linker may vary from  system to system, though the utility  program plld
described in section 9.7 often hides these problems from the user.

Loading  shared objects  (DLL  files on  Windows) provides  sharing  and
protection  and is  generally the  best choice.    If  a saved-state  is
created using qsave_program/[1,2], an initialization/1 directive  may be
used to load the appropriate library at startup.

Note  that  the  definition of  the  foreign  predicates  is  the  same,
regardless of the linking type used.


99..33 DDyynnaammiicc LLiinnkkiinngg ooff sshhaarreedd lliibbrraarriieess

The interface  defined in this  section allows the  user to load  shared
libraries  (.so files  on most  Unix systems,  .dll  files on  Windows).
This  interface is  portable to  Windows  as well  as to  Unix  machines
providing  dlopen(2)  (Solaris,  Linux,  FreeBSD, Irix  and  many  more)
or  shl_open(2) (HP/UX).  It  is  advised to  use  the  predicates  from
section 9.4 in your application.


ooppeenn__sshhaarreedd__oobbjjeecctt((_+_F_i_l_e_, _-_H_a_n_d_l_e))
    _F_i_l_e  is  the name  of a  shared object  file  (called dynamic  load
    library  in  MS-Windows).   This  file is  attached  to the  current
    process  and  _H_a_n_d_l_e  is  unified  with a  handle  to  the  library.
    Equivalent   to  open_shared_object(File, [], Handle).      See  also
    load_foreign_library/[1,2].

    On  errors, an  exception shared_object(_A_c_t_i_o_n_, _M_e_s_s_a_g_e) is  raised.
    _M_e_s_s_a_g_e is the return value from dlerror().


ooppeenn__sshhaarreedd__oobbjjeecctt((_+_F_i_l_e_, _-_H_a_n_d_l_e_, _+_O_p_t_i_o_n_s))
    As  open_shared_object/2, but  allows  for  additional flags  to  be
    passed.   _O_p_t_i_o_n_s is a list of  atoms.  now implies the  symbols are
    resolved  immediately rather  than lazy (default).   global  implies
    symbols of the  loaded object are visible while loading other shared
    objects (by default they  are local).  Note that these flags may not
    be  supported by your operating system.  Check the  documentation of
    dlopen() or equivalent  on your operating system.  Unsupported flags
    are silently ignored.


cclloossee__sshhaarreedd__oobbjjeecctt((_+_H_a_n_d_l_e))
    Detach the shared object identified by _H_a_n_d_l_e.


ccaallll__sshhaarreedd__oobbjjeecctt__ffuunnccttiioonn((_+_H_a_n_d_l_e_, _+_F_u_n_c_t_i_o_n))
    Call the named function  in the loaded shared library.  The function
    is  called  without  arguments  and  the  return-value  is  ignored.
    Normally  this function installs  foreign language predicates  using
    calls to PL_register_foreign().


99..44 UUssiinngg tthhee lliibbrraarryy sshhlliibb ffoorr .DLL aanndd .so ffiilleess

This  section discusses  the  functionality  of the  (autoload)  library
shlib.pl, providing an interface to shared libraries.   This library can
only be used if the prolog-flag open_shared_object is enabled.


llooaadd__ffoorreeiiggnn__lliibbrraarryy((_+_L_i_b_, _+_E_n_t_r_y))
    Search  for the  given foreign library  and link  it to the  current
    SWI-Prolog  instance.  The library may be specified with  or without
    the  extension.    First,  absolute_file_name/3 is  used  to  locate
    the  file.    If  this succeeds,  the  full path  is passed  to  the
    low-level  function  to open  the library.    Otherwise,  the  plain
    library  name  is passed,  exploiting  the operating-system  defined
    search  mechanism for  the shared  library.   The file_search_path/2
    alias  mechanism  defines the  alias foreign,  which  refers to  the
    directories <_p_l_h_o_m_e>/lib/<_a_r_c_h>and <_p_l_h_o_m_e>/lib, in this order.

    If  the library  can be loaded,  the function  called _E_n_t_r_y will  be
    called  without arguments.    The return  value of  the function  is
    ignored.

    The  _E_n_t_r_y  function  will  normally  call  PL_register_foreign()  to
    declare functions in the library as foreign predicates.


llooaadd__ffoorreeiiggnn__lliibbrraarryy((_+_L_i_b))
    Equivalent  to load_foreign_library/2.   For  the entry-point,  this
    function  first identifies the `base-name' of the library,  which is
    defined  to be the file-name with path nor extension.  It  will then
    try  the  entry-point install-<_b_a_s_e>.    On  failure it  will try  to
    function install().  Otherwise no install function will be called.


uunnllooaadd__ffoorreeiiggnn__lliibbrraarryy((_+_L_i_b))
    If  the foreign  library defines  the function  uninstall_<_b_a_s_e>() or
    uninstall(),  this  function will  be called  without arguments  and
    its  return value is  ignored.   Next, abolish/2  is used to  remove
    all  known foreign predicates defined in  the library.  Finally  the
    library itself is detached from the process.


ccuurrrreenntt__ffoorreeiiggnn__lliibbrraarryy((_-_L_i_b_, _-_P_r_e_d_i_c_a_t_e_s))
    Query  the currently loaded foreign libraries and  their predicates.
    _P_r_e_d_i_c_a_t_e_s  is  a  list  with  elements  of  the  form  _M_o_d_u_l_e_:_H_e_a_d,
    indicating  the predicates installed with  PL_register_foreign()  when
    the entry-point of the library was called.

Figure  9.1 connects  a Windows  message-box using  a foreign  function.
This example was tested using Windows NT and Microsoft Visual C++ 2.0.

#include <windows.h>
#include <SWI-Prolog.h>

static foreign_t
pl_say_hello(term_t to)
{ char *a;

  if ( PL_get_atom_chars(to, &a) )
  { MessageBox(NULL, a, "DLL test", MB_OK|MB_TASKMODAL);

    PL_succeed;
  }

  PL_fail;
}

install_t
install()
{ PL_register_foreign("say_hello", 1, pl_say_hello, 0);
}

            Figure 9.1:  MessageBox() example in Windows NT


99..44..11 SSttaattiicc LLiinnkkiinngg

Below  is an  outline of  the files  structure  required for  statically
linking  SWI-Prolog  with  foreign extensions.     \ldots/pl  refers  to
the  SWI-Prolog  home  directory (see  current_prolog_flag/2).    <_a_r_c_h>
refers  to  the  architecture identifier  that  may  be  obtained  using
current_prolog_flag/2.

      .../pl/runtime/<_a_r_c_h>/libpl.a SWI-Library
      .../pl/include/SWI-Prolog.h   Include file
      .../pl/include/SWI-Stream.h   Stream I/O include file
      .../pl/include/SWI-Exports    Export declarations (AIX only)
      .../pl/include/stub.c         Extension stub

The  definition   of  the  foreign  predicates   is  the  same  as   for
dynamic  linking.   Unlike  with dynamic  linking however,  there is  no
initialisation  function.   Instead,  the file  \ldots/pl/include/stub.c
may  be copied  to  your project  and  modified  to define  the  foreign
extensions.   Below is  stub.c, modified to  link the lowercase  example
described later in this chapter:

#include <stdio.h>
#include <SWI-Prolog.h>

extern foreign_t pl_lowercase(term, term);

PL_extension predicates[] =
{
/*{ "name",      arity,  function,      PL_FA_<flags> },*/

  { "lowercase", 2       pl_lowercase,  0 },
  { NULL,        0,      NULL,          0 }     /* terminating line */
};

int
main(int argc, char **argv)
{ PL_register_extensions(predicates);

  if ( !PL_initialise(argc, argv) )
    PL_halt(1);

  PL_install_readline();                /* delete if not required */

  PL_halt(PL_toplevel() ? 0 : 1);
}

Now,  a  new  executable may  be  created  by compiling  this  file  and
linking  it to  libpl.a from  the runtime  directory  and the  libraries
required  by both  the  extensions and  the  SWI-Prolog  kernel.    This
may  be  done  by   hand,  or  using  the  plld  utility   described  in
secrefplld.   If the  linking is performed  `by hand', the  command-line
option -dump-runtime-variables (see  section 2.4) can be used to  obtain
the  required paths,  libraries  and linking  options  to link  the  new
executable.


99..55 IInntteerrffaaccee DDaattaa ttyyppeess


99..55..11 TTyyppee term_t::  aa rreeffeerreennccee ttoo aa PPrroolloogg tteerrmm

The principal  data-type is term_t.   Type term_t is what Quintus  calls
QP_term_ref.   This name indicates  better what the type represents:   it
is a _h_a_n_d_l_e for a  term rather than the term itself.  Terms can  only be
represented and manipulated  using this type, as  this is the only  safe
way to  ensure the  Prolog kernel is  aware of  all terms referenced  by
foreign code  and thus allows the  kernel to perform  garbage-collection
and/or stack-shifts while foreign  code is active, for example  during a
callback from C.

A term  reference is  a C unsigned  long, representing  the offset of  a
variable on the Prolog environment-stack.  A foreign  function is passed
term references for the predicate-arguments, one for each  argument.  If
references for intermediate  results are needed, such references  may be
created using  PL_new_term_ref() or  PL_new_term_refs().   These  references
normally live till the foreign function returns control  back to Prolog.
Their scope  can be explicitly  limited using PL_open_foreign_frame()  and
PL_close_foreign_frame()/PL_discard_foreign_frame().

A term_t  always refers to a valid Prolog term (variable,  atom, integer,
float  or compound  term).    A  term  lives either  until  backtracking
takes  us back  to a  point before  the term  was created,  the  garbage
collector  has collected  the  term or  the  term  was created  after  a
PL_open_foreign_frame() and PL_discard_foreign_frame() has been called.

The  foreign-interface functions  can either  _r_e_a_d,  _u_n_i_f_y or  _w_r_i_t_e  to
term-references.   In the  this document we  use the following  notation
for arguments of type term_t:

     term_t +t  Accessed   in  read-mode.       The   `+'
                indicates the argument is `input'.
     term_t -t  Accessed in write-mode.
     term_t ?t  Accessed in unify-mode.

Term references are obtained in any of the following ways.

  o _P_a_s_s_e_d _a_s _a_r_g_u_m_e_n_t
    The  C-functions implementing  foreign predicates  are passed  their
    arguments  as term-references.    These  references may  be read  or
    unified.  Writing to these variables causes undefined behaviour.

  o _C_r_e_a_t_e_d _b_y _P_L___n_e_w___t_e_r_m___r_e_f_(_)
    A  term  created  by  PL_new_term_ref()  is  normally  used  to  build
    temporary  terms or be  written by one  of the interface  functions.
    For example,  PL_get_arg() writes a reference  to the term-argument in
    its last argument.

  o _C_r_e_a_t_e_d _b_y _P_L___n_e_w___t_e_r_m___r_e_f_s_(_i_n_t _n_)
    This   function  returns  a   set  of  term   refs  with  the   same
    characteristics as PL_new_term_ref().  See PL_open_query().

  o _C_r_e_a_t_e_d _b_y _P_L___c_o_p_y___t_e_r_m___r_e_f_(_t_e_r_m___t _t_)
    Creates a new term-reference  to the same term as the argument.  The
    term may be written to.  See figure 9.3.

Term-references  can safely  be  copied  to other  C-variables  of  type
term_t, but all copies will always refer to the same term.


term_t PPLL__nneeww__tteerrmm__rreeff()
    Return  a fresh reference  to a  term.   The reference is  allocated
    on  the _l_o_c_a_l  stack.   Allocating  a term-reference  may trigger  a
    stack-shift  on machines  that cannot  use sparse-memory  management
    for allocation the  Prolog stacks.  The returned reference describes
    a variable.


term_t PPLL__nneeww__tteerrmm__rreeffss(_i_n_t _n)
    Return  _n  new  term  references.     The  first  term-reference  is
    returned.   The others are _t +1, _t +2, etc.   There are two reasons
    for  using this function.   PL_open_query() expects the arguments  as
    a  set of  consecutive term references  and _v_e_r_y time-critical  code
    requiring a number of term-references can be written as:

    pl_mypredicate(term_t a0, term_t a1)
    { term_t t0 = PL_new_term_refs(2);
      term_t t1 = t0+1;

      ...
    }


term_t PPLL__ccooppyy__tteerrmm__rreeff(_t_e_r_m___t _f_r_o_m)
    Create a new term  reference and make it point initially to the same
    term  as _f_r_o_m.  This function  is commonly used to copy  a predicate
    argument to a term reference that may be written.


void PPLL__rreesseett__tteerrmm__rreeffss(_t_e_r_m___t _a_f_t_e_r)
    Destroy  all term  references that  have been  created after  _a_f_t_e_r,
    including  _a_f_t_e_r itself.    Any  reference to  the invalidated  term
    references after this call results in undefined behaviour.

    Note  that  returning  from  the  foreign  context  to  Prolog  will
    reclaim  all references  used in  the foreign  context.   This  call
    is  only necessary  if  references are  created inside  a loop  that
    never  exits  back  to  Prolog.    See  also  PL_open_foreign_frame(),
    PL_close_foreign_frame() and PL_discard_foreign_frame().


99..55..11..11 IInntteerraaccttiioonn wwiitthh tthhee ggaarrbbaaggee ccoolllleeccttoorr aanndd ssttaacckk--sshhiifftteerr

Prolog implements two  mechanisms for avoiding stack overflow:   garbage
collection and stack expansion.   On machines that allow for  it, Prolog
will use virtual  memory management to detect stack overflow  and expand
the  runtime stacks.    On  other machines  Prolog will  reallocate  the
stacks and update all pointers to them.  To do so,  Prolog needs to know
which data is  referenced by C-code.  As  all Prolog data known by  C is
referenced through term references (term_t), Prolog has  all information
necessary to perform  its memory management without special  precautions
from the C-programmer.


99..55..22 OOtthheerr ffoorreeiiggnn iinntteerrffaaccee ttyyppeess

aattoomm__tt An atom in Prologs  internal representation.  Atoms  are pointers
    to  an  opaque structure.    They are  a  unique representation  for
    represented  text, which  implies that  atom A  represents the same
    text as atom B  if-and-only-if A and B are the same pointer.

    Atoms  are  the  central  representation for  textual  constants  in
    Prolog  The  transformation  of C  a  character  string to  an  atom
    implies  a hash-table lookup.  If the same atom is needed  often, it
    is  advised to  store its reference  in a  global variable to  avoid
    repeated lookup.

ffuunnccttoorr__tt A  functor is  the  internal  representation of  a  name/arity
    pair.   They are used to find the name and arity of  a compound term
    as  well as to construct new compound  terms.  Like atoms  they live
    for the whole Prolog session and are unique.

pprreeddiiccaattee__tt Handle  to a  Prolog  predicate.    Predicate  handles  live
    forever (although they can loose their definition).

qqiidd__tt Query         Identifier.                          Used         by
    PL_open_query()/PL_next_solution()/PL_close_query()  to  handle   back-
    tracking from C.

ffiidd__tt Frame         Identifier.                          Used         by
    PL_open_foreign_frame()/PL_close_foreign_frame().

mmoodduullee__tt A module is  a unique handle to a  Prolog module.  Modules  are
    used only to call predicates in a specific module.

ffoorreeiiggnn__tt Return type for a C-function implementing a Prolog predicate.

ccoonnttrrooll__tt Passed  as additional  argument to  non-deterministic  foreign
    functions.  See PL_retry*() and PL_foreign_context*().

iinnssttaallll__tt Type for the install() and uninstall() functions  of shared or
    dynamic link libraries.  See secrefshlib.

iinntt6644__tt Actually  part of  the C99  standard  rather than  Prolog.    As
    of  version  5.5.6,  Prolog integers  are  64-bit on  all  hardware.
    The  C99 type  int64_t  is defined  in the  stdint.h standard  header
    and  provides platform independent 64-bit  integers.  Portable  code
    accessing  Prolog should use this  type to exchange integer  values.
    Please  note that PL_get_long() can  return FALSE on Prolog  integers
    outside  the long domain.  Robust code should not assume any  of the
    integer fetching functions  to succeed if the Prolog term is know to
    be an integer.


99..66 TThhee FFoorreeiiggnn IInncclluuddee FFiillee


99..66..11 AArrgguummeenntt PPaassssiinngg aanndd CCoonnttrrooll

If  Prolog encounters  a foreign  predicate  at run  time it  will  call
a  function  specified  in  the  predicate  definition  of  the  foreign
predicate.   The arguments 1;:::; <_a_r_i_t_y>pass the  Prolog arguments to the
goal as  Prolog terms.   Foreign  functions should  be declared of  type
foreign_t.   Deterministic foreign  functions have  two alternatives  to
return control back to Prolog:


_(_r_e_t_u_r_n_) _f_o_r_e_i_g_n___t PPLL__ssuucccceeeedd(())
    Succeed deterministically.  PL_succeed is defined as return TRUE.


_(_r_e_t_u_r_n_) _f_o_r_e_i_g_n___t PPLL__ffaaiill(())
    Fail  and  start  Prolog  backtracking.     PL_fail  is  defined  as
    return FALSE.


99..66..11..11 NNoonn--ddeetteerrmmiinniissttiicc FFoorreeiiggnn PPrreeddiiccaatteess

By  default   foreign  predicates   are  deterministic.      Using   the
PL_FA_NONDETERMINISTIC   attribute  (see   PL_register_foreign())  it   is
possible  to register  a  predicate  as a  non-deterministic  predicate.
Writing   non-deterministic   foreign  predicates   is   slightly   more
complicated  as  the  foreign function  needs  context  information  for
generating  the next  solution.   Note  that the  same foreign  function
should  be  prepared  to be  simultaneously  active  in  more  than  one
goal.     Suppose  the  natural_number_below_n/2  is  a  non-deterministic
foreign predicate, backtracking over all natural numbers  lower than the
first argument.  Now consider the following predicate:

quotient_below_n(Q, N) :-
        natural_number_below_n(N, N1),
        natural_number_below_n(N, N2),
        Q =:= N1 / N2, !.

In  this predicate  the function  natural_number_below_n/2  simultaneously
generates solutions for both its invocations.

Non-deterministic foreign functions  should be prepared to handle  three
different calls from Prolog:

  o _I_n_i_t_i_a_l _c_a_l_l _(PL_FIRST_CALL_)
    Prolog  has just created a frame  for the foreign function and  asks
    it to produce the first answer.

  o _R_e_d_o _c_a_l_l _(PL_REDO_)
    The previous invocation  of the foreign function associated with the
    current  goal indicated it was possible  to backtrack.  The  foreign
    function should produce the next solution.

  o _T_e_r_m_i_n_a_t_e _c_a_l_l _(PL_CUTTED_)
    The choice point  left by the foreign function has been destroyed by
    a  cut.  The foreign function is given the opportunity to  clean the
    environment.

Both  the  context  information  and  the  type  of   call  is  provided
by  an  argument  of  type  control_t  appended  to  the  argument  list
for  deterministic foreign  functions.    The macro  PL_foreign_control()
extracts  the   type  of  call   from  the  control   argument.      The
foreign  function  can  pass  a context  handle  using  the  PL_retry*()
macros  and  extract  the handle  from  the  extra  argument  using  the
PL_foreign_context*() macro.


_(_r_e_t_u_r_n_) _f_o_r_e_i_g_n___t PPLL__rreettrryy((_l_o_n_g))
    The  foreign function  succeeds while leaving  a choice  point.   On
    backtracking  over this  goal the  foreign function  will be  called
    again,  but the control argument now  indicates it is a `Redo'  call
    and  the  macro PL_foreign_context() returns  the  handle passed  via
    PL_retry().   This handle is  a 30 bits signed  value (two bits  are
    used  for status indication).  Defined  as return _PL_retry(n).   See
    also PL_succeed().


_(_r_e_t_u_r_n_) _f_o_r_e_i_g_n___t PPLL__rreettrryy__aaddddrreessss((_v_o_i_d _*))
    As  PL_retry(), but  ensures  an  address as  returned  by  malloc()
    is  correctly recovered by  PL_foreign_context_address().  Defined  as
    return  _PL_retry_address(n).  See also PL_succeed().


_i_n_t PPLL__ffoorreeiiggnn__ccoonnttrrooll((_c_o_n_t_r_o_l___t))
    Extracts  the type of  call from the control  argument.  The  return
    values  are  described above.    Note that  the  function should  be
    prepared  to handle the PL_CUTTED case and should be aware  that the
    other arguments are not valid in this case.


_l_o_n_g PPLL__ffoorreeiiggnn__ccoonntteexxtt((_c_o_n_t_r_o_l___t))
    Extracts  the context from the context  argument.  In the call  type
    is  PL_FIRST_CALL the context value is 0L. Otherwise it  is the value
    returned  by the last PL_retry() associated with this goal (both  if
    the call type is PL_REDO as PL_CUTTED).


_v_o_i_d _* PPLL__ffoorreeiiggnn__ccoonntteexxtt__aaddddrreessss((_c_o_n_t_r_o_l___t))
    Extracts an address as passed in by PL_retry_address().

Note:  If a  non-deterministic foreign function returns using PL_succeed
or  PL_fail,   Prolog  assumes  the  foreign  function  has  cleaned  its
environment.  NNoo call with control argument PL_CUTTED will follow.

The code of figure 9.2 shows a skeleton for  a non-deterministic foreign
predicate definition.

typedef struct                  /* define a context structure */
{ ...
} context;

foreign_t
my_function(term_t a0, term_t a1, control_t handle)
{ struct context * ctxt;

  switch( PL_foreign_control(handle) )
  { case PL_FIRST_CALL:
        ctxt = malloc(sizeof(struct context));
        ...
        PL_retry_address(ctxt);
    case PL_REDO:

        ctxt = PL_foreign_context_address(handle);
        ...
        PL_retry_address(ctxt);
    case PL_CUTTED:
        ctxt = PL_foreign_context_address(handle);
        ...
        free(ctxt);
        PL_succeed;

  }
}

     Figure 9.2:  Skeleton for non-deterministic foreign functions


99..66..22 AAttoommss aanndd ffuunnccttoorrss

The  following  functions  provide for  communication  using  atoms  and
functors.


atom_t PPLL__nneeww__aattoomm(_c_o_n_s_t _c_h_a_r _*)
    Return an atom handle  for the given C-string.  This function always
    succeeds.    The returned handle  is valid  as long as  the atom  is
    referenced (see section 9.6.2.1).


const char* PPLL__aattoomm__cchhaarrss(_a_t_o_m___t _a_t_o_m)
    Return  a C-string for the text represented by the given atom.   The
    returned  text will not be changed by Prolog.  It is  not allowed to
    modify  the contents, not even `temporary' as the string  may reside
    in  read-only memory.   The returned  string becomes invalid if  the
    atom is garbage-collected  (see section 9.6.2.1).  Foreign functions
    that  require the text from an atom passed in  a term_t  normally use
    PL_get_atom_chars() or PL_get_atom_nchars().


functor_t PPLL__nneeww__ffuunnccttoorr(_a_t_o_m___t _n_a_m_e_, _i_n_t _a_r_i_t_y)
    Returns  a _f_u_n_c_t_o_r  _i_d_e_n_t_i_f_i_e_r, a  handle for  the name/arity  pair.
    The returned handle is valid for the entire Prolog session.


atom_t PPLL__ffuunnccttoorr__nnaammee(_f_u_n_c_t_o_r___t _f)
    Return an atom representing the name of the given functor.


int PPLL__ffuunnccttoorr__aarriittyy(_f_u_n_c_t_o_r___t _f)
    Return the arity of the given functor.


99..66..22..11 AAttoommss aanndd aattoomm--ggaarrbbaaggee ccoolllleeccttiioonn

With  the introduction  of  atom-garbage  collection in  version  3.3.0,
atoms  no  longer  live  as  long  as  the  process.    Instead,   their
lifetime is  guaranteed only as  long as  they are referenced.   In  the
single-threaded version,  atom garbage collections  are only invoked  at
the  _c_a_l_l_-_p_o_r_t.   In  the multi-threaded  version (see  section 8,  they
appear asynchronously, except for the invoking thread.

For dealing with  atom garbage collection, two additional  functions are
provided:


void PPLL__rreeggiisstteerr__aattoomm(_a_t_o_m___t _a_t_o_m)
    Increment  the reference  count of the  atom by  one.   PL_new_atom()
    performs  this automatically,  returning  an atom  with a  reference
    count of at least one.


void PPLL__uunnrreeggiisstteerr__aattoomm(_a_t_o_m___t _a_t_o_m)
    Decrement  the reference count of the atom.  If  the reference-count
    drops below zero, an assertion error is raised.

Please note that the  following two calls are different with  respect to
atom garbage collection:

PL_unify_atom_chars(t, "text");
PL_unify_atom(t, PL_new_atom("text"));

The  latter increments  the  reference count  of  the atom  text,  which
effectively ensures the atom will never be collected.   It is advised to
use the *_chars() or *_nchars() functions whenever applicable.


99..66..33 AAnnaallyyssiinngg TTeerrmmss vviiaa tthhee FFoorreeiiggnn IInntteerrffaaccee

Each argument  of a foreign function  (except for the control  argument)
is of type term_t, an opaque  handle to a Prolog term.  Three  groups of
functions are  available for  the analysis  of terms.    The first  just
validates the type,  like the Prolog  predicates var/1, atom/1, etc  and
are  called PL_is_*().    The second  group  attempts to  translate  the
argument into a C primitive type.   These predicates take a term_t and a
pointer to the appropriate C-type and return TRUE or  FALSE depending on
successful or unsuccessful translation.   If the translation fails,  the
pointed-to data is never modified.


99..66..33..11 TTeessttiinngg tthhee ttyyppee ooff aa tteerrmm


int PPLL__tteerrmm__ttyyppee(_t_e_r_m___t)
    Obtain  the type  of a  term,  which should  be a  term returned  by
    one  of the  other interface  predicates or passed  as an  argument.
    The  function  returns the  type  of  the Prolog  term.    The  type
    identifiers  are listed below.   Note that the extraction  functions
    PL_ge_t*() also validate  the type and  thus the two sections  below
    are equivalent.

            if ( PL_is_atom(t) )
            { char *s;

              PL_get_atom_chars(t, &s);
              ...;
            }

    or

            char *s;
            if ( PL_get_atom_chars(t, &s) )
            { ...;
            }

    ___________________________________________________________________
    | PL_VARIABLE            |An unbound variable.   The value  of term|
    |                        |as such  is a  unique identifier  for the|

    |                        |variable.                                |
    | PL_ATOM                |A Prolog atom.                           |
    | PL_STRING              |A Prolog string.                         |
    | PL_INTEGER             |A Prolog integer.                        |
    | PL_FLOAT               |A Prolog floating point number.          |
    | PL_TERM                |A compound term.   Note that a  list is a|
    |________________________|compound_term_./2._______________________|

The functions PL_is_<_t_y_p_e> are an alternative to PL_term_type().  The test
PL_is_variable(term) is equivalent to  PL_term_type(term) == PL_VARIABLE,
but  the first  is considerably  faster.   On  the other  hand, using  a
switch over  PL_term_type() is  faster and  more readable  then using  an
if-then-else using  the functions  below.   All  these functions  return
either TRUE or FALSE.


int PPLL__iiss__vvaarriiaabbllee(_t_e_r_m___t)
    Returns non-zero if _t_e_r_m is a variable.


int PPLL__iiss__ggrroouunndd(_t_e_r_m___t)
    Returns  non-zero if  _t_e_r_m is  a ground term.    See also  ground/1.
    This function is cycle-safe.


int PPLL__iiss__aattoomm(_t_e_r_m___t)
    Returns non-zero if _t_e_r_m is an atom.


int PPLL__iiss__ssttrriinngg(_t_e_r_m___t)
    Returns non-zero if _t_e_r_m is a string.


int PPLL__iiss__iinntteeggeerr(_t_e_r_m___t)
    Returns non-zero if _t_e_r_m is an integer.


int PPLL__iiss__ffllooaatt(_t_e_r_m___t)
    Returns non-zero if _t_e_r_m is a float.


int PPLL__iiss__ccoommppoouunndd(_t_e_r_m___t)
    Returns non-zero if _t_e_r_m is a compound term.


int PPLL__iiss__ffuunnccttoorr(_t_e_r_m___t_, _f_u_n_c_t_o_r___t)
    Returns  non-zero if _t_e_r_m  is compound and  its functor is  _f_u_n_c_t_o_r.
    This test  is equivalent to PL_get_functor(),  followed by testing the
    functor, but easier to write and faster.


int PPLL__iiss__lliisstt(_t_e_r_m___t)
    Returns non-zero if _t_e_r_m  is a compound term with functor ./2 or the
    atom [].


int PPLL__iiss__aattoommiicc(_t_e_r_m___t)
    Returns non-zero if _t_e_r_m is atomic (not variable or compound).


int PPLL__iiss__nnuummbbeerr(_t_e_r_m___t)
    Returns non-zero if _t_e_r_m is an integer or float.


99..66..33..22 RReeaaddiinngg ddaattaa ffrroomm aa tteerrmm

The functions PL_get_*() read information from  a Prolog term.  Most  of
them take two arguments.  The first is the input  term and the second is
a pointer to the output value or a term-reference.


int PPLL__ggeett__aattoomm(_t_e_r_m___t _+_t_, _a_t_o_m___t _*_a)
    If _t is an atom,  store the unique atom identifier over _a.  See also
    PL_atom_chars() and PL_new_atom().  If there is no need  to access the
    data  (characters) of  an atom,  it is advised  to manipulate  atoms
    using  their handle.  As the  atom is referenced by _t, it  will live
    at  least as long as _t does.   If longer live-time is required,  the
    atom should be locked using PL_register_atom().


int PPLL__ggeett__aattoomm__cchhaarrss(_t_e_r_m___t _+_t_, _c_h_a_r _*_*_s)
    If  _t is an atom, store a  pointer to a 0-terminated C-string  in _s.
    It is explicitly  nnoott allowed to modify the contents of this string.
    Some  built-in  atoms may  have the  string  allocated in  read-only
    memory, so `temporary manipulation' can cause an error.


int PPLL__ggeett__ssttrriinngg__cchhaarrss(_t_e_r_m___t _+_t_, _c_h_a_r _*_*_s_, _i_n_t _*_l_e_n)
    If  _t  is  a  string  object,  store a  pointer  to  a  0-terminated
    C-string  in _s  and the  length of  the string in  _l_e_n.   Note  that
    this pointer  is invalidated by backtracking, garbage-collection and
    stack-shifts,  so generally the only save operations are to  pass it
    immediately to a C-function that doesn't involve Prolog.


int PPLL__ggeett__cchhaarrss(_t_e_r_m___t _+_t_, _c_h_a_r _*_*_s_, _u_n_s_i_g_n_e_d _f_l_a_g_s)
    Convert  the argument term _t to  a 0-terminated C-string.  _f_l_a_g_s  is
    a  bitwise disjunction  from two  groups of  constants.   The  first
    specifies  which term-types should converted and the second  how the
    argument  is stored.  Below  is a specification of these  constants.
    BUF_RING implies, if  the data is not static (as from an  atom), the
    data  is copied to the next buffer from a ring of 16 buffers.   This
    is  a convenient way  of converting multiple  arguments passed to  a
    foreign  predicate to C-strings.    If BUF_MALLOC  is used, the  data
    must be freed using PL_free() when not needed any longer.

    With  the introduction of wide-characters (see section 2.17.1),  not
    all  atoms can be converted into a char*.  This function  fails if _t
    is  of the wrong type, but  also if the text cannot  be represented.
    See the REP_* flags below for details.
    ___________________________________________________________________
    | CVT_ATOM               |Convert if term is an atom               |
    | CVT_STRING             |Convert if term is a string              |
    | CVT_LIST               |Convert  if term  is a  list of  integers|
    |                        |between 1 and 255                        |

    | CVT_INTEGER            |Convert if term is an integer (using %d) |
    | CVT_FLOAT              |Convert if term is a float (using %f)    |
    | CVT_NUMBER             |Convert if term is a integer or float    |
    | CVT_ATOMIC             |Convert if term is atomic                |
    | CVT_VARIABLE           |Convert variable to print-name           |
    | CVT_WRITE              |Convert any  term that  is not  converted|
    |                        |by any of the  other flags using write/1.|
    |                        |If  no  BUF_*  is  provided,  BUF_RING  is|

    |                        |implied.                                 |
    | CVT_ALL                |Convert  if term  is  any  of the  above,|
    |________________________|except_for_CVT_VARIABLE_and_CVT_WRITE____|
    | CVT_EXCEPTION          |If conversion fails due  to a type error,|
    |                        |raise a  Prolog type  error exception  in|
    |________________________|addition_to_failure______________________|
    | BUF_DISCARDABLE        |Data must copied immediately             |

    | BUF_RING               |Data is stored in a ring of buffers      |
    | BUF_MALLOC             |Data is  copied to a new  buffer returned|
    |                        |by PL_malloc(3).   When no  longer needed|
    |________________________|the_user_must_call_PL_free()_on_the_data.|_
    | REP_ISO_LATIN_1        |(0,  default).   Text  is in ISO  Latin-1|
    |                        |encoding  and  the  call  fails  if  text|
    |                        |cannot be represented.                   |

    | REP_UTF8               |Convert  the  text  to  a  UTF-8  string.|
    |                        |This works for all text.                 |
    | REP_MB                 |Convert to  default locale-defined  8-bit|
    |                        |string.   Success depends on  the locale.|
    |                        |Conversion  is done  using the  wcrtomb()|
    |________________________|C-library_function.______________________|


int PPLL__ggeett__lliisstt__cchhaarrss(_+_t_e_r_m___t _l_, _c_h_a_r _*_*_s_, _u_n_s_i_g_n_e_d _f_l_a_g_s)
    Same  as PL_get_chars(l, s, CVT_LIST|flags), provided _f_l_a_g_s  contains
    no of the CVT_* flags.


int PPLL__ggeett__iinntteeggeerr(_+_t_e_r_m___t _t_, _i_n_t _*_i)
    If  _t is  a Prolog  integer, assign  its value over  _i.   On  32-bit
    machines,  this is  the same as  PL_get_long(),  but avoids a  warning
    from the compiler.  See also PL_get_long().


int PPLL__ggeett__lloonngg(_t_e_r_m___t _+_t_, _l_o_n_g _*_i)
    If  _t is a Prolog integer that can be represented as a  long, assign
    its  value over _i.   If _t is  an integer that cannot be  represented
    by  a C long, this function returns FALSE. If _t is a  floating point
    number that can  be represented as a long, this function succeeds as
    well.  See also PL_get_int64()


int PPLL__ggeett__iinntt6644(_t_e_r_m___t _+_t_, _i_n_t_6_4___t _*_i)
    If  _t is  a Prolog integer  or float  that can be  represented as  a
    int64_t,  assign its value over  _i.   Currently all Prolog  integers
    can  be  represented  using this  type,  but  this might  change  if
    SWI-Prolog introduces unbounded integers.


int PPLL__ggeett__bbooooll(_t_e_r_m___t _+_t_, _i_n_t _*_v_a_l)
    If _t has the value  true or false, set _v_a_l to the C constant TRUE or
    FALSE and return success.  otherwise return failure.


int PPLL__ggeett__ppooiinntteerr(_t_e_r_m___t _+_t_, _v_o_i_d _*_*_p_t_r)
    In   the  current  system,   pointers  are  represented  by   Prolog
    integers,   but  need  some  manipulation  to  make  sure   they  do
    not  get  truncated   due  to  the  limited  Prolog  integer  range.
    PL_put_pointer()/PL_get_pointer() guarantees pointers in the  range of
    malloc() are handled without truncating.


int PPLL__ggeett__ffllooaatt(_t_e_r_m___t _+_t_, _d_o_u_b_l_e _*_f)
    If _t is a float or integer, its value is assigned over _f.


int PPLL__ggeett__ffuunnccttoorr(_t_e_r_m___t _+_t_, _f_u_n_c_t_o_r___t _*_f)
    If  _t  is  compound  or  an  atom,   the  Prolog  representation  of
    the   name-arity  pair  will  be  assigned   over  _f.     See   also
    PL_get_name_arity() and PL_is_functor().


int PPLL__ggeett__nnaammee__aarriittyy(_t_e_r_m___t _+_t_, _a_t_o_m___t _*_n_a_m_e_, _i_n_t _*_a_r_i_t_y)
    If  _t is  compound or  an atom,  the functor-name  will be  assigned
    over  _n_a_m_e and the arity over  _a_r_i_t_y.  See  also PL_get_functor()  and
    PL_is_functor().


int PPLL__ggeett__mmoodduullee(_t_e_r_m___t _+_t_, _m_o_d_u_l_e___t _*_m_o_d_u_l_e)
    If _t is an  atom, the system will lookup or create the corresponding
    module and assign an opaque pointer to it over _m_o_d_u_l_e,.


int PPLL__ggeett__aarrgg(_i_n_t _i_n_d_e_x_, _t_e_r_m___t _+_t_, _t_e_r_m___t _-_a)
    If  _t is  compound and  index is  between 1  and arity  (including),
    assign _a with a term-reference to the argument.


int _PPLL__ggeett__aarrgg(_i_n_t _i_n_d_e_x_, _t_e_r_m___t _+_t_, _t_e_r_m___t _-_a)
    Same as  PL_get_arg(), but no checking is performed,  nor whether _t is
    actually a term, nor whether _i_n_d_e_x is a valid argument-index.


99..66..33..33 EExxcchhaannggiinngg tteexxtt uussiinngg lleennggtthh aanndd ssttrriinngg

All  internal text-representation  of  SWI-Prolog is  represented  using
char * plus length and  allow for _0_-_b_y_t_e_s in them.  The  foreign library
supports this by implementing  a *_nchars() function for  each applicable
*_chars()  function.   Below we briefly present  the signatures of  these
functions.  For full documentation consult the *_chars() function.


int PPLL__ggeett__aattoomm__nncchhaarrss(_t_e_r_m___t _t_, _s_i_z_e___t _*_l_e_n_, _c_h_a_r _*_*_s)
    See PL_get_atom_chars().


int PPLL__ggeett__lliisstt__nncchhaarrss(_t_e_r_m___t _t_, _s_i_z_e___t _*_l_e_n_, _c_h_a_r _*_*_s)
    See PL_get_list_chars().


int PPLL__ggeett__nncchhaarrss(_t_e_r_m___t _t_, _s_i_z_e___t _*_l_e_n_, _c_h_a_r _*_*_s_, _u_n_s_i_g_n_e_d _i_n_t _f_l_a_g_s)
    See PL_get_chars().


int PPLL__ppuutt__aattoomm__nncchhaarrss(_t_e_r_m___t _t_, _s_i_z_e___t _l_e_n_, _c_o_n_s_t _c_h_a_r _*_s)
    See PL_put_atom_chars().


int PPLL__ppuutt__ssttrriinngg__nncchhaarrss(_t_e_r_m___t _t_, _s_i_z_e___t _l_e_n_, _c_o_n_s_t _c_h_a_r _*_s)
    See PL_put_string_chars().


int PPLL__ppuutt__lliisstt__nnccooddeess(_t_e_r_m___t _t_, _s_i_z_e___t _l_e_n_, _c_o_n_s_t _c_h_a_r _*_s)
    See PL_put_list_codes().


int PPLL__ppuutt__lliisstt__nncchhaarrss(_t_e_r_m___t _t_, _s_i_z_e___t _l_e_n_, _c_o_n_s_t _c_h_a_r _*_s)
    See PL_put_list_chars().


int PPLL__uunniiffyy__aattoomm__nncchhaarrss(_t_e_r_m___t _t_, _s_i_z_e___t _l_e_n_, _c_o_n_s_t _c_h_a_r _*_s)
    See PL_unify_atom_chars().


int PPLL__uunniiffyy__ssttrriinngg__nncchhaarrss(_t_e_r_m___t _t_, _s_i_z_e___t _l_e_n_, _c_o_n_s_t _c_h_a_r _*_s)
    See PL_unify_string_chars().


int PPLL__uunniiffyy__lliisstt__nnccooddeess(_t_e_r_m___t _t_, _s_i_z_e___t _l_e_n_, _c_o_n_s_t _c_h_a_r _*_s)
    See PL_unify_codes().


int PPLL__uunniiffyy__lliisstt__nncchhaarrss(_t_e_r_m___t _t_, _s_i_z_e___t _l_e_n_, _c_o_n_s_t _c_h_a_r _*_s)
    See PL_unify_list_chars().

In  addition, the  following functions  are available  for creating  and
inspecting atoms:


atom_t PPLL__nneeww__aattoomm__nncchhaarrss(_s_i_z_e___t _l_e_n_, _c_o_n_s_t _c_h_a_r _*_s)
    Create a new atom as PL_new_atom(), but from length and characters.


const char * PPLL__aattoomm__nncchhaarrss(_a_t_o_m___t _a_, _s_i_z_e___t _*_l_e_n)
    Extract text and length of an atom.


99..66..33..44 WWiiddee cchhaarraacctteerr vveerrssiioonnss

Support  for  exchange   of  wide  character  strings  is  still   under
considerations.    The functions  dealing with  8-bit character  strings
return failure when operating on a wide character atom  or Prolog string
object.  The  functions below can extract and unify bith 8-bit  and wide
atoms and  string objects.   Wide character  strings are represented  as
C arrays  of objects of the  type pl_wchar_t,  which is guaranteed to  be
the same as wchar_t on platforms supporting this type.   For example, on
MS-Windows, this represents 16-bit UCS2 characters, while  using the GNU
C library (glibc) this represents 32-bit UCS4 characters.


atom_t PPLL__nneeww__aattoomm__wwcchhaarrss(_i_n_t _l_e_n_, _c_o_n_s_t _p_l___w_c_h_a_r___t _*_s)
    Create  atom from wide-character  string as PL_new_atom_nchars()  does
    for ISO-Latin-1 strings.   It _s only contains ISO-Latin-1 characters
    a normal byte-array atom is created.


pl_wchar_t* PPLL__aattoomm__wwcchhaarrss(_a_t_o_m___t _a_t_o_m_, _i_n_t _*_l_e_n)
    Extract  characters  from a  wide-character atom.    Fails  (returns
    NULL)  if  _a_t_o_m  is  not  a  wide-character  atom.     This  is  the
    wide-character  version of PL_atom_nchars().   Note that only one  of
    these  functions succeeds on a  particular atom.  Especially,  after
    creating an atom with  PL_new_atom_wchars(), extracting the text using
    PL_atom_wchars()  will fail  of the  atom  only contains  ISO-Latin-1
    characters.


int PPLL__ggeett__wwcchhaarrss(_t_e_r_m___t _t_, _s_i_z_e___t _*_l_e_n_, _p_l___w_c_h_a_r___t _*_*_s_, _u_n_s_i_g_n_e_d _f_l_a_g_s)
    Wide-character version  of PL_get_chars().  The  _f_l_a_g_s argument is the
    same as for PL_get_chars().


int PPLL__uunniiffyy__wwcchhaarrss(_t_e_r_m___t _t_, _i_n_t _t_y_p_e_, _s_i_z_e___t _l_e_n_, _c_o_n_s_t _p_l___w_c_h_a_r___t _*_s)
    Unify _t with  a textual representation of the C wide character array
    _s.   The _a_r_gtype argument  defines the Prolog representation and  is
    one of PL_ATOM, PL_STRING, PL_CODE_LIST or PL_CHAR_LIST.


int PPLL__uunniiffyy__wwcchhaarrss__ddiiffff(_t_e_r_m___t _+_t_, _t_e_r_m___t _-_t_a_i_l_, _i_n_t _t_y_p_e_, _s_i_z_e___t _l_e_n_, _c_o_n_s_t _p_l___w_c_h_a_r___t _*_s)

    Difference   list  version   of   PL_unify_wchars(),   only   support-
    ing the  types PL_CODE_LIST and PL_CHAR_LIST. It serves two purposes.
    It  allows  for returning  very long  lists from  data  read from  a
    stream  without the  need for  a resizing buffer  in C  and the  use
    of  difference lists  is often practical  for further processing  in
    Prolog.   Examples can be found in packages/clib/readutil.c from the
    source distribution.


99..66..33..55 RReeaaddiinngg aa lliisstt

The functions from this section are intended to read  a Prolog list from
C. Suppose we expect a list of atoms, the following  code will print the
atoms, each on a line:

foreign_t
pl_write_atoms(term_t l)
{ term_t head = PL_new_term_ref();      /* variable for the elements */
  term_t list = PL_copy_term_ref(l);    /* copy as we need to write */

  while( PL_get_list(list, head, list) )
  { char *s;

    if ( PL_get_atom_chars(head, &s) )
      Sprintf("%s\n", s);
    else
      PL_fail;
  }

  return PL_get_nil(list);              /* test end for [] */
}


int PPLL__ggeett__lliisstt(_t_e_r_m___t _+_l_, _t_e_r_m___t _-_h_, _t_e_r_m___t _-_t)
    If  _l is a list and not [] assign a term-reference to the  head to _h
    and to the tail to _t.


int PPLL__ggeett__hheeaadd(_t_e_r_m___t _+_l_, _t_e_r_m___t _-_h)
    If _l is a list and not [] assign a term-reference to the head to _h.


int PPLL__ggeett__ttaaiill(_t_e_r_m___t _+_l_, _t_e_r_m___t _-_t)
    If _l is a list and not [] assign a term-reference to the tail to _t.


int PPLL__ggeett__nniill(_t_e_r_m___t _+_l)
    Succeeds if  represents the atom [].


99..66..33..66 AAnn eexxaammppllee::  ddeeffiinniinngg write/1 iinn CC

Figure  9.3 shows  a  simplified  definition of  write/1  to  illustrate
the described  functions.   This simplified version  does not deal  with
operators.    It is  called  display/1, because  it mimics  closely  the
behaviour of this Edinburgh predicate.
foreign_t
pl_display(term_t t)

{ functor_t functor;
  int arity, len, n;
  char *s;

  switch( PL_term_type(t) )
  { case PL_VARIABLE:
    case PL_ATOM:

    case PL_INTEGER:
    case PL_FLOAT:
      PL_get_chars(t, &s, CVT_ALL);
      Sprintf("%s", s);
      break;
    case PL_STRING:
      PL_get_string_chars(t, &s, &len);
      Sprintf("\"%s\"", s);

      break;
    case PL_TERM:
    { term_t a = PL_new_term_ref();

      PL_get_name_arity(t, &name, &arity);
      Sprintf("%s(", PL_atom_chars(name));
      for(n=1; n<=arity; n++)

      { PL_get_arg(n, t, a);
        if ( n > 1 )
          Sprintf(", ");
        pl_display(a);
      }
      Sprintf(")");
      break;
    default:

      PL_fail;                          /* should not happen */
  }

  PL_succeed;
}

             Figure 9.3:  A Foreign definition of display/1


99..66..44 CCoonnssttrruuccttiinngg TTeerrmmss

Terms  can  be  constructed  using  functions from  the  PL_put_*()  and
PL_cons_*()  families.    This  approach  builds the  term  `inside-out',
starting  at  the  leaves  and  subsequently  creating  compound  terms.
Alternatively,   terms  may  be  created   `top-down',  first   creating
a  compound  holding  only  variables  and   subsequently  unifying  the
arguments.   This section  discusses functions  for the first  approach.
This approach is generally used for creating arguments for PL_call() and
PL_open_query.


void PPLL__ppuutt__vvaarriiaabbllee(_t_e_r_m___t _-_t)
    Put  a fresh variable in  the term.   The new variable lives on  the
    global  stack.   Note that the initial  variable lives on the  local
    stack  and is  lost after  a write to  the term-references.    After
    using this function, the variable will continue to live.


void PPLL__ppuutt__aattoomm(_t_e_r_m___t _-_t_, _a_t_o_m___t _a)
    Put  an  atom  in the  term  reference from  a  handle.    See  also
    PL_new_atom() and PL_atom_chars().


void PPLL__ppuutt__aattoomm__cchhaarrss(_t_e_r_m___t _-_t_, _c_o_n_s_t _c_h_a_r _*_c_h_a_r_s)
    Put an atom  in the term-reference constructed from the 0-terminated
    string.  The  string itself will never be references by Prolog after
    this function.


void PPLL__ppuutt__ssttrriinngg__cchhaarrss(_t_e_r_m___t _-_t_, _c_o_n_s_t _c_h_a_r _*_c_h_a_r_s)
    Put  a zero-terminated string in the term-reference.  The  data will
    be copied.  See also PL_put_string_nchars().


void PPLL__ppuutt__ssttrriinngg__nncchhaarrss(_t_e_r_m___t _-_t_, _s_i_z_e___t _l_e_n_, _c_o_n_s_t _c_h_a_r _*_c_h_a_r_s)

    Put  a string,  represented by  a length/start pointer  pair in  the
    term-reference.   The data will be copied.  This interface  can deal
    with 0-bytes in the string.  See also section 9.6.19.


void PPLL__ppuutt__lliisstt__cchhaarrss(_t_e_r_m___t _-_t_, _c_o_n_s_t _c_h_a_r _*_c_h_a_r_s)
    Put a list of ASCII values in the term-reference.


void PPLL__ppuutt__iinntteeggeerr(_t_e_r_m___t _-_t_, _l_o_n_g _i)
    Put a Prolog integer in the term reference.


void PPLL__ppuutt__iinntt6644(_t_e_r_m___t _-_t_, _i_n_t_6_4___t _i)
    Put a Prolog integer in the term reference.


void PPLL__ppuutt__ppooiinntteerr(_t_e_r_m___t _-_t_, _v_o_i_d _*_p_t_r)
    Put a Prolog integer  in the term-reference.  Provided ptr is in the
    `malloc()-area', PL_get_pointer() will get the pointer back.


void PPLL__ppuutt__ffllooaatt(_t_e_r_m___t _-_t_, _d_o_u_b_l_e _f)
    Put a floating-point value in the term-reference.


void PPLL__ppuutt__ffuunnccttoorr(_t_e_r_m___t _-_t_, _f_u_n_c_t_o_r___t _f_u_n_c_t_o_r)
    Create  a new compound  term from _f_u_n_c_t_o_r and  bind _t to this  term.
    All arguments of the  term will be variables.  To create a term with
    instantiated  arguments, either instantiate the arguments  using the
    PL_unify_*() functions or use PL_cons_functor().


void PPLL__ppuutt__lliisstt(_t_e_r_m___t _-_l)
    Same as PL_put_functor(l, PL_new_functor(PL_new_atom("."), 2)).


void PPLL__ppuutt__nniill(_t_e_r_m___t _-_l)
    Same as PL_put_atom_chars("[]").


void PPLL__ppuutt__tteerrmm(_t_e_r_m___t _-_t_1_, _t_e_r_m___t _+_t_2)
    Make _t_1 point to the same term as _t_2.


void PPLL__ccoonnss__ffuunnccttoorr(_t_e_r_m___t _-_h_, _f_u_n_c_t_o_r___t _f_, _._._.)
    Create  a term,  whose arguments are  filled from variable  argument
    list  holding the same number of term_t objects as the arity  of the
    functor.  To create the term animal(gnu, 50), use:

    { term_t a1 = PL_new_term_ref();
      term_t a2 = PL_new_term_ref();
      term_t t  = PL_new_term_ref();
      functor_t animal2;

      /* animal2 is a constant that may be bound to a global
         variable and re-used
      */
      animal2 = PL_new_functor(PL_new_atom("animal"), 2);

      PL_put_atom_chars(a1, "gnu");
      PL_put_integer(a2, 50);
      PL_cons_functor(t, animal2, a1, a2);
    }

    After  this sequence, the term-references _a_1 and _a_2 may be  used for
    other purposes.


void PPLL__ccoonnss__ffuunnccttoorr__vv(_t_e_r_m___t _-_h_, _f_u_n_c_t_o_r___t _f_, _t_e_r_m___t _a_0)
    Creates  a compound term like  PL_cons_functor(),  but _a_0 is an  array
    of  term references as returned by PL_new_term_refs().  The  length of
    this  array should  match the  number of arguments  required by  the
    functor.


void PPLL__ccoonnss__lliisstt(_t_e_r_m___t _-_l_, _t_e_r_m___t _+_h_, _t_e_r_m___t _+_t)
    Create  a list (cons-) cell in _l from  the head and tail.   The code
    below  creates a list of  atoms from a char **.   The list is  built
    tail-to-head.   The  PL_unify_*()  functions can be  used to build  a
    list head-to-tail.

    void
    put_list(term_t l, int n, char **words)
    { term_t a = PL_new_term_ref();

      PL_put_nil(l);
      while( --n >= 0 )
      { PL_put_atom_chars(a, words[n]);
        PL_cons_list(l, a, l);
      }
    }

    Note  that _l can  be redefined  within a PL_cons_list call as  shown
    here because operationally  its old value is consumed before its new
    value is set.


99..66..55 UUnniiffyyiinngg ddaattaa

The  functions  of  this  sections  _u_n_i_f_y  terms  with  other  terms  or
translated C-data structures.   Except for  PL_unify(), the functions  of
this section  are specific  to SWI-Prolog.   They  have been  introduced
to make translation  of old code easier,  but also because they  provide
for a faster mechanism  for returning data to Prolog that  requires less
term-references.  Consider the case where we want  a foreign function to
return the  host name of the  machine Prolog is running  on.  Using  the
PL_get_*() and PL_put_*() functions, the code becomes:

foreign_t
pl_hostname(term_t name)
{ char buf[100];

  if ( gethostname(buf, sizeof(buf)) )
  { term_t tmp = PL_new_term_ref();

    PL_put_atom_chars(tmp, buf);
    return PL_unify(name, tmp);
  }

  PL_fail;
}

Using PL_unify_atom_chars(), this becomes:

foreign_t
pl_hostname(term_t name)
{ char buf[100];

  if ( gethostname(buf, sizeof(buf)) )
    return PL_unify_atom_chars(name, buf);

  PL_fail;
}


int PPLL__uunniiffyy(_t_e_r_m___t _?_t_1_, _t_e_r_m___t _?_t_2)
    Unify two Prolog terms and return non-zero on success.


int PPLL__uunniiffyy__aattoomm(_t_e_r_m___t _?_t_, _a_t_o_m___t _a)
    Unify _t with the atom _a and return non-zero on success.


int PPLL__uunniiffyy__cchhaarrss(_t_e_r_m___t _?_t_, _i_n_t _f_l_a_g_s_, _s_i_z_e___t _l_e_n_, _c_o_n_s_t _c_h_a_r _*_c_h_a_r_s)
    New  function  do  deal  with  unification  of  char*  with  various
    encodings  to a  Prolog representation.    The _f_l_a_g_s  argument is  a
    bitwise  _o_r specifying the  Prolog target type  and the encoding  of
    _c_h_a_r_s.   Prolog types  is one of PL_ATOM, PL_STRING, PL_CODE_LIST  or
    PL_CHAR_LIST. Representations is one of  REP_ISO_LATIN_T, REP_UTF8 or
    REP_MB.  See PL_get_chars() for  a definition  of the  representation
    types.  If _l_e_n is -1, _c_h_a_r_s is assumed to be null-terminated.


int PPLL__uunniiffyy__aattoomm__cchhaarrss(_t_e_r_m___t _?_t_, _c_o_n_s_t _c_h_a_r _*_c_h_a_r_s)
    Unify  _t with  an atom  created from  _c_h_a_r_s and  return non-zero  on
    success.


int PPLL__uunniiffyy__lliisstt__cchhaarrss(_t_e_r_m___t _?_t_, _c_o_n_s_t _c_h_a_r _*_c_h_a_r_s)
    Unify _t with a list of ASCII characters constructed from _c_h_a_r_s.


void PPLL__uunniiffyy__ssttrriinngg__cchhaarrss(_t_e_r_m___t _?_t_, _c_o_n_s_t _c_h_a_r _*_c_h_a_r_s)
    Unify  _t  with  a  Prolog  string  object  created  from  the  zero-
    terminated  string  _c_h_a_r_s.   The  data will  be copied.    See  also
    PL_unify_string_nchars().


void PPLL__uunniiffyy__ssttrriinngg__nncchhaarrss(_t_e_r_m___t _?_t_, _s_i_z_e___t _l_e_n_, _c_o_n_s_t _c_h_a_r _*_c_h_a_r_s)
    Unify _t with  a Prolog string object created from the string created
    from  the _l_e_n/_c_h_a_r_s pair.  The data will be copied.   This interface
    can deal with 0-bytes in the string.  See also section 9.6.19.


int PPLL__uunniiffyy__iinntteeggeerr(_t_e_r_m___t _?_t_, _l_o_n_g _n)
    Unify _t with a Prolog integer from _n.


int PPLL__uunniiffyy__iinntt6644(_t_e_r_m___t _?_t_, _i_n_t_6_4___t _n)
    Unify _t with a Prolog integer from _n.


int PPLL__uunniiffyy__ffllooaatt(_t_e_r_m___t _?_t_, _d_o_u_b_l_e _f)
    Unify _t with a Prolog float from _f.


int PPLL__uunniiffyy__ppooiinntteerr(_t_e_r_m___t _?_t_, _v_o_i_d _*_p_t_r)
    Unify  _t with a  Prolog integer  describing the pointer.   See  also
    PL_put_pointer() and PL_get_pointer().


int PPLL__uunniiffyy__ffuunnccttoorr(_t_e_r_m___t _?_t_, _f_u_n_c_t_o_r___t _f)
    If  _t is a compound term with  the given functor, just succeed.   If
    it  is unbound,  create a term  and bind the  variable, else  fails.
    Not  that this function  does not create a  term if the argument  is
    already instantiated.


int PPLL__uunniiffyy__lliisstt(_t_e_r_m___t _?_l_, _t_e_r_m___t _-_h_, _t_e_r_m___t _-_t)
    Unify  _l with a list-cell (./2).   If successful, write  a reference
    to  the head of  the list to _h  and a reference  to the tail of  the
    list  in _t.   This  reference may  be used for  subsequent calls  to
    this  function.   Suppose we  want to  return a list  of atoms  from
    a  char **.   We could  use the  example described  by PL_put_list(),
    followed by  a call to PL_unify(), or we can use the code  below.  If
    the  predicate argument is unbound,  the difference is minimal  (the
    code  based on PL_put_list() is  probably slightly faster).   If  the
    argument  is bound, the code below may fail before reaching  the end
    of  the word-list, but even  if the unification succeeds, this  code
    avoids a duplicate (garbage) list and a deep unification.

    foreign_t
    pl_get_environ(term_t env)
    { term_t l = PL_copy_term_ref(env);
      term_t a = PL_new_term_ref();
      extern char **environ;
      char **e;

      for(e = environ; *e; e++)
      { if ( !PL_unify_list(l, a, l) ||
             !PL_unify_atom_chars(a, *e) )
          PL_fail;
      }

      return PL_unify_nil(l);
    }


int PPLL__uunniiffyy__nniill(_t_e_r_m___t _?_l)
    Unify _l with the atom [].


int PPLL__uunniiffyy__aarrgg(_i_n_t _i_n_d_e_x_, _t_e_r_m___t _?_t_, _t_e_r_m___t _?_a)
    Unifies the _i_n_d_e_x_-_t_h argument (1-based) of _t with _a.


int PPLL__uunniiffyy__tteerrmm(_t_e_r_m___t _?_t_, _._._.)
    Unify  _t with a (normally) compound  term.  The remaining  arguments
    is  a  sequence  of a  type  identifier,  followed by  the  required
    arguments.      This  predicate  is  an  extension  to  the  Quintus
    and  SICStus foreign  interface  from which  the SWI-Prolog  foreign
    interface  has been  derived, but has  proved to  be a powerful  and
    comfortable  way to create compound terms from C. Due to  the vararg
    packing/unpacking and  the required type-switching this interface is
    slightly  slower than using the primitives.   Please note that  some
    bad  C-compilers have fairly low  limits on the number of  arguments
    that may be passed to a function.

    Special  attention is required when  passing numbers.  C  `promotes'
    any  integral  smaller than  int  to  int.    I.e. the  types  char,
    short  and  int  are all  passed  as int.    In  addition,  on  most
    32-bit  platforms int and long are  the same.  Up-to version  4.0.5,
    only  PL_INTEGER  could be specified which  was taken from the  stack
    as  long.   Such  code fails when  passing small  integral types  on
    machines  where int  is smaller than  long.   It is  advised to  use
    PL_SHORT, PL_INT or  PL_LONG  as appropriate.   Similar, C  compilers
    promote  float to  double and  therefore PL_FLOAT and  PL_DOUBLE  are
    synonyms.

    The type identifiers are:

    PL_VARIABLE nnoonnee
         No op.  Used in arguments of PL_FUNCTOR.

    PL_BOOL iinntt
         Unify the argument with true or false.

    PL_ATOM aattoomm__tt
         Unify the argument with an atom, as in PL_unify_atom().

    PL_CHARS ccoonnsstt cchhaarr **
         Unify the argument  with an atom,  constructed from the C  char
         *, as in PL_unify_atom_chars().

    PL_NCHARS ssiizzee__tt,, ccoonnsstt cchhaarr **
         Unify the argument  with an atom,  constructed from length  and
         char* as in PL_unify_atom_nchars().

    PL_UTF8_CHARS ccoonnsstt cchhaarr **
         Create an atom from a UTF-8 string.

    PL_UTF8_STRING ccoonnsstt cchhaarr **
         Create a packed string object from a UTF-8 string.

    PL_MBCHARS ccoonnsstt cchhaarr **
         Create an atom from a multi-byte string in the current locale.

    PL_MBCODES ccoonnsstt cchhaarr **
         Create a list  of character codes  from a multi-byte string  in
         the current locale.

    PL_MBSTRING ccoonnsstt cchhaarr **
         Create a packed string  object from a multi-byte string  in the
         current locale.

    PL_NWCHARS ssiizzee__tt,, ccoonnsstt wwcchhaarr__tt **
         Create an atom from a length and a wide character pointer.

    PL_NWCODES ssiizzee__tt,, ccoonnsstt wwcchhaarr__tt **
         Create an  list of  character codes from  a length  and a  wide
         character pointer.

    PL_NWSTRING ssiizzee__tt,, ccoonnsstt wwcchhaarr__tt **
         Create  a  packed  string object  from  a  length  and  a  wide
         character pointer.

    PL_SHORT sshhoorrtt
         Unify the  argument with an  integer, as  in PL_unify_integer().
         As short is promoted to int, PL_SHORT is a synonym for PL_INT.

    PL_INT iinntt
         Unify the argument with an integer, as in PL_unify_integer().

    PL_LONG lloonngg
         Unify the argument with an integer, as in PL_unify_integer().

    PL_INTEGER lloonngg
         Unify the argument with an integer, as in PL_unify_integer().

    PL_DOUBLE ddoouubbllee
         Unify  the  argument  with  a  float,  as  in  PL_unify_float().
         Note  that,  as the  argument  is  passed using  the  C  vararg
         conventions, a float must be casted to a double explicitly.

    PL_FLOAT ddoouubbllee
         Unify the argument with a float, as in PL_unify_float().

    PL_POINTER vvooiidd **
         Unify the argument with a pointer, as in PL_unify_pointer().

    PL_STRING ccoonnsstt cchhaarr **
         Unify   the   argument   with   a   string   object,    as   in
         PL_unify_string_chars().

    PL_TERM tteerrmm__tt
         Unify  a  subterm.    Note  this  may the  return  value  of  a
         PL_new_term_ref() call to get access to a variable.

    PL_FUNCTOR ffuunnccttoorr__tt,, ......
         Unify the argument  with a compound  term.  This  specification
         should be  followed by  exactly as many  specifications as  the
         number of arguments of the compound term.

    PL_FUNCTOR_CHARS ccoonnsstt cchhaarr **nnaammee,, iinntt aarriittyy,, ......
         Create a functor from the given name and arity  and then behave
         as PL_FUNCTOR.

    PL_LIST iinntt lleennggtthh,, ......
         Create  a  list  of  the  indicated  length.     The  following
         arguments contain the elements of the list.

    For  example, to  unify an argument  with the term  language(dutch),
    the following skeleton may be used:

    static functor_t FUNCTOR_language1;

    static void
    init_constants()
    { FUNCTOR_language1 = PL_new_functor(PL_new_atom("language"), 1);
    }

    foreign_t
    pl_get_lang(term_t r)
    { return PL_unify_term(r,
                           PL_FUNCTOR, FUNCTOR_language1,
                               PL_CHARS, "dutch");
    }

    install_t
    install()
    { PL_register_foreign("get_lang", 1, pl_get_lang, 0);
      init_constants();
    }


int PPLL__cchhaarrss__ttoo__tteerrmm(_c_o_n_s_t _c_h_a_r _*_c_h_a_r_s_, _t_e_r_m___t _-_t)
    Parse  the string _c_h_a_r_s  and put the  resulting Prolog term into  _t.
    _c_h_a_r_s  may or may not  be closed using  a Prolog full-stop (i.e.,  a
    dot  followed by  a blank).   Returns  FALSE if a  syntax error  was
    encountered  and TRUE after successful  completion.  In addition  to
    returning  FALSE, the exception-term  is returned in  _t on a  syntax
    error.  See also term_to_atom/2.

    The following example build a goal-term from a string and calls it.

    int
    call_chars(const char *goal)
    { fid_t fid = PL_open_foreign_frame();
      term_t g = PL_new_term_ref();
      BOOL rval;

      if ( PL_string_to_term(goal, g) )
        rval = PL_call(goal, NULL);
      else
        rval = FALSE;

      PL_discard_foreign_frame(fid);
      return rval;
    }

      ...
      call_chars("consult(load)");
      ...


char * PPLL__qquuoottee(_i_n_t _c_h_r_, _c_o_n_s_t _c_h_a_r _*_s_t_r_i_n_g)
    Return a quoted version of  _s_t_r_i_n_g.  If _c_h_r is '\'', the result is a
    quoted  atom.  If _c_h_r  is '"', the result is  a string.  The  result
    string  is stored in the same ring of buffers as described  with the
    BUF_RING argument of PL_get_chars();

    In  the  current implementation,  the string  is  surrounded by  _c_h_r
    and  any  occurrence  of  _c_h_r  is  doubled.     In  the  future  the
    behaviour  will depend  on the  character_escape prolog-flag.    See
    current_prolog_flag/2.


99..66..66 BBLLOOBBSS:: UUssiinngg aattoommss ttoo ssttoorree aarrbbiittrraarryy bbiinnaarryy ddaattaa

SWI-Prolog atoms as well as strings can represent  arbitrary binary data
of arbitrary length.   This facility  is attractive for storing  foreign
data such  as images in an  atom.   An atom is  a unique handle to  this
data and the  atom garbage collector is  able to destroy atoms that  are
no longer  referenced by  the Prolog  engine.   This  property of  atoms
makes them  attractive as a  handle to foreign  resources, such as  Java
atoms, Microsoft's  COM objects, etc.,  providing safe combined  garbage
collection.

To  exploit   these  features   safely  and  in   an  organised   manner
the  SWI-Prolog  foreign  interface allows  for  creating  `atoms'  with
additional type  information.   The type is  represented by a  structure
holding C  function pointers that  tell Prolog  how to handle  releasing
the  atom,  writing it,  sorting  it,  etc.    Two  atoms  created  with
different types  can represent the  same sequence of bytes.   Atoms  are
first ordered on the rank  number of the type and then on the  result of
the compare()  function.   Rank numbers  are assigned  when the type  is
registered.


99..66..66..11 DDeeffiinniinngg aa BBLLOOBB ttyyppee

The  type PL_blob_t represents  a structure  with  the layout  displayed
above.  The structure contains additional fields at  the ...for internal
bookkeeping as well as future extension.

typedef struct PL_blob_t
{ unsigned long         magic;          /* PL_BLOB_MAGIC */
  unsigned long         flags;          /* Bitwise or of PL_BLOB_* */
  char *                name;           /* name of the type */
  int                   (*release)(atom_t a);
  int                   (*compare)(atom_t a, atom_t b);
  int                   (*write)(IOSTREAM *s, atom_t a, int flags);
  ...
} PL_blob_t;

For each  type exactly  one such  structure should  be allocated.    Its
first field must be initialised to PL_BLOB_MAGIC. The _f_l_a_g_s is a bitwise
or of the following constants:

PPLL__BBLLOOBB__TTEEXXTT
    If  specified the blob is assumed to contain text and  is considered
    a normal Prolog atom.

PPLL__BBLLOOBB__UUNNIIQQUUEE
    If  specified the system  ensures that the  blob-handle is a  unique
    reference  for a blob with the given  type, length and content.   If
    this flag is not specified each lookup creates a new blob.

PPLL__BBLLOOBB__NNOOCCOOPPYY
    By  default the  content of the  blob is  copied.   Using this  flag
    the  blob references  the external  data directly.    The user  must
    ensure  the provided  pointer is valid  as long  as the atom  lives.
    If  PL_BLOB_UNIQUE  is  also specified  uniqueness is  determined  by
    comparing the pointer rather than the data pointed at.

The _n_a_m_e  field represents the type  name as available  to Prolog.   See
also current_blob/2.   The other field  are function pointers that  must
be initialised to proper functions or NULL to get  the default behaviour
of built-in atoms.  Below are the defined member functions:


void aaccqquuiirree(_a_t_o_m___t _a)
    Called  if a new blob  of this type is  created through PL_put_blob()
    or  PL_unify_blob().   This callback  may be used  together with  the
    release hook to deal with reference counted external objects.


int rreelleeaassee(_a_t_o_m___t _a)
    The  blob (atom)  _a is  about to  be released.    This function  can
    retrieve  the data of the blob  using PL_blob_data().  If it  returns
    FALSE the atom garbage collector will _n_o_t reclaim the atom.


int ccoommppaarree(_a_t_o_m___t _a_, _a_t_o_m___t _b)
    Compare the blobs _a  and _b, both of which are of the type associated
    to  this blob-type.   Return values  are, as memcmp(),  <0  if _a is
    less then _b, = 0 if both are equal and >0 otherwise.


int wwrriittee(_I_O_S_T_R_E_A_M _*_s_, _a_t_o_m___t _a_, _i_n_t _f_l_a_g_s)
    Write  the content of the blob _a to the stream _s and  respecting the
    _f_l_a_g_s.   The _f_l_a_g_s are a bitwise or  of zero or more of the PL_WRT_*
    flags  defined in SWI-Prolog.h.  This prototype is available if  the
    undocumented SWI-Stream.h is included _b_e_f_o_r_e SWI-Prolog.h.

    If  this function is not provided, write/1 emits the content  of the
    blob  for blobs of type PL_BLOB_TEXT or a string of  the format <#_h_e_x
    _d_a_t_a> for binary blobs.

If  a blob  type is  registered from  a loadable  object (shared  object
or DLL)  the blob-type  must be  deregistered before the  object may  be
released.


int PPLL__uunnrreeggiisstteerr__bblloobb__ttyyppee(_P_L___b_l_o_b___t _*_t_y_p_e)
    Unlink  the blob  type from  the registered type  and transform  the
    type  of possible  living blobs  to  unregistered, avoiding  further
    reference  to the type structure,  functions referred by it as  well
    as  the data.  This function  returns TRUE if no blobs of  this type
    existed  and FALSE otherwise.   PL_unregister_blob_type() is  intended
    for  the  uninstall()  hook of  foreign  modules,  avoiding  further
    references to the module.


99..66..66..22 AAcccceessssiinngg bblloobbss

The blob access  functions are similar to the atom  accessing functions.
Blobs being atoms, the  atom functions operate on blobs and  visa versa.
For clarity and  possible future compatibility issues however it  is not
advised to rely on this.


int PPLL__iiss__bblloobb(_t_e_r_m___t _t_, _P_L___b_l_o_b___t _*_*_t_y_p_e)
    Succeeds  if _t refers to a blob,  in which case _t_y_p_e is  filled with
    the type of the blob.


int PPLL__uunniiffyy__bblloobb(_t_e_r_m___t _t_, _v_o_i_d _*_b_l_o_b_, _s_i_z_e___t _l_e_n_, _P_L___b_l_o_b___t _*_t_y_p_e)
    Unify  _t  to  a  new  blob  constructed  from  the  given  data  and
    associated to the given type.  See also PL_unify_atom_nchars().


int PPLL__ppuutt__bblloobb(_t_e_r_m___t _t_, _v_o_i_d _*_b_l_o_b_, _s_i_z_e___t _l_e_n_, _P_L___b_l_o_b___t _*_t_y_p_e)
    Store  the described blob in _t.  The return value  indicates whether
    a  new blob  was allocated  (FALSE) or the  blob is  a reference  to
    an  existing  blob (TRUE).  Reporting new/existing  can  be used  to
    deal  with external objects having their  own reference counts.   If
    the  return is  TRUE this  reference count must  be incremented  and
    it  must be  decremented on  blob destruction  callback.   See  also
    PL_put_atom_nchars().


int PPLL__ggeett__bblloobb(_t_e_r_m___t _t_, _v_o_i_d _*_*_b_l_o_b_, _s_i_z_e___t _*_l_e_n_, _P_L___b_l_o_b___t _*_*_t_y_p_e)
    If  _t holds a blob  or atom get the  data and type and return  TRUE.
    Otherwise  return FALSE. Each result  pointer may be NULL, in  which
    case the requested information is ignored.


void * PPLL__bblloobb__ddaattaa(_a_t_o_m___t _a_, _s_i_z_e___t _*_l_e_n_, _P_L___b_l_o_b___t _*_*_t_y_p_e)
    Get  the  data  and  type associated  to  a  blob.    This  function
    is   mainly  used   from   the  callback   functions  described   in
    section 9.6.6.1.


99..66..77 EExxcchhaannggiinngg GGMMPP nnuummbbeerrss

If  SWI-Prolog is  linked  with the  GNU Multiple  Precision  Arithmetic
Library  (GMP,  used   by  default),  the  foreign   interface  provides
functions for exchanging numeric  values to GMP types.  To  access these
functions the  header <gmp.h>  must be  included _b_e_f_o_r_e  <SWI-Prolog.h>.
Here is an example exploiting the function mpz_nextprime():

#include <gmp.h>
#include <SWI-Prolog.h>

static foreign_t
next_prime(term_t n, term_t prime)
{ mpz_t mpz;
  int rc;

  mpz_init(mpz);
  if ( PL_get_mpz(n, mpz) )
  { mpz_nextprime(mpz, mpz);

    rc = PL_unify_mpz(prime, mpz);
  } else
    rc = FALSE;

  mpz_clear(mpz);
  return rc;
}

install_t
install()
{ PL_register_foreign("next_prime", 2, next_prime, 0);
}


int PPLL__ggeett__mmppzz(_t_e_r_m___t _t_, _m_p_z___t _m_p_z)
    If  _t represents an  integer _m_p_z  is filled with  the value and  the
    function  returns TRUE. Otherwise _m_p_z is untouched and  the function
    returns  FALSE.  Note that  _m_p_z must  have  been initialised  before
    calling  this  function and  must  be cleared  using  mpz_clear()  to
    reclaim any storage associated with it.


int PPLL__ggeett__mmppqq(_t_e_r_m___t _t_, _m_p_q___t _m_p_q)
    If  _t is an integer or  rational number (term rdiv/2) _m_p_q is  filled
    with  the _n_o_r_m_a_l_i_s_e rational number  and the function returns  TRUE.
    Otherwise  _m_p_q is  untouched and  the function  returns FALSE.  Note
    that  _m_p_q must  have been initialised  before calling this  function
    and  must  be  cleared  using  mpq_clear()  to  reclaim  any  storage
    associated with it.


int PPLL__uunniiffyy__mmppzz(_t_e_r_m___t _t_, _m_p_z___t _m_p_z)
    Unify  _t with the integer value  represented by _m_p_z and return  _T_R_U_E
    on success.  The _m_p_z argument is not changed.


int PPLL__uunniiffyy__mmppqq(_t_e_r_m___t _t_, _m_p_q___t _m_p_q)
    Unify  _t with a rational number  represented by _m_p_q and return  _T_R_U_E
    on  success.    Note  that  _t is  unified  with  an integer  if  the
    denominator is 1.  The _m_p_q argument is not changed.


99..66..88 CCaalllliinngg PPrroolloogg ffrroomm CC

The Prolog  engine can be  called from C. There  are two interfaces  for
this.    For the  first, a  term is  created that  could be  used as  an
argument to  call/1 and  next PL_call()  is used  to call Prolog.    This
system is simple,  but does not allow  to inspect the different  answers
to  a non-deterministic  goal  and is  relatively  slow as  the  runtime
system needs  to find the predicate.   The  other interface is based  on
PL_open_query(), PL_next_solution() and  PL_cut_query()  or PL_close_query().
This mechanism is more powerful, but also more complicated to use.


99..66..88..11 PPrreeddiiccaattee rreeffeerreenncceess

This  section  discusses   the  functions  used  to  communicate   about
predicates.   Though a Prolog predicate  may defined or not,  redefined,
etc., a Prolog predicate has a handle that is  not destroyed, nor moved.
This handle is known by the type predicate_t.


predicate_t PPLL__pprreedd(_f_u_n_c_t_o_r___t _f_, _m_o_d_u_l_e___t _m)
    Return  a handle to a predicate for the specified name/arity  in the
    given module.   This function always succeeds, creating a handle for
    an  undefined predicate if no handle  was available.  If the  module
    argument _m is NULL, the current context module is used.


predicate_t PPLL__pprreeddiiccaattee(_c_o_n_s_t _c_h_a_r _*_n_a_m_e_, _i_n_t _a_r_i_t_y_, _c_o_n_s_t _c_h_a_r_* _m_o_d_u_l_e)

    Same   a  PL_pred(),   but  provides  a  more  convenient   interface
    to the C-programmer.


void PPLL__pprreeddiiccaattee__iinnffoo(_p_r_e_d_i_c_a_t_e___t _p_, _a_t_o_m___t _*_n_, _i_n_t _*_a_, _m_o_d_u_l_e___t _*_m)
    Return  information on the  predicate _p.   The  name is stored  over
    _n,  the  arity  over _a,  while  _m  receives the  definition  module.
    Note  that  the  latter need  not  be  the same  as  specified  with
    PL_predicate().  If the predicate is imported into  the module given
    to  PL_predicate(), this  function will return  the module where  the
    predicate is defined.


99..66..88..22 IInniittiiaattiinngg aa qquueerryy ffrroomm CC

This  section discusses  the  functions  for creating  and  manipulating
queries from C. Note that a foreign context can have  at most one active
query.    This  implies it  is allowed  to  make strictly  nested  calls
between  C and  Prolog (Prolog  calls C,  calls Prolog,  calls C,  etc.,
but it  is nnoott  allowed to  open multiple queries  and start  generating
solutions for  each of them  by calling PL_next_solution().   Be sure  to
call PL_cut_query()  or PL_close_query() on  any query  you opened  before
opening the next or returning control back to Prolog.


qid_t PPLL__ooppeenn__qquueerryy(_m_o_d_u_l_e___t _c_t_x_, _i_n_t _f_l_a_g_s_, _p_r_e_d_i_c_a_t_e___t _p_, _t_e_r_m___t _+_t_0)

    Opens  a query  and returns  an identifier for  it.   This  function
    always  succeeds,  regardless whether  the predicate  is defined  or
    not.    _c_t_x is  the _c_o_n_t_e_x_t  _m_o_d_u_l_e of  the goal.    When NULL,  the
    context  module of  the calling  context will  be used,  or user  if
    there  is no calling  context (as may  happen in embedded  systems).
    Note  that the  context module  only matters  for _m_o_d_u_l_e___t_r_a_n_s_p_a_r_e_n_t
    predicates.   See context_module/1 and module_transparent/1.   The _p
    argument  specifies the  predicate, and  should be the  result of  a
    call  to PL_pred() or PL_predicate().   Note that  it is allowed  to
    store  this handle as global data  and reuse it for future  queries.
    The  term-reference _t_0 is the  first of a vector of  term-references
    as returned by PL_new_term_refs(n).

    The  _f_l_a_g_s  arguments provides  some additional  options  concerning
    debugging  and  exception handling.    It  is a  bitwise or  of  the
    following values:

    PL_Q_NORMAL
         Normal operation.    The  debugger inherits  its settings  from
         the environment.   If an exception  occurs that is not  handled
         in Prolog, a  message is printed and  the tracer is started  to
         debug the error.

    PL_Q_NODEBUG
         Switch off the debugger while executing the goal.   This option
         is used by many  calls to hook-predicates to avoid  tracing the
         hooks.   An example is  print/1 calling portray/1 from  foreign
         code.

    PL_Q_CATCH_EXCEPTION
         If an  exception is  raised while  executing the  goal, do  not
         report it, but make it available for PL_exception().

    PL_Q_PASS_EXCEPTION
         As PL_Q_CATCH_EXCEPTION,  but do  not invalidate the  exception-
         term  while   calling  PL_close_query().       This  option   is
         experimental.

    The  example below opens a query to the predicate is_a/2  to find the
    ancestor of for some name.

    char *
    ancestor(const char *me)
    { term_t a0 = PL_new_term_refs(2);
      static predicate_t p;

      if ( !p )
        p = PL_predicate("is_a", 2, "database");

      PL_put_atom_chars(a0, me);
      PL_open_query(NULL, PL_Q_NORMAL, p, a0);
      ...
    }


int PPLL__nneexxtt__ssoolluuttiioonn(_q_i_d___t _q_i_d)
    Generate the first (next)  solution for the given query.  The return
    value  is TRUE if  a solution  was found, or  FALSE to indicate  the
    query  could not be proven.  This function may be  called repeatedly
    until it fails to generate all solutions to the query.


void PPLL__ccuutt__qquueerryy(_q_i_d)
    Discards  the query, but does not delete any of the data  created by
    the  query.   It just  invalidate _q_i_d,  allowing for a  new call  to
    PL_open_query() in this context.


void PPLL__cclloossee__qquueerryy(_q_i_d)
    As  PL_cut_query(),  but all  data and bindings  created by the  query
    are destroyed.


int PPLL__ccaallll__pprreeddiiccaattee(_m_o_d_u_l_e___t _m_, _i_n_t _f_l_a_g_s_, _p_r_e_d_i_c_a_t_e___t _p_r_e_d_, _t_e_r_m___t _+_t_0)

    Shorthand   for  PL_open_query(),   PL_next_solution(),  PL_cut_query(),
    generating  a single solution.   The arguments  are the same as  for
    PL_open_query(), the return value is the same as PL_next_solution().


int PPLL__ccaallll(_t_e_r_m___t_, _m_o_d_u_l_e___t)
    Call term just like  the Prolog predicate once/1.  _T_e_r_m is called in
    the  specified module, or in the context  module if module_t  = NULL.
    Returns  TRUE if  the call succeeds,  FALSE otherwise.   Figure  9.4
    shows an example to  obtain the number of defined atoms.  All checks
    are omitted to improve readability.


99..66..99 DDiissccaarrddiinngg DDaattaa

The Prolog  data created and  term-references needed  to setup the  call
and/or analyse  the result can  in most cases  be discarded right  after
the  call.    PL_close_query()  allows  for destructing  the  data,  while
leaving the  term-references.   The calls below may  be used to  destroy
term-references and data.  See figure 9.4 for an example.


fid_t PPLL__ooppeenn__ffoorreeiiggnn__ffrraammee()
    Created  a foreign  frame,  holding a  mark that  allows the  system
    to  undo  bindings and  destroy data  created after  it  as well  as
    providing  the  environment  for  creating term-references.     This
    function   is  called  by  the  kernel  before  calling   a  foreign
    predicate.


void PPLL__cclloossee__ffoorreeiiggnn__ffrraammee(_f_i_d___t _i_d)
    Discard  all term-references  created  after the  frame was  opened.
    All  other Prolog data is retained.  This function is called  by the
    kernel whenever a foreign function returns control back to Prolog.


void PPLL__ddiissccaarrdd__ffoorreeiiggnn__ffrraammee(_f_i_d___t _i_d)
    Same  as PL_close_foreign_frame(), but  also undo  all bindings  made
    since the open and destroy all Prolog data.


void PPLL__rreewwiinndd__ffoorreeiiggnn__ffrraammee(_f_i_d___t _i_d)
    Undo all bindings  and discard all term-references created since the
    frame was created, but  does not pop the frame.  I.e. the same frame
    can  be rewinded multiple  times, and must  eventually be closed  or
    discarded.

It is obligatory to call either of the two  closing functions to discard
a foreign frame.  Foreign frames may be nested.

int
count_atoms()
{ fid_t fid = PL_open_foreign_frame();
  term_t goal  = PL_new_term_ref();
  term_t a1    = PL_new_term_ref();
  term_t a2    = PL_new_term_ref();
  functor_t s2 = PL_new_functor(PL_new_atom("statistics"), 2);

  int atoms;

  PL_put_atom_chars(a1, "atoms");
  PL_cons_functor(goal, s2, a1, a2);
  PL_call(goal, NULL);         /* call it in current module */

  PL_get_integer(a2, &atoms);
  PL_discard_foreign_frame(fid);

  return atoms;
}

                      Figure 9.4:  Calling Prolog


99..66..1100 FFoorreeiiggnn CCooddee aanndd MMoodduulleess

Modules are  identified via a  unique handle.   The following  functions
are available to query and manipulate modules.


module_t PPLL__ccoonntteexxtt()
    Return the module  identifier of the context module of the currently
    active foreign predicate.


int PPLL__ssttrriipp__mmoodduullee(_t_e_r_m___t _+_r_a_w_, _m_o_d_u_l_e___t _*_m_, _t_e_r_m___t _-_p_l_a_i_n)
    Utility  function.  If  _r_a_w is a  term, possibly holding the  module
    construct  <_m_o_d_u_l_e>:<_r_e_s_t>this function will make _p_l_a_i_n a  reference
    to  <_r_e_s_t>  and fill  _m_o_d_u_l_e _*  with <_m_o_d_u_l_e>.    For further  nested
    module  constructs  the inner  most module  is  returned via  _m_o_d_u_l_e
    _*.   If  _r_a_w is  not a module  construct _a_r_g will  simply be put  in
    _p_l_a_i_n.   If _m_o_d_u_l_e _* is NULL  it will be set to the  context module.
    Otherwise  it will be left untouched.   The following example  shows
    how  to obtain the  plain term and module  if the default module  is
    the user module:

    { module m = PL_new_module(PL_new_atom("user"));
      term_t plain = PL_new_term_ref();

      PL_strip_module(term, &m, plain);
      ...


atom_t PPLL__mmoodduullee__nnaammee(_m_o_d_u_l_e___t)
    Return the name of _m_o_d_u_l_e as an atom.


module_t PPLL__nneeww__mmoodduullee(_a_t_o_m___t _n_a_m_e)
    Find  an existing or create a new module with name specified  by the
    atom _n_a_m_e.


99..66..1111 PPrroolloogg eexxcceeppttiioonnss iinn ffoorreeiiggnn ccooddee

This     section     discusses    PL_exception(),      PL_throw()     and
PL_raise_exception(),  the   interface  functions  to  detect  and   gen-
erate Prolog exceptions from C-code.  PL_throw() and PL_raise_exception()
from  the  C-interface   to  raise  an  exception  from  foreign   code.
PL_throw()  exploits the  C-function longjmp() to  return immediately  to
the  innermost PL_next_solution().    PL_raise_exception()  registers  the
exception term and returns FALSE. If a foreign  predicate returns FALSE,
while  and exception-term  is  registered  a Prolog  exception  will  be
raised by the virtual machine.

Calling these functions  outside the context of a function  implementing
a foreign predicate results in undefined behaviour.

PL_exception()  may be used after a call to  PL_next_solution() fails,  and
returns  a term  reference to  an  exception term  if an  exception  was
raised, and 0 otherwise.

If a  C-function, implementing a predicate  calls Prolog and detects  an
exception using PL_exception(), it can handle this exception,  or return
with  the exception.    Some caution  is required  though.    It is  nnoott
allowed to call PL_close_query() or PL_discard_foreign_frame() afterwards,
as this  will invalidate the  exception term.   Below  is the code  that
calls a Prolog defined arithmetic function (see arithmetic_function/1).

If PL_next_solution()  succeeds, the  result is  analysed and  translated
to  a number,  after  which the  query  is closed  and all  Prolog  data
created  after  PL_open_foreign_frame()  is  destroyed.     On  the  other
hand,  if  PL_next_solution()  fails and  if  an  exception  was  raised,
just  pass  it.    Otherwise  generate an  exception  (PL_error()  is  an
internal  call  for  building  the  standard  error  terms  and  calling
PL_raise_exception()).     After  this,  the  Prolog  environment  should
be  discarded using  PL_cut_query()  and PL_close_foreign_frame() to  avoid
invalidating the exception term.

static int
prologFunction(ArithFunction f, term_t av, Number r)
{ int arity = f->proc->definition->functor->arity;
  fid_t fid = PL_open_foreign_frame();
  qid_t qid;
  int rval;

  qid = PL_open_query(NULL, PL_Q_NORMAL, f->proc, av);

  if ( PL_next_solution(qid) )
  { rval = valueExpression(av+arity-1, r);
    PL_close_query(qid);
    PL_discard_foreign_frame(fid);
  } else
  { term_t except;

    if ( (except = PL_exception(qid)) )
    { rval = PL_throw(except);          /* pass exception */
    } else
    { char *name = stringAtom(f->proc->definition->functor->name);

                                        /* generate exception */
      rval = PL_error(name, arity-1, NULL, ERR_FAILED, f->proc);
    }

    PL_cut_query(qid);                  /* donot destroy data */
    PL_close_foreign_frame(fid);        /* same */
  }

  return rval;
}


int PPLL__rraaiissee__eexxcceeppttiioonn(_t_e_r_m___t _e_x_c_e_p_t_i_o_n)
    Generate  an exception (as  throw/1) and return  FALSE. Below is  an
    example returning an exception from foreign predicate:

    foreign_t
    pl_hello(term_t to)
    { char *s;

      if ( PL_get_atom_chars(to, &s) )
      { Sprintf("Hello \"%s\"\n", s);

        PL_succeed;
      } else
      { term_t except = PL_new_term_ref();

        PL_unify_term(except,
                      PL_FUNCTOR_CHARS, "type_error", 2,
                        PL_CHARS, "atom",
                        PL_TERM, to);

        return PL_raise_exception(except);
      }
    }


int PPLL__tthhrrooww(_t_e_r_m___t _e_x_c_e_p_t_i_o_n)
    Similar  to PL_raise_exception(), but returns  using the C  longjmp()
    function to the innermost PL_next_solution().


term_t PPLL__eexxcceeppttiioonn(_q_i_d___t _q_i_d)
    If  PL_next_solution()  fails, this  can  be  due to  normal  failure
    of  the  Prolog  call, or  because  an  exception was  raised  using
    throw/1.   This function returns  a handle to the exception term  if
    an exception was raised, or 0 if the Prolog goal simply failed..


99..66..1122 CCaattcchhiinngg SSiiggnnaallss ((SSooffttwwaarree IInntteerrrruuppttss))

SWI-Prolog offers both  a C and Prolog  interface to deal with  software
interrupts (signals).   The Prolog mapping  is defined in section  4.10.
This subsection deals with handling signals from C.

If a signal is not  used by Prolog and the handler does not  call Prolog
in any way, the native signal interface routines may be used.

Some versions of SWI-Prolog, notably running on  popular Unix platforms,
handle SIG_SEGV  for guarding  the Prolog stacks.    If the  application
wishes to handle  this signal too, it should  use PL_signal() to  install
its handler  after initialising Prolog.   SWI-Prolog will  pass SIG_SEGV
to the user  code if it detected the  signal is not related to a  Prolog
stack overflow.

Any handler that  wishes to call one  of the Prolog interface  functions
should call PL_signal() for its installation.


void (*)() PPLL__ssiiggnnaall(_s_i_g_, _f_u_n_c)
    This  function  is equivalent  to  the BSD-Unix  signal()  function,
    regardless  of the  platform used.   The  signal handler is  blocked
    while  the signal routine  is active, and automatically  reactivated
    after the handler returns.

    After  a  signal handler  is  registered using  this  function,  the
    native  signal interface  redirects the signal  to a generic  signal
    handler  inside  SWI-Prolog.   This  generic  handler validates  the
    environment,   creates  a  suitable  environment  for   calling  the
    interface functions described  in this chapter and finally calls the
    registered user-handler.

    By  default, signals  are handled asynchronously  (i.e. at the  time
    they  arrive).   It is inherently  dangerous to call extensive  code
    fragments,  and especially exception related code  from asynchronous
    handlers.     The  interface  allows  for  _s_y_n_c_h_r_o_n_o_u_s  handling  of
    signals.   In  this case  the native OS  handler just schedules  the
    signal  using PL_raise(),  which is checked by  PL_handle_signals()  at
    the  call- and redo-port.  This behaviour is realised by  or-ing _s_i_g
    with the constant PL_SIGSYNC.

    Signal    handling    routines    may   raise    exceptions    using
    PL_raise_exception().      The   use  of  PL_throw()  is  not   safe.
    If  a  synchronous handler  raises an  exception,  the exception  is
    delayed to the next call to PL_handle_signals();


int PPLL__rraaiissee(_i_n_t _s_i_g)
    Register  _s_i_g  for  _s_y_n_c_h_r_o_n_o_u_s handling  by  Prolog.    Synchronous
    signals  are  handled at  the  call-port or  if foreign  code  calls
    PL_handle_signals().  See also thread_signal/2.


int PPLL__hhaannddllee__ssiiggnnaallss(_v_o_i_d)
    Handle  any signals pending from  PL_raise().   PL_handle_signals()  is
    called  at  each pass  through the  call- and  redo-port  at a  safe
    point.   Exceptions raised by the  handler using PL_raise_exception()
    are properly passed to the environment.

    The  user  may   call  this  function  inside  long-running  foreign
    functions to handle  scheduled interrupts.  This routine returns the
    number  of signals handled.   If a handler raises an exception,  the
    return value is  -1 and the calling routine should return with FALSE
    as soon as possible.


99..66..1133 MMiisscceellllaanneeoouuss


99..66..1133..11 TTeerrmm CCoommppaarriissoonn


int PPLL__ccoommppaarree(_t_e_r_m___t _t_1_, _t_e_r_m___t _t_2)
    Compares  two terms using  the standard order  of terms and  returns
    -1, 0 or 1.  See also compare/3.


int PPLL__ssaammee__ccoommppoouunndd(_t_e_r_m___t _t_1_, _t_e_r_m___t _t_2)
    Yields TRUE if _t_1  and _t_2 refer to physically the same compound term
    and FALSE otherwise.


99..66..1133..22 RReeccoorrddeedd ddaattaabbaassee

In some  applications it is  useful to store  and retrieve Prolog  terms
from C-code.  For example, the XPCE graphical  environment does this for
storing arbitrary Prolog data as slot-data of XPCE objects.

Please note  that the  returned handles  have no meaning  at the  Prolog
level  and  the recorded  terms  are  not  visible from  Prolog.     The
functions PL_recorded() and PL_erase() are the  only functions that  can
operate on the stored term.

Two groups of functions are  provided.  The first group (PL_record() and
friends) store Prolog terms on the Prolog heap for  retrieval during the
same session.   These functions are also used by recorda/3  and friends.
The recorded  database may be used  to communicate Prolog terms  between
threads.


record_t PPLL__rreeccoorrdd(_t_e_r_m___t _+_t)
    Record  the term _t into the Prolog database as recorda/3  and return
    an  opaque handle to  the term.   The returned handle remains  valid
    until  PL_erase()  is called on  it.   PL_recorded()  is used to  copy
    recorded terms back to the Prolog stack.


void PPLL__rreeccoorrddeedd(_r_e_c_o_r_d___t _r_e_c_o_r_d_, _t_e_r_m___t _-_t)
    Copy a recorded term  back to the Prolog stack.  The same record may
    be used to copy  multiple instances at any time to the Prolog stack.
    See also PL_record() and PL_erase().


void PPLL__eerraassee(_r_e_c_o_r_d___t _r_e_c_o_r_d)
    Remove  the recorded term from  the Prolog database, reclaiming  all
    associated memory resources.

The  second group  (headed  by  PL_record_external())  provides  the  same
functionality, but the returned data has properties  that enable storing
the  data on  an external  device.   It  has been  designed  to make  it
possible to store Prolog terms fast an compact in  an external database.
Here are the main features:

  o _I_n_d_e_p_e_n_d_e_n_t _o_f _s_e_s_s_i_o_n
    Records  can  be communicated  to another  Prolog  session and  made
    visible using PL_recorded_external().

  o _B_i_n_a_r_y
    The representation is  binary for maximum performance.  The returned
    data may contain 0-bytes.

  o _B_y_t_e_-_o_r_d_e_r _i_n_d_e_p_e_n_d_e_n_t
    The   representation  can  be  transferred  between   machines  with
    different byte-order.

  o _N_o _a_l_i_g_n_m_e_n_t _r_e_s_t_r_i_c_t_i_o_n_s
    There are no  memory alignment restrictions and copies of the record
    can  thus be  moved freely.    For example,  it is  possible to  use
    this  representation to exchange  terms using shared memory  between
    different Prolog processes.

  o _C_o_m_p_a_c_t
    It  is  assumed  that a  smaller  memory footprint  will  eventually
    outperform slightly faster representations.

  o _S_t_a_b_l_e
    The  format is  designed  for future  enhancements without  breaking
    compatibility with older records.


char * PPLL__rreeccoorrdd__eexxtteerrnnaall(_t_e_r_m___t _+_t_, _s_i_z_e___t _*_l_e_n)
    Record  the term _t into the Prolog database as recorda/3  and return
    an  opaque handle to  the term.   The returned handle remains  valid
    until PL_erase_external() is called on it.

    It  is allowed  to copy  the data  and use  PL_recorded_external()  on
    the  copy.   The user  is responsible for  the memory management  of
    the  copy.    After copying,  the original  may  be discarded  using
    PL_erase_external().

    PL_recorded_external() is  used to copy  such recorded terms back  to
    the Prolog stack.


int PPLL__rreeccoorrddeedd__eexxtteerrnnaall(_c_o_n_s_t _c_h_a_r _*_r_e_c_o_r_d_, _t_e_r_m___t _-_t)
    Copy a recorded term  back to the Prolog stack.  The same record may
    be used to copy  multiple instances at any time to the Prolog stack.
    See also PL_record_external() and PL_erase_external().


int PPLL__eerraassee__eexxtteerrnnaall(_c_h_a_r _*_r_e_c_o_r_d)
    Remove  the recorded term from  the Prolog database, reclaiming  all
    associated memory resources.


99..66..1133..33 GGeettttiinngg ffiillee nnaammeess

The function  PL_get_file_name() provides access  to Prolog filenames  and
its  file-search mechanism  described  with absolute_file_name/3.    Its
existence  is motivated  to realise  a uniform  interface  to deal  with
file-properties, search, naming conventions etc. from foreign code.


int PPLL__ggeett__ffiillee__nnaammee(_t_e_r_m___t _s_p_e_c_, _c_h_a_r _*_*_n_a_m_e_, _i_n_t _f_l_a_g_s)
    Translate   a  Prolog  term  into  a  file  name.     The   name  is
    stored  in  the  static  buffer  ring described  with  PL_get_chars()
    option  BUF_RING.  Conversion from the  internal UNICODE encoding  is
    done  using  standard C  library functions.    _f_l_a_g_s  is a  bit-mask
    controlling the conversion process.  Options are:

    PL_FILE_ABSOLUTE
         Return an absolute path to the requested file.

    PL_FILE_OSPATH
         Return  a the  name  using the  hosting  OS  conventions.    On
         MS-Windows, \ is  used to separate directories rather than  the
         canonical /.

    PL_FILE_SEARCH
         Invoke  absolute_file_name/3.      This   implies  rules   from
         file_search_path/2are used.

    PL_FILE_EXIST
         Demand the path to refer to an existing entity.

    PL_FILE_READ
         Demand read-access on the result.

    PL_FILE_WRITE
         Demand write-access on the result.

    PL_FILE_EXECUTE
         Demand execute-access on the result.

    PL_FILE_NOERRORS
         Do not raise any exceptions.


99..66..1144 EErrrroorrss aanndd wwaarrnniinnggss

PL_warning()  prints a standard  Prolog warning  message to the  standard
error (user_error) stream.   Please note  that new code should  consider
using  PL_raise_exception() to  raise  a  Prolog exception.     See  also
section 4.9.


int PPLL__wwaarrnniinngg(_f_o_r_m_a_t_, _a_1_, _._._.)
    Print  an  error message  starting  with `[WARNING: ',  followed  by
    the  output from  _f_o_r_m_a_t, followed  by a `]'  and a newline.    Then
    start  the tracer.   _f_o_r_m_a_t and  the arguments are  the same as  for
    printf(2).  Always returns FALSE.


99..66..1155 EEnnvviirroonnmmeenntt CCoonnttrrooll ffrroomm FFoorreeiiggnn CCooddee


int PPLL__aaccttiioonn(_i_n_t_, _._._.)
    Perform  some  action on  the  Prolog system.    _i_n_t  describes  the
    action,  Remaining arguments depend  on the requested  action.   The
    actions are listed in table 9.1.
    ___________________________________________________________________
    | PL_ACTION_TRACE        |Start Prolog tracer (trace/0).   Requires|
    |                        |no arguments.                            |
    | PL_ACTION_DEBUG        |Switch  on Prolog  debug mode  (debug/0).|
    |                        |Requires no arguments.                   |
    | PL_ACTION_BACKTRACE    |Print   backtrace   on   current   output|
    |                        |stream.   The  argument (an  int) is  the|
    |                        |number of frames printed.                |

    | PL_ACTION_HALT         |Halt  Prolog  execution.     This  action|
    |                        |should be called rather  than Unix exit()|
    |                        |to give  Prolog the opportunity  to clean|
    |                        |up.   This  call does  not return.    The|
    |                        |argument (an int) is the  exit code.  See|
    |                        |halt/1.                                  |
    | PL_ACTION_ABORT        |Generate a Prolog abort  (abort/0).  This|

    |                        |call  does  not  return.     Requires  no|
    |                        |arguments.                               |
    | PL_ACTION_BREAK        |Create  a  standard  Prolog  break  envi-|
    |                        |ronment  (break/0).    Returns after  the|
    |                        |user  types  the  end-of-file  character.|
    ||                       |Requires|no arguments.                   ||

    | PL_ACTION_GUIAPP       |Win32:  Used to  indicate the kernel that|
    |                        |the application  is a GUI  application if|
    |                        |the  argument  is  not 0  and  a  console|
    |                        |application if  the argument  is 0.    If|
    |                        |a  fatal error  occurs,  the system  uses|
    |                        |a windows  messagebox to  report this  on|
    |                        |a GUI  application and simply  prints the|
    |                        |error and exits otherwise.               |
    | PL_ACTION_WRITE        |Write  the  argument,  a  char *  to  the|

    |                        |current output stream.                   |
    | PL_ACTION_FLUSH        |Flush  the current  output stream.    Re-|
    |                        |quires no arguments.                     |
    | PL_ACTION_ATTACH_CONSOLE|Attach a console  to a thread if  it does|
    |________________________|not_have_one.__See_attach_console/0._____|

                     Table 9.1:  PL_action() options


99..66..1166 QQuueerryyiinngg PPrroolloogg


long PPLL__qquueerryy(_i_n_t)
    Obtain  status  information  on  the  Prolog system.     The  actual
    argument  type depends on the  information required.  _i_n_t  describes
    what information is wanted.  The options are given in table 9.2.
    ___________________________________________________________________
    | PL_QUERY_ARGC          |Return an  integer holding the  number of|
    |                        |arguments given to Prolog from Unix.     |
    | PL_QUERY_ARGV          |Return  a char  **  holding the  argument|
    |                        |vector given to Prolog from Unix.        |
    | PL_QUERY_SYMBOLFILE    |Return  a  char  *  holding  the  current|

    |                        |symbol file of the running process.      |
    | PL_MAX_INTEGER         |Return a  long, representing  the maximal|
    |                        |integer  value  represented by  a  Prolog|
    |                        |integer.                                 |
    | PL_MIN_INTEGER         |Return a  long, representing  the minimal|
    |                        |integer value.                           |
    | PL_QUERY_VERSION       |Return a  long, representing  the version|

    |                        |as 10; 000M* +100m* +p, where  M is  the |
    |                        |major,  m the  minor version  number and |
    |                        |p the  patch-level.   For  example, 20717|
    |                        |means 2.7.17.                            |
    | PL_QUERY_MAX_THREADS   |Return  the  maximum  number  of  threads|
    |                        |that  can  be created  in  this  version.|
    |                        |Return  values   of  PL_thread_self()  are|
    |                        |between 0 and this number.               |

    | PL_QUERY_ENCODING      |Return  the  default stream  encoding  of|
    |                        |Prolog (of type IOENC).                  |
    | PL_QUERY_USER_CPU      |Get  amount  of  user  CPU  time  of  the|
    |________________________|process_in_milliseconds._________________|

                     Table 9.2:  PL_query() options


99..66..1177 RReeggiisstteerriinngg FFoorreeiiggnn PPrreeddiiccaatteess


int PPLL__rreeggiisstteerr__ffoorreeiiggnn__iinn__mmoodduullee(_c_o_n_s_t _c_h_a_r _*_m_o_d_u_l_e_, _c_o_n_s_t _c_h_a_r _*_n_a_m_e_, _i_n_t _a_r_i_t_y_, _f_o_r_e_i_g_n___t _(_*_f_u_n_c_t_i_o_n_)_(_)_, _i_n_t _f_l_a_g_s)

    Register   a   C-function   to   implement   a   Prolog   predicate.
    After  this call returns successfully a predicate with name  _n_a_m_e (a
    char  *) and  arity _a_r_i_t_y  (a C int)  is created  in module  _m_o_d_u_l_e.
    If  _m_o_d_u_l_e is NULL,  the predicate is created  in the module of  the
    calling context or if no context is present in the module user.

    When  called in  Prolog, Prolog  will call  _f_u_n_c_t_i_o_n.   _f_l_a_g_s  forms
    bitwise or'ed list of options for the installation.  These are:
    ___________________________________________________________________
    | PL_FA_NOTRACE          |Predicate cannot be seen in the tracer   |
    | PL_FA_TRANSPARENT      |Predicate is module transparent          |
    | PL_FA_NONDETERMINISTIC |Predicate  is  non-deterministic.     See|
    |                        |also PL_retry().                         |

    |_PL_FA_VARARGS__________|Use_alternative_calling_convention.______|
    Predicates    may   be   registered    either   before   or    after
    PL_initialise().      When  registered  before  initialisation   the
    registration  is recorded and  executed after installing the  system
    predicates and before loading the saved state.

    Default  calling (i.e. without PL_FA_VARARGS) _f_u_n_c_t_i_o_n is  passed the
    same  number of term_t arguments  as the arity of the  predicate and,
    if  the predicate is  non-deterministic, an  extra argument of  type
    control_t  (see section  9.6.1.1).    If PL_FA_VARARGS is  provided,
    _f_u_n_c_t_i_o_n  is called  with three arguments.    The first argument  is
    a  term_t  handle to the  first argument.   Further arguments can  be
    reached  by adding  the  offset (see  also PL_new_term_refs()).    The
    second  argument is  the arity,  which defines the  number of  valid
    term-references  in the argument vector.  The last argument  is used
    for  non-deterministic  calls.   It  is  currently undocumented  and
    should be defined of type void*.  Here is an example:

    static foreign_t
    atom_checksum(term_t a0, int arity, void* context)
    { char *s;

      if ( PL_get_atom_chars(a0, &s) )
      { int sum;

        for(sum=0; *s; s++)
          sum += *s&0xff;

        return PL_unify_integer(a0+1, sum&0xff);
      }

      return FALSE;
    }

    install_t
    install()
    { PL_register_foreign("atom_checksum", 2, atom_checksum, PL_FA_VARARGS);
    }


int PPLL__rreeggiisstteerr__ffoorreeiiggnn(_c_o_n_s_t _c_h_a_r _*_n_a_m_e_, _i_n_t _a_r_i_t_y_, _f_o_r_e_i_g_n___t _(_*_f_u_n_c_t_i_o_n_)_(_)_, _i_n_t _f_l_a_g_s)

    Same   as  PL_register_foreign_in_module(),  passing   NULL  for   the
    _m_o_d_u_l_e.


void PPLL__rreeggiisstteerr__eexxtteennssiioonnss__iinn__mmoodduullee(_c_o_n_s_t _c_h_a_r _*_m_o_d_u_l_e_, _P_L___e_x_t_e_n_s_i_o_n _*_e)

    Register   a  series  of  predicates   from  an  array  of   defini-
    tions  of the type PL_extension in the  given _m_o_d_u_l_e.  If _m_o_d_u_l_e  is
    NULL, the predicate  is created in the module of the calling context
    or  if no context is present  in the module user.   The PL_extension
    type is defined as

    typedef struct PL_extension
    { char          *predicate_name;        /* Name of the predicate */
      short         arity;                  /* Arity of the predicate */
      pl_function_t function;               /* Implementing functions */
      short         flags;                  /* Or of PL_FA_... */
    } PL_extension;

    For  details, see PL_register_foreign_in_module().  Here  is an example
    of its usage:

    static PL_extension predicates[] = {
    { "foo",        1,      pl_foo, 0 },
    { "bar",        2,      pl_bar, PL_FA_NONDETERMINISTIC },
    { NULL,         0,      NULL,   0 }
    };

    main(int argc, char **argv)
    { PL_register_extensions_in_module("user", predicates);

      if ( !PL_initialise(argc, argv) )
        PL_halt(1);

      ...
    }


void PPLL__rreeggiisstteerr__eexxtteennssiioonnss( _P_L___e_x_t_e_n_s_i_o_n _*_e)
    Same  as PL_register_extensions_in_module()  using NULL for the  _m_o_d_u_l_e
    argument.


99..66..1188 FFoorreeiiggnn CCooddee HHooookkss

For various specific applications some hooks re provided.


PL_dispatch_hook_t PPLL__ddiissppaattcchh__hhooookk(_P_L___d_i_s_p_a_t_c_h___h_o_o_k___t)
    If  this hook  is not  NULL, this  function is  called when  reading
    from  the  terminal.     It is  supposed  to  dispatch  events  when
    SWI-Prolog  is connected  to a window  environment.   It can  return
    two  values:  PL_DISPATCH_INPUT indicates Prolog input is  available
    on  file descriptor 0 or PL_DISPATCH_TIMEOUT to indicate a  timeout.
    The  old hook is  returned.  The  type PL_dispatch_hook_t is  defined
    as:

    typedef int  (*PL_dispatch_hook_t)(void);


void PPLL__aabboorrtt__hhooookk(_P_L___a_b_o_r_t___h_o_o_k___t)
    Install  a hook  when abort/0 is  executed.   SWI-Prolog abort/0  is
    implemented  using C  setjmp()/longjmp() construct.   The hooks  are
    executed  in  the  reverse order  of  their registration  after  the
    longjmp()  took place and before the Prolog top-level  is reinvoked.
    The type PL_abort_hook_t is defined as:

    typedef void (*PL_abort_hook_t)(void);


int PPLL__aabboorrtt__uunnhhooookk(_P_L___a_b_o_r_t___h_o_o_k___t)
    Remove  a hook installed with  PL_abort_hook().   Returns FALSE if  no
    such hook is found, TRUE otherwise.


void PPLL__oonn__hhaalltt(_v_o_i_d _(_*_f_)_(_i_n_t_, _v_o_i_d _*_)_, _v_o_i_d _*_c_l_o_s_u_r_e)
    Register  the function _f to be called if SWI-Prolog is halted.   The
    function  is  called with  two  arguments:   the  exit code  of  the
    process  (0 if this cannot  be determined on your operating  system)
    and  the _c_l_o_s_u_r_e argument passed to the PL_on_halt() call.   See also
    at_halt/1.


PL_agc_hook_t PPLL__aaggcc__hhooookk(_P_L___a_g_c___h_o_o_k___t _n_e_w)
    Register    a   hook   with   the   atom-garbage    collector   (see
    garbage_collect_atoms/0  that  is   called  on  any  atom  that   is
    reclaimed.    The old hook  is returned.   If  no hook is  currently
    defined,  NULL is returned.  The argument of the called hook  is the
    atom  that is to be garbage collected.  The return value  is an int.
    If  the return value is zero, the  atom is nnoott reclaimed.   The hook
    may invoke any Prolog predicate.

    The  example  below  defines  a foreign  library  for  printing  the
    garbage collected atoms for debugging purposes.

    #include <SWI-Stream.h>
    #include <SWI-Prolog.h>

    static int
    atom_hook(atom_t a)
    { Sdprintf("AGC: deleting %s\n", PL_atom_chars(a));

      return TRUE;
    }

    static PL_agc_hook_t old;

    install_t
    install()
    { old = PL_agc_hook(atom_hook);
    }

    install_t
    uninstall()
    { PL_agc_hook(old);
    }


99..66..1199 SSttoorriinngg ffoorreeiiggnn ddaattaa

This section  provides some hints for  handling foreign data in  Prolog.
With foreign  data, we refer  to data that is  used by foreign  language
predicates  and  needs  to  be passed  around  in  Prolog.     Excluding
combinations, there are three principal options for storing such data

  o _N_a_t_u_r_a_l _P_r_o_l_o_g _d_a_t_a
    E.i.  using the  representation  one would  choose if  there was  no
    foreign interface required.

  o _O_p_a_q_u_e _p_a_c_k_e_d _P_r_o_l_o_g _d_a_t_a
    Data  can also be represented in  a foreign structure and stored  on
    the  Prolog stacks  using PL_put_string_nchars()  and retrieved  using
    PL_get_string_chars().   It is  generally good practice  to wrap  the
    string  in a compound term with arity 1, so Prolog can  identify the
    type.    portray/1 rules  may be  used to  streamline printing  such
    terms during development.

  o _N_a_t_u_r_a_l _f_o_r_e_i_g_n _d_a_t_a_, _p_a_s_s_i_n_g _a _p_o_i_n_t_e_r
    An  alternative is to pass  a pointer to the  foreign data.   Again,
    this functor may be wrapped in a compound term.

The choice may be guided using the following distinctions

  o _I_s _t_h_e _d_a_t_a _o_p_a_q_u_e _t_o _P_r_o_l_o_g
    With  `opaque' data, we refer to data handled in  foreign functions,
    passed  around in  Prolog, but  of which Prolog  never examines  the
    contents  of the  data itself.   If  the data is  opaque to  Prolog,
    the  chosen representation  does not  depend on  simple analysis  by
    Prolog,  and the selection  will be driven  solely by simplicity  of
    the interface and performance (both in time and space).

  o _H_o_w _b_i_g _i_s _t_h_e _d_a_t_a
    Is  efficient encoding required?   For examine, a boolean array  may
    be  expressed as  a compound  term, holding integers  each of  which
    contains a number of bits, or as a list of true and false.

  o _W_h_a_t _i_s _t_h_e _n_a_t_u_r_e _o_f _t_h_e _d_a_t_a
    For  examples in C,  constants are often  expressed using `enum'  or
    #define'd  integer values.   If  prolog needs  to handle this  data,
    atoms  are a more logical  choice.  Whether  or not this mapping  is
    used  depends on  whether Prolog  needs to interpret  the data,  how
    important debugging is and how important performance is.

  o _W_h_a_t _i_s _t_h_e _l_i_f_e_t_i_m_e _o_f _t_h_e _d_a_t_a
    We can distinguish three cases.

     1.  The lifetime is  dictated by the  accessibility of the data  on
         the Prolog stacks.   Their is no way by which the  foreign code
         when the  data becomes `garbage',  and the  data thus needs  to
         be represented  on the Prolog  stacks using Prolog  data-types.
         (2),

     2.  The  data lives  on  the  `heap' and  is  explicitly  allocated
         and deallocated.   In  this case,  representing the data  using
         native foreign representation and passing a pointer to  it is a
         sensible choice.

     3.  The data lives as  during the lifetime of a  foreign predicate.
         If the predicate is deterministic, foreign  automatic variables
         are suitable.  if the predicate is  non-deterministic, the data
         may be allocated  using malloc() and  a pointer may be  passed.
         See section 9.6.1.1.


99..66..1199..11 EExxaammpplleess ffoorr ssttoorriinngg ffoorreeiiggnn ddaattaa

In this section, we will outline some examples,  covering typical cases.
In  the  first  example,  we will  deal  with  extending  Prolog's  data
representation with integer-sets, represented as bit-vectors.   Finally,
we discuss the outline of the DDE interface.

IInntteeggeerr  sseettss with  not-too-far-apart upper-  and  lower-bounds  can  be
represented using  bit-vectors.  Common  set operations, such as  union,
intersection,  etc.    are  reduced to  simple  and'ing and  or'ing  the
bit-vectors.    This  can  be done  in  Prolog,  using a  compound  term
holding  integer  arguments.    Especially  if  the  integers  are  kept
below  the maximum  tagged  integer  value (see  current_prolog_flag/2),
this representation  is fairly space-efficient (wasting  1 word for  the
functor and and  7 bits per integer for  the tags).  Arithmetic can  all
be performed in Prolog too.

For really demanding  applications, foreign representation will  perform
better,  especially  time-wise.    Bit-vectors are  naturally  expressed
using  string  objects.    If  the  string is  wrapped  in  bitvector/1,
lower-bound of the vector  is 0, and the upper-bound is not  defined, an
implementation for  getting and putting  the sets as  well as the  union
predicate for it is below.

#include <SWI-Prolog.h>

#define max(a, b) ((a) > (b) ? (a) : (b))
#define min(a, b) ((a) < (b) ? (a) : (b))

static functor_t FUNCTOR_bitvector1;

static int
get_bitvector(term_t in, int *len, unsigned char **data)
{ if ( PL_is_functor(in, FUNCTOR_bitvector1) )
  { term_t a = PL_new_term_ref();

    PL_get_arg(1, in, a);
    return PL_get_string(a, (char **)data, len);
  }

  PL_fail;
}

static int
unify_bitvector(term_t out, int len, const unsigned char *data)
{ if ( PL_unify_functor(out, FUNCTOR_bitvector1) )
  { term_t a = PL_new_term_ref();

    PL_get_arg(1, out, a);

    return PL_unify_string_nchars(a, len, (const char *)data);
  }

  PL_fail;
}

static foreign_t
pl_bitvector_union(term_t t1, term_t t2, term_t u)
{ unsigned char *s1, *s2;
  int l1, l2;

  if ( get_bitvector(t1, &l1, &s1) &&
       get_bitvector(t2, &l2, &s2) )
  { int l = max(l1, l2);
    unsigned char *s3 = alloca(l);

    if ( s3 )
    { int n;
      int ml = min(l1, l2);

      for(n=0; n<ml; n++)
        s3[n] = s1[n] | s2[n];
      for( ; n < l1; n++)
        s3[n] = s1[n];
      for( ; n < l2; n++)
        s3[n] = s2[n];

      return unify_bitvector(u, l, s3);
    }

    return PL_warning("Not enough memory");
  }

  PL_fail;
}

install_t
install()
{ PL_register_foreign("bitvector_union", 3, pl_bitvector_union, 0);

  FUNCTOR_bitvector1 = PL_new_functor(PL_new_atom("bitvector"), 1);
}

TThhee DDDDEE  iinntteerrffaaccee (see section  4.42) represents  another common  usage
of  the foreign  interface:   providing communication  to new  operating
system features.  The DDE interface requires knowledge  about active DDE
server and  client channels.   These  channels contains various  foreign
data-types.  Such an interface is normally achieved  using an open/close
protocol that creates and destroys a _h_a_n_d_l_e.  The  handle is a reference
to a foreign data-structure containing the relevant information.

There are a  couple of possibilities for  representing the handle.   The
choice  depends on  responsibilities  and  debugging facilities.     The
simplest  approach is  to using  PL_unify_pointer() and  PL_get_pointer().
This  approach is  fast and  easy, but  has the  drawbacks of  (untyped)
pointers:   there  is no  reliable way  to detect  the  validity of  the
pointer, not  to verify  it is pointing  to a  structure of the  desired
type.   The pointer  may be wrapped  into a compound  term with arity  1
(i.e., dde_channel(<_P_o_i_n_t_e_r>)), making the type-problem less serious.

Alternatively  (used in  the  DDE  interface),  the interface  code  can
maintain a  (preferably variable  length) array of  pointers and  return
the index in this  array.  This provides better protection.   Especially
for debugging  purposes, wrapping  the handle  in a compound  is a  good
suggestion.


99..66..2200 EEmmbbeeddddiinngg SSWWII--PPrroolloogg iinn ootthheerr aapppplliiccaattiioonnss

With embedded Prolog we refer to the situation where  the `main' program
is not the Prolog application.  Prolog is sometimes  embedded in C, C++,
Java or  other languages  to provide  logic based services  in a  larger
application.   Embedding  loads the Prolog  engine as  a library to  the
external language.   Prolog  itself only provides  for embedding in  the
C-language (compatible  to C++).   Embedding in  Java is achieved  using
JPL using a C-glue between the Java and Prolog C-interfaces.

The  most   simple  embedded   program  is   below.      The   interface
function  PL_initialise()  mmuusstt  be   called  before  any  of  the  other
SWI-Prolog  foreign  language  functions  described  in   this  chapter,
except  for   PL_initialise_hook(),   PL_new_atom(),   PL_new_functor()   and
PL_register_foreign().   PL_initialise()  interprets all the  command-line
arguments,  except  for the  -t toplevel  flag  that is  interpreted  by
PL_toplevel().

int
main(int argc, char **argv)
{
#ifdef READLINE /* Remove if you don't want readline */
  PL_initialise_hook(install_readline);
#endif

  if ( !PL_initialise(argc, argv) )
    PL_halt(1);

  PL_halt(PL_toplevel() ? 0 : 1);
}


int PPLL__iinniittiiaalliissee(_i_n_t _a_r_g_c_, _c_h_a_r _*_*_a_r_g_v)
    Initialises  the SWI-Prolog  heap and  stacks,  restores the  Prolog
    state, loads the  system and personal initialisation files, runs the
    at_initialization/1 hooks and finally runs the -g goal hook.

    Special  consideration  is required  for argv[0].    On  UUnniixx,  this
    argument passes the  part of the command-line that is used to locate
    the  executable.   Prolog  uses this to  find the  file holding  the
    running executable.   The WWiinnddoowwss version uses this to find a _m_o_d_u_l_e
    of  the  running executable.    If the  specified  module cannot  be
    found, it tries  the module libpl.dll, containing the Prolog runtime
    kernel.   In  all these cases,  the resulting file  is used for  two
    purposes

      o  See whether a Prolog saved-state  is appended to the file.   If
         this is  the case,  this state  will be loaded  instead of  the
         default boot.prc file from the SWI-Prolog home directory.   See
         also qsave_program/[1,2] and section 9.7.

      o  Find the Prolog home  directory.  This process is  described in
         detail in section 9.8.

    PL_initialise() returns  1  if all  initialisation succeeded  and  0
    otherwise.

    In  most cases, _a_r_g_c and _a_r_g_v will be passed from the  main program.
    It  is allowed to create your own argument vector,  provided argv[0]
    is constructed according to the rules above.  For example:

    int
    main(int argc, char **argv)
    { char *av[10];
      int ac = 0;

      av[ac++] = argv[0];
      av[ac++] = "-x";
      av[ac++] = "mystate";
      av[ac]   = NULL;

      if ( !PL_initialise(ac, av) )
        PL_halt(1);
      ...
    }

    Please  note that the  passed argument vector  may be referred  from
    Prolog  at any time  and should  therefore be valid  as long as  the
    Prolog engine is used.

    A  good setup  in Windows is  to add  SWI-Prolog's bin directory  to
    your  PATH  and  either pass  a  module holding  a  saved-state,  or
    "libpl.dll"  as argv[0].  If the  Prolog state is attached to  a DLL
    (see the -dll option of plld, pass the name of this DLL.


int PPLL__iiss__iinniittiiaalliisseedd(_i_n_t _*_a_r_g_c_, _c_h_a_r _*_*_*_a_r_g_v)
    Test  whether the  Prolog engine  is already initialised.    Returns
    FALSE  if Prolog  is not  initialised and TRUE  otherwise.   If  the
    engine is initialised  and _a_r_g_c is not NULL, the argument count used
    with  PL_initialise()  is stored  in _a_r_g_c.    Same  for the  argument
    vector _a_r_g_v.


void PPLL__iinnssttaallll__rreeaaddlliinnee()
    Installs  the GNU-readline line-editor.  Embedded  applications that
    do  not use the Prolog  top-level should normally delete this  line,
    shrinking  the Prolog kernel significantly.   Note that the  Windows
    version does not use GNU readline.


int PPLL__ttoopplleevveell()
    Runs  the  goal of  the -t toplevel  switch  (default prolog/0)  and
    returns 1 if successful, 0 otherwise.


void PPLL__cclleeaannuupp(_i_n_t _s_t_a_t_u_s)
    This  function performs  the reverse  of PL_initialise().   It  runs
    the  PL_on_halt()  and at_halt/1 handlers, closes all streams  (except
    for the `standard  I/O' streams which are flushed only), deallocates
    all  memory and restores all signal  handlers.  The _s_t_a_t_u_s  argument
    is  passed  to  the  various termination  hooks  and  indicates  the
    _e_x_i_t_-_s_t_a_t_u_s.

    This  function allows deleting and  restarting the Prolog system  in
    the  same process.  Use it with care, as PL_initialise()  is a costly
    function.   Unix  users should consider  using exec() (available  as
    part of the clib package,).


int PPLL__hhaalltt(_i_n_t _s_t_a_t_u_s)
    Cleanup  the Prolog environment using  PL_cleanup() and calls  exit()
    with  the  status argument.    As  PL_cleanup()  can only  be  called
    from  the main thread, this function returns FALSE when  called from
    another thread as the main one.


99..66..2200..11 TThhrreeaaddiinngg,, SSiiggnnaallss aanndd eemmbbeeddddeedd PPrroolloogg

This section  applies to  Unix-based environments that  have signals  or
multi-threading.   The Windows version  is compiled for  multi-threading
and Windows lacks proper signals.

We  can distinguish  two classes  of embedded  executables.   There  are
small C/C++-programs  that act  as an interfacing  layer around  Prolog.
Most  of  these  programs  can  be  replaced  using  the  normal  Prolog
executable extended with  a dynamically loaded foreign extension and  in
most cases  this is  the preferred  route.   In other  cases, Prolog  is
embedded in a complex application that---like  Prolog---wants to control
the process  environment.   A good  example is Java.   Embedding  Prolog
is generally  the only  way to  get these environments  together in  one
process image.   Java applications however are by  nature multi-threaded
and appear to do signal-handling (software interrupts).

To  make  Prolog  operate smoothly  in  such  environments  it  must  be
told not  to alter  the process  environment.   This is  partly done  at
build-time and  partly execution time.   At  build-time we must  specify
the  use of  software stack-overflow  rather then  the default  hardware
checks.  This is done using

sh configure --disable-segv-handling

The resulting  Prolog executable  is about  10% slower  than the  normal
executable,  but  behaves much  more  reliable in  complicated  embedded
situations.  In addition, as the process  no longer handles segmentation
violations, debugging foreign code linked to it is much easier.

At runtime,  it is advised to pass  the flag -nosignals, which  inhibits
all default signal handling.  This has a few consequences though:

  o It  is  no  longer  possible  to break  into  the  tracer  using  an
    interrupt signal (Control-C).

  o SIGPIPE is normally set  to be ignored.  Prolog uses return-codes to
    diagnose  broken pipes.  Depending on the situation one  should take
    appropriate action if Prolog streams are connected to pipes.

  o Fatal  errors normally cause Prolog to call PL_cleanup()  and exit().
    It  is advised to call PL_cleanup() as part of  the exit-procedure of
    your application.


99..77 LLiinnkkiinngg eemmbbeeddddeedd aapppplliiccaattiioonnss uussiinngg pplllldd

The  utility program  plld (Win32:   plld.exe)  may be  used  to link  a
combination of C-files  and Prolog files into a stand-alone  executable.
plld automates most of what is described in the previous sections.

In the normal  usage, a copy is  made of the default embedding  template
\ldots/pl/include/stub.c.    The  main()  routine is  modified  to  suit
your  application.    PL_initialise()  mmuusstt  be passed  the  program-name
(_a_r_g_v_[_0_])  (Win32:    the  executing  program  can   be  obtained  using
GetModuleFileName()).   The other  elements of  the command-line may  be
modified.  Next, plld is typically invoked as:

plld -o output stubfile.c [other-c-or-o-files] [plfiles]

plld  will first  split the  options into  various groups  for both  the
C-compiler and the Prolog  compiler.  Next, it will add  various default
options to the  C-compiler and call it  to create an executable  holding
the  user's C-code  and the  Prolog kernel.    Then,  it  will call  the
SWI-Prolog compiler  to create a  saved state  from the provided  Prolog
files  and finally,  it  will attach  this saved  state to  the  created
emulator to create the requested executable.

Below, it  is described how the options  are split and which  additional
options are passed.

--hheellpp
    Print brief synopsis.

--ppll _p_r_o_l_o_g
    Select  the prolog to use.   This prolog  is used for two  purposes:
    get  the home-directory as well  as the compiler/linker options  and
    create a saved state of the Prolog code.

--lldd _l_i_n_k_e_r
    Linker  used to  link the raw  executable.   Default is  to use  the
    C-compiler (Win32:  link.exe).

--cccc _C_-_c_o_m_p_i_l_e_r
    Compiler  for  .c files  found  on the  command-line.    Default  is
    the  compiler used  to build SWI-Prolog  (see current_prolog_flag/2)
    (Win32:  cl.exe).

--cc++++ _C_+_+_-_c_o_m_p_i_l_e_r
    Compiler  for C++ sources (extensions  .cpp, .cxx, .cc or .C)  files
    found on the command-line.   Default is c++ or g++ if the C-compiler
    is gcc) (Win32:  cl.exe).

--nnoossttaattee
    Just  relink  the  kernel,  do  not  add  any  Prolog  code  to  the
    new  kernel.     This  is  used  to  create  a  new  kernel  holding
    additional  foreign predicates on machines  that do not support  the
    shared-library  (DLL) interface, or if building the state  cannot be
    handled  by the default procedure used by plld.  In the  latter case
    the  state is created  separately and appended  to the kernel  using
    cat <_k_e_r_n_e_l> <_s_t_a_t_e> > <_o_u_t>(Win32:  copy /b <_k_e_r_n_e_l>+<_s_t_a_t_e> <_o_u_t>)

--sshhaarreedd
    Link  C, C++ or object files into a shared object (DLL) that  can be
    loaded  by the  load_foreign_library/1predicate.    If used with  -c
    it  sets the proper  options to compile  a C or  C++ file ready  for
    linking into a shared object

--ddllll
    _W_i_n_d_o_w_s  _o_n_l_y.     Embed  SWI-Prolog  into  a  DLL  rather  than  an
    executable.

--cc
    Compile  C  or C++  source-files  into object  files.    This  turns
    plld  into a  replacement for  the C  or C++  compiler where  proper
    options  such as the  location of the  include directory are  passed
    automatically to the compiler.

--EE
    Invoke the C preprocessor.   Used to make plld a replacement for the
    C or C++ compiler.

--ppll--ooppttiioonnss _,_._._.
    Additional  options passed to Prolog when creating the  saved state.
    The  first character  immediately  following pl-options  is used  as
    separator  and  translated to  spaces when  the  argument is  built.
    Example:  -pl-options,-F,xpce  passed -F xpce as additional flags to
    Prolog.

--lldd--ooppttiioonnss _,_._._.
    Passes options to the linker, similar to -pl-options.

--cccc--ooppttiioonnss _,_._._.
    Passes options to the C/C++ compiler, similar to -pl-options.

--vv
    Select  verbose operation,  showing the  various programs and  their
    options.

--oo _o_u_t_f_i_l_e
    Reserved to specify the final output file.

--ll_l_i_b_r_a_r_y
    Specifies  a library for the C-compiler.   By default, -lpl  (Win32:
    libpl.lib) and the libraries needed by the Prolog kernel are given.

--LL_l_i_b_r_a_r_y_-_d_i_r_e_c_t_o_r_y
    Specifies  a  library directory  for  the C-compiler.    By  default
    the  directory  containing  the  Prolog C-library  for  the  current
    architecture is passed.

-g | -Iinclude-directory | -Ddefinition
    These  options  are passed  to  the C-compiler.    By  default,  the
    include directory containing SWI-Prolog.h  is passed.  plld adds two
    additional * -Ddef flags:

    --DD____SWI_PROLOG__
         Indicates the code is to be connected to SWI-Prolog.

    --DD____SWI_EMBEDDED__
         Indicates the creation of an embedded program.

 _*_._o | _*_._c | _*_._C | _*_._c_x_x | _*_._c_p_p
    Passed as input files to the C-compiler

 _*_._p_l |_*_._q_l_f
    Passed  as  input  files  to  the  Prolog  compiler  to  create  the
    saved-state.

 *
    I.e.  all other options.  These are passed as linker options  to the
    C-compiler.


99..77..11 AA ssiimmppllee eexxaammppllee

The  following is  a very  simple example  going through  all the  steps
outlined above.   It  provides an arithmetic expression  evaluator.   We
will call  the application calc  and define it in  the files calc.c  and
calc.pl.  The Prolog file is simple:

calc(Atom) :-
        term_to_atom(Expr, Atom),
        A is Expr,
        write(A),
        nl.

The  C-part  of   the  application  parses  the  command-line   options,
initialises the  Prolog engine, locates  the calc/1 predicate and  calls
it.  The coder is in figure 9.5.

#include <stdio.h>
#include <SWI-Prolog.h>

#define MAXLINE 1024

int
main(int argc, char **argv)
{ char expression[MAXLINE];
  char *e = expression;
  char *program = argv[0];
  char *plav[2];
  int n;

  /* combine all the arguments in a single string */

  for(n=1; n<argc; n++)
  { if ( n != 1 )
      *e++ = ' ';
    strcpy(e, argv[n]);
    e += strlen(e);

  }

  /* make the argument vector for Prolog */

  plav[0] = program;
  plav[1] = NULL;

  /* initialise Prolog */

  if ( !PL_initialise(1, plav) )
    PL_halt(1);

  /* Lookup calc/1 and make the arguments and call */

  { predicate_t pred = PL_predicate("calc", 1, "user");

    term_t h0 = PL_new_term_refs(1);
    int rval;

    PL_put_atom_chars(h0, expression);
    rval = PL_call_predicate(NULL, PL_Q_NORMAL, pred, h0);

    PL_halt(rval ? 0 : 1);
  }

  return 0;
}

             Figure 9.5:  C-source for the calc application

The application is now created using the following command-line:

% plld -o calc calc.c calc.pl

The following indicates the usage of the application:

% calc pi/2
1.5708


99..88 TThhee PPrroolloogg ``hhoommee'' ddiirreeccttoorryy

Executables  embedding SWI-Prolog  should  be able  to find  the  `home'
directory  of  the  development  environment  unless   a  self-contained
saved-state has  been added to  the executable  (see qsave_program/[1,2]
and section 9.7).

If Prolog starts up, it will try to  locate the development environment.
To do so, it will try the following steps until one succeeds.

 1. If  the environment variable  SWI_HOME_DIRis  defined and points  to
    an existing directory, use this.

 2. If  the  environment variable  SWIPL  is defined  and points  to  an
    existing directory, use this.

 3. Locate  the  primary   executable  or  (Windows  only)  a  component
    (_m_o_d_u_l_e)  thereof  and check  whether the  parent  directory of  the
    directory  holding this file contains the  file swipl.  If so,  this
    file  contains the (relative) path to  the home directory.  If  this
    directory  exists, use this.   This is the normal mechanism  used by
    the binary distribution.

 4. If  the precompiled path exists, use it.  This is only  useful for a
    source installation.

If  all fails  and  there is  no state  attached  to the  executable  or
provided Windows module (see  PL_initialise()), SWI-Prolog gives up.   If
a state is attached, the current working directory is used.

The file_search_path/2 alias swi is set  to point to the home  directory
located.


99..99 EExxaammppllee ooff UUssiinngg tthhee FFoorreeiiggnn IInntteerrffaaccee

Below is an example  showing all stages of the declaration of  a foreign
predicate that transforms atoms possibly holding  uppercase letters into
an atom only holding lower case letters.  Figure  9.6 shows the C-source
file, figure 9.7 illustrates compiling and loading of foreign code.
/*  Include file depends on local installation */

#include <SWI-Prolog.h>
#include <stdlib.h>
#include <ctype.h>

foreign_t
pl_lowercase(term_t u, term_t l)
{ char *copy;
  char *s, *q;

  int rval;

  if ( !PL_get_atom_chars(u, &s) )
    return PL_warning("lowercase/2: instantiation fault");
  copy = malloc(strlen(s)+1);

  for( q=copy; *s; q++, s++)

    *q = (isupper(*s) ? tolower(*s) : *s);
  *q = '\0';

  rval = PL_unify_atom_chars(l, copy);
  free(copy);

  return rval;
}

install_t
install()
{ PL_register_foreign("lowercase", 2, pl_lowercase, 0);
}

                   Figure 9.6:  Lowercase source file

% gcc -I/usr/local/lib/pl-\plversion/include -fpic -c lowercase.c
% gcc -shared -o lowercase.so lowercase.o

% pl
Welcome to SWI-Prolog (Version \plversion)
Copyright (c) 1993-1996 University of Amsterdam.  All rights reserved.

For help, use ?- help(Topic). or ?- apropos(Word).

1 ?- load_foreign_library(lowercase).

Yes
2 ?- lowercase('Hello World!', L).

L = 'hello world!'

Yes

    Figure 9.7:  Compiling the C-source and loading the object file


99..1100 NNootteess oonn UUssiinngg FFoorreeiiggnn CCooddee


99..1100..11 MMeemmoorryy AAllllooccaattiioonn

SWI-Prolog's heap  memory allocation is based  on the malloc(3)  library
routines.     The  stacks  are  allocated  using  mmap()  on  most  Unix
machines and using  VirtualAlloc() on windows.  SWI-Prolog  provides the
functions below  as a  wrapper around  malloc().   Allocation errors  in
these functions  trap SWI-Prolog's  fatal-error handler,  in which  case
PL_malloc() or PL_realloc() do not return.

Portable applications must use  PL_free() to release strings  returned by
PL_get_chars() using the BUF_MALLOC argument.  Portable  applications may
use both PL_malloc() and friends or malloc() and friends but  should not
mix these two sets of functions on the same memory.


void * PPLL__mmaalllloocc(_s_i_z_e___t _b_y_t_e_s)
    Allocate  _b_y_t_e_s of  memory.    On failure  SWI-Prolog's fatal  error
    handler  is  called  and  PL_malloc()  does  not  return.     Memory
    allocated  using these functions must use  PL_realloc() and PL_free()
    rather than realloc() and free().


void * PPLL__rreeaalllloocc(_v_o_i_d _*_m_e_m_, _s_i_z_e___t _s_i_z_e)
    Change  the size of the  allocated chunk, possibly  moving it.   The
    _m_e_m  argument  must  be  obtained  from  a  previous PL_malloc()  or
    PL_realloc() call.


void PPLL__ffrreeee(_v_o_i_d _*_m_e_m)
    Release  memory.  The _m_e_m argument must be obtained from  a previous
    PL_malloc() or PL_realloc() call.


99..1100..22 CCoommppaattiibbiilliittyy bbeettwweeeenn PPrroolloogg vveerrssiioonnss

Great  care  is   taken  to  ensure  binary  compatibility  of   foreign
extensions  between  different   Prolog  versions.     Only  much   less
frequently  used  stream  interface  has  been  responsible  for  binary
incompatibilities.

Source-code that  relies on new  features of  the foreign interface  can
use  the  macro  PLVERSION to  find  the  version  of  SWI-Prolog.h  and
PL_query()  using  the option  PL_QUERY_VERSION  to  find the  version  of
the  attached Prolog  system.   Both  follow the  same numbering  schema
explained with PL_query().


99..1100..33 DDeebbuuggggiinngg FFoorreeiiggnn CCooddee

Statically  linked foreign  code  or embedded  systems can  be  debugged
normally.     Most  modern  environments  provide  debugging  tools  for
dynamically  loaded shared  objects  or dynamic  load  libraries.    The
following example traces  the code of lowercase  using gdb(1) in a  Unix
environment.

% gcc -I/usr/local/lib/pl-2.2.0/include -fpic -c -g lowercase.c
% gcc -shared -o lowercase.so lowercase.o
% gdb pl
(gdb) r
Welcome to SWI-Prolog (Version \plversion)
Copyright (c) 1993-1996 University of Amsterdam.  All rights reserved.

For help, use ?- help(Topic). or ?- apropos(Word).

?- load_foreign_library(lowercase).
<type Control-C>
(gdb) shared                    % loads symbols for shared objects
(gdb) break pl_lowercase
(gdb) continue
?- lowercase('HELLO', X).


99..1100..44 NNaammee CCoonnfflliiccttss iinn CC mmoodduulleess

In  the  current  version  of the  system  all  public  C  functions  of
SWI-Prolog are in the symbol table.  This can lead  to name clashes with
foreign code.    Someday I  should write a  program to  strip all  these
symbols from the symbol table  (why does Unix not have that?).   For now
I can only suggest to give your function another name.   You can do this
using the C preprocessor.  If---for example---your  foreign package uses
a function warning(), which happens to exist in SWI-Prolog  as well, the
following macro should fix the problem.

#define warning warning_

Note  that shared  libraries do  not  have this  problem as  the  shared
library loader  will only look  for symbols in  the main executable  for
symbols that are not defined in the library itself.


99..1100..55 CCoommppaattiibbiilliittyy ooff tthhee FFoorreeiiggnn IInntteerrffaaccee

The term-reference  mechanism was first used  by Quintus Prolog  version
3.     SICStus  Prolog version  3  is  strongly  based  on  the  Quintus
interface.  The  described SWI-Prolog interface is similar to  using the
Quintus or SICStus interfaces, defining all  foreign-predicate arguments
of  type  +term.    SWI-Prolog  explicitly uses  type  functor_t,  while
Quintus  and SICStus  uses <_n_a_m_e>  and  <_a_r_i_t_y>.   As  the  names of  the
functions differ  from Prolog to  Prolog, a  simple macro layer  dealing
with the names can also deal with this detail.  For example:

#define QP_put_functor(t, n, a) PL_put_functor(t, PL_new_functor(n, a))

The  PL_unify_*()  functions are  lacking from  the  Quintus and  SICStus
interface.    They can  easily  be emulated  or the  put/unify  approach
should be used to write compatible code.

The PL_open_foreign_frame()/PL_close_foreign_frame() combination  is lacking
from both  other Prologs.   SICStus has  PL_new_term_refs(0), followed  by
PL_reset_term_refs() that allows for discarding term references.

The  Prolog interface  for  the graphical  user interface  package  XPCE
shares about  90% of the code  using a simple  macro layer to deal  with
different naming and calling conventions of the interfaces.


CChhaapptteerr 1100..  GGEENNEERRAATTIINNGG RRUUNNTTIIMMEE AAPPPPLLIICCAATTIIOONNSS

This  chapter  describes  the  features  of  SWI-Prolog  for  delivering
applications that can run without the development version  of the system
installed.

A SWI-Prolog runtime executable is a file consisting of two  parts.  The
first part  is the  _e_m_u_l_a_t_o_r, which is  machine dependent.   The  second
part is the _r_e_s_o_u_r_c_e  _a_r_c_h_i_v_e, which contains the compiled program  in a
machine-independent format,  startup options  and possibly  user-defined
_r_e_s_o_u_r_c_e_s, see resource/3 and open_resource/3.

These two parts  can be connected in various  different ways.  The  most
common way  for distributed runtime applications  is to _c_o_n_c_a_t_e_n_a_t_e  the
two parts.   This can be achieved  using external commands (Unix:   cat,
Windows:   copy), or  using the  stand_alone option  to qsave_program/2.
The  second option  is  to  attach a  startup  script  in front  of  the
resource that  starts the  emulator with the  proper options.   This  is
the default  under Unix.   Finally,  an emulator  can be told  to use  a
specified resource file using the -x command-line switch.


qqssaavvee__pprrooggrraamm((_+_F_i_l_e_, _+_L_i_s_t_O_f_O_p_t_i_o_n_s))
    Saves   the  current  state  of  the  program  to  the   file  _F_i_l_e.
    The  result  is a  resource archive  containing  a saved-state  that
    expresses  all  Prolog   data  from  the  running  program  and  all
    user-defined  resources.   Depending on the stand_alone option,  the
    resource is headed by the emulator, a Unix shell-script or nothing.

    _L_i_s_t_O_f_O_p_t_i_o_n_s  is a  list of  <_K_e_y>=<_V_a_l_u_e> or  <_K_e_y>(<_V_a_l_u_e>)  pairs.
    The available keys are described in table 10.1.
_________________________________________________________________________
|__KKeeyy________________||OOppttiioonn__||________TTyyppee____________||DDeessccrriippttiioonn__________________________________________________||
|| local      | --LL   ||    K-bytes    |Size (Limit) of local stack         |
| global     | --GG   ||    K-bytes    |Size (Limit) of global stack        |
| trail      | --TT   ||    K-bytes    |Size (Limit) of trail stack         |

| argument   | --AA   ||    K-bytes    |Size (Limit) of argument stack      |
| goal       | --gg   ||     atom      |Initialisation goal                 |
| toplevel   | --tt   ||     atom      |Prolog top-level goal               |
|_init_file___|--ff___||_____atom______|Personal_initialisation_file________||||||
| class      |      |     atom      |If  runtime,  only  read  resources |
|            |      |               |from  the  state  (default).     If |

|            |      |               |kernel,  lock  all   predicates  as |
|            |      |               |system predicates  If  development, |
|            |      |               |save  the   predicates   in   their |
|            |      |               |current  state  and  keep   reading |
|            |      |               |resources  from  their  source  (if |
|            |      |               |present).  See also resource/3.     |
| autoload   |      |     bool      |If true, run autoload/0 first       |
| map        |      |     file      |File to write info on dump          |
| op         |      | save/standard |Save operator declarations?         |

| stand_alone |     |     bool      |Include the emulator in the state   |
| emulator   |      |     file      |Emulator attached  to  the  (stand- |
|            |      |               |alone) executable.  Default  is the |
|____________|______|_______________|running_emulator.___________________|

          Table 10.1:  <_K_e_y> = <_V_a_l_u_e> pairs for qsave_program/2

    Before   writing  the  data   to  file,  qsave_program/2  will   run
    autoload/0  to all  required  autoloading the  system can  discover.
    See autoload/0.

    Provided  the  application  does  not  require  any  of  the  Prolog
    libraries  to  be  loaded  at  runtime,   the  only  file  from  the
    SWI-Prolog development  environment required is the emulator itself.
    The  emulator may  be built in  two flavours.   The  default is  the
    _d_e_v_e_l_o_p_m_e_n_t  _e_m_u_l_a_t_o_r.  The  _r_u_n_t_i_m_e _e_m_u_l_a_t_o_r is similar, but  lacks
    the tracer.

    If  the option  stand_alone(true) is  present, the  emulator is  the
    first  part of the state.   If the emulator is started it  will test
    whether  a  boot-file (state)  is attached  to  the emulator  itself
    and  load this state.   Provided  the application has all  libraries
    loaded,  the resulting executable  is completely independent of  the
    runtime environment or location where it was build.

    See also section 2.10.2.4.


qqssaavvee__pprrooggrraamm((_+_F_i_l_e))
    Equivalent to qsave_program(File, []).


aauuttoollooaadd
    Check  the current Prolog program  for predicates that are  referred
    to,  are  undefined and  have a  definition in  the Prolog  library.
    Load the appropriate libraries.

    This  predicate is used  by qsave_program/[1,2] to ensure the  saved
    state  will not  depend  on one  of the  libraries.   The  predicate
    autoload/0  will find all ddiirreecctt references to predicates.   It does
    not  find predicates referenced via meta-predicates.   The predicate
    log/2  is  defined  in the  library(quintus)  to provide  a  quintus
    compatible means to compute  the natural logarithm of a number.  The
    following program will  behave correctly if its state is executed in
    an environment where the library(quintus) is not available:

    logtable(From, To) :-
            From > To, !.
    logtable(From, To) :-
            log(From, Value),
            format('~d~t~8|~2f~n', [From, Value]),
            F is From + 1,
            logtable(F, To).

    However,  the following implementation  refers to log/2 through  the
    meta-predicate  maplist/3.   Autoload will not  be able to find  the
    reference.   This problem may be fixed either by loading  the module
    library(quintus)  explicitly or  use  require/1 to  tell the  system
    that the predicate log/2 is required by this module.

    logtable(From, To) :-
            findall(X, between(From, To, X), Xlist),
            maplist(log, Xlist, SineList),
            write_table(Xlist, SineList).

    write_table([], []).
    write_table([I|IT], [V|VT]) :-
            format('~d~t~8|~2f~n', [I, V]),
            write_table(IT, VT).


vvoollaattiillee _+_N_a_m_e_/_A_r_i_t_y_, _._._.
    Declare  that  the clauses  of specified  predicates  should nnoott  be
    saved  to the program.   The  volatile declaration is normally  used
    to avoid that  the clauses of dynamic predicates that represent data
    for the current session is saved in the state file.


1100..11 LLiimmiittaattiioonnss ooff qqssaavvee__pprrooggrraamm

There  are  three  areas  that  require  special  attention  when  using
qsave_program/[1,2].

  o If  the  program  is an  embedded  Prolog  application or  uses  the
    foreign  language interface,  care has  to be taken  to restore  the
    appropriate foreign context.  See section 10.2 for details.

  o If  the program uses directives (:- goal. lines) that  perform other
    actions then  setting predicate attributes (dynamic, volatile, etc.)
    or  loading files  (consult,  etc.), the  directive may  need to  be
    prefixed with initialization/1.

  o Database  references as returned by clause/3, recorded/3, etc.   are
    not preserved and may thus not be part of the database when saved.


1100..22 RRuunnttiimmeess aanndd FFoorreeiiggnn CCooddee

Some  applications may  need  to  use the  foreign  language  interface.
Object code is  by definition machine-dependent and thus cannot  be part
of the saved program file.

To complicate the matter even further there are various  ways of loading
foreign code:

  o _U_s_i_n_g _t_h_e _l_i_b_r_a_r_y_(_s_h_l_i_b_) _p_r_e_d_i_c_a_t_e_s
    This  is the preferred way of dealing  with foreign code.   It loads
    quickly and ensures  an acceptable level of independence between the
    versions  of the emulator and the foreign code loaded.  It  works on
    Unix  machines supporting shared libraries and library  functions to
    load  them.  Most  modern Unixes, as  well as Win32 (Windows  95/NT)
    satisfy this constraint.

  o _S_t_a_t_i_c _l_i_n_k_i_n_g
    This  mechanism works on  all machines,  but generally requires  the
    same  C-compiler and linker to be  used for the external code as  is
    used to build SWI-Prolog itself.

To make  a runtime  executable that  can run on  multiple platforms  one
must make runtime  checks to find the correct  way of linking.   Suppose
we have a  source-file myextension.c defining the installation  function
install().

If this file  is compiled into a  shared library, load_foreign_library/1
will load this library and call the installation  function to initialise
the  foreign code.    If it  is  loaded as  a static  extension,  define
install() as the predicate install/0:

static foreign_t
pl_install()
{ install();

  PL_succeed;
}

PL_extension PL_extensions [] =
{
/*{ "name",     arity,  function,       PL_FA_<flags> },*/

  { "install",  0,      pl_install,     0 },
  { NULL,       0,      NULL,           0 }     /* terminating line */
};

Now, use the following Prolog code to load the foreign library:

load_foreign_extensions :-
        current_predicate(install, install), !, % static loaded
        install.
load_foreign_extensions :-                      % shared library
        load_foreign_library(foreign(myextension)).

:- initialization load_foreign_extensions.

The path  alias foreign  is defined by  file_search_path/2.   By  default
it  searches  the  directories <_h_o_m_e>/lib/<_a_r_c_h> and  <_h_o_m_e>/lib.     The
application can specify additional rules for file_search_path/2.


1100..33 UUssiinngg pprrooggrraamm rreessoouurrcceess

A  _r_e_s_o_u_r_c_e is  very  similar to  a  file.    Resources however  can  be
represented in two different formats:  on files, as well  as part of the
resource _a_r_c_h_i_v_e of a saved-state (see qsave_program/2).

A resource has a _n_a_m_e  and a _c_l_a_s_s.  The _s_o_u_r_c_e data of the  resource is
a file.   Resources are declared by declaring the  predicate resource/3.
They are accessed using the predicate open_resource/3.

Before  going into  details,  let  us start  with  an  example.    Short
texts can  easily be  expressed in Prolog  source code,  but long  texts
are cumbersome.   Assume our application  defines a command `help'  that
prints a  helptext to the screen.   We put  the content of the  helptext
into a  file called help.txt.   The following  code implements our  help
command such that help.txt is incorporated into the runtime executable.

resource(help, text, 'help.txt').

help :-
        open_resource(help, text, In),
        copy_stream(In, user_output),
        close(In).

copy_stream(In, Out) :-
        get0(In, C),
        copy_stream(C, In, Out).

copy_stream(-1, _, _) :- !.
copy_stream(C, In, Out) :-
        put(Out, C),
        get0(In, C2),
        copy_stream(C2, In, Out).

The predicate  help/0 opens  the resource  as a Prolog  stream.   If  we
are executing this from the development environment,  this will actually
return a  stream to the file  help.txt itself.   When executed from  the
saved-state, the stream will actually be a stream  opened on the program
resource file, taking care of the offset and length of the resource.


1100..33..11 PPrreeddiiccaatteess DDeeffiinniittiioonnss


rreessoouurrccee((_+_N_a_m_e_, _+_C_l_a_s_s_, _+_F_i_l_e_S_p_e_c))
    This  predicate is  defined  as a  dynamic predicate  in the  module
    user.    Clauses for  it may  be  defined in  any module,  including
    the  user module.    _N_a_m_e is  the name  of the  resource (an  atom).
    A  resource name  may contain  any character,  except for  $ and  :,
    which  are  reserved for  internal usage  by  the resource  library.
    _C_l_a_s_s  describes the what kind of object is stored in  the resource.
    In  the  current implementation,  it  is just  an  atom.    _F_i_l_e_S_p_e_c
    is  a file  specification that  may exploit  file_search_path/2 (see
    absolute_file_name/2).

    Normally,  resources are  defined as unit  clauses (facts), but  the
    definition  of this  predicate also allows  for rules.   For  proper
    generation  of the  saved state,  it must be  possible to  enumerate
    the  available  resources by  calling this  predicate  with all  its
    arguments unbound.

    Dynamic  rules are useful to turn  all files in a certain  directory
    into  resources, without specifying a resources for each file.   For
    example,  assume the file_search_path/2icons refers to  the resource
    directory  containing icon-files.   The  following definition  makes
    all these images available as resources:

    resource(Name, image, icons(XpmName)) :-
            atom(Name), !,
            file_name_extension(Name, xpm, XpmName).
    resource(Name, image, XpmFile) :-
            var(Name),
            absolute_file_name(icons(.), [type(directory)], Dir)
            concat(Dir, '/*.xpm', Pattern),
            expand_file_name(Pattern, XpmFiles),
            member(XpmFile, XpmFiles).


ooppeenn__rreessoouurrccee((_+_N_a_m_e_, _?_C_l_a_s_s_, _-_S_t_r_e_a_m))
    Opens  the resource specified by _N_a_m_e and  _C_l_a_s_s.  If the  latter is
    a  variable, it will be unified  to the class of the  first resource
    found that has the  specified _N_a_m_e.  If successful, _S_t_r_e_a_m becomes a
    handle to a  binary input stream, providing access to the content of
    the resource.

    The  predicate  open_resource/3  first  checks  resource/3.     When
    successful   it  will  open   the  returned  resource   source-file.
    Otherwise  it will  look in the  programs resource  database.   When
    creating  a  saved-state,  the system  normally saves  the  resource
    contents  into the resource archive, but does not save  the resource
    clauses.

    This   way,  the  development   environment  uses  the  files   (and
    modifications   to   the   resource/3  declarations   and/or   files
    containing  resource   info  thus  immediately  affect  the  running
    environment,  while the runtime  system quickly accesses the  system
    resources.


1100..33..22 TThhee plrc pprrooggrraamm

The  utility program  plrc can  be used  to examine  and manipulate  the
contents of  a SWI-Prolog resource  file.  The  options are inspired  by
the Unix ar program.  The basic command is:

% plrc option resource-file member ...

The options are described below.

ll
    List contents of the archive.

xx
    Extract  named (or  all)  members of  the archive  into the  current
    directory.

aa
    Add  files  to the  archive.    If the  archive already  contains  a
    member  with the  same name,  the contents  is replaced.    Anywhere
    in   the  sequence  of  members,   the  options  --class=_c_l_a_s_s   and
    --encoding=_e_n_c_o_d_i_n_g may appear.   They affect the class and encoding
    of subsequent files.  The initial class is data and encoding none.

dd
    Delete named members from the archive.

This command is also described in the pl(1) Unix manual page.


1100..44 FFiinnddiinngg AApppplliiccaattiioonn ffiilleess

If your application  uses files that are  not part of the saved  program
such as database  files, configuration files, etc., the  runtime version
has to be able to  locate these files.  The file_search_path/2 mechanism
in combination with  the -palias command-line argument is the  preferred
way to  locate runtime  files.   The first  step is to  define an  alias
for the  top-level directory  of your application.    We will call  this
directory gnatdir in our examples.

A  good  place  for storing  data  associated  with  SWI-Prolog  runtime
systems is  below the emulator's  home-directory.   swi is a  predefined
alias for this directory.  The following is  a useful default definition
for the search path.

user:file_search_path(gnatdir, swi(gnat)).

The  application  should  locate   all  files  using  absolute_file_name.
Suppose   gnatdir   contains   a  file   config.pl   to   define   local
configuration.  Then use the code below to load this file:

configure_gnat :-
        (   absolute_file_name(gnatdir('config.pl'), ConfigFile)
            ->  consult(ConfigFile)
            ;   format(user_error, 'gnat: Cannot locate config.pl~n'),
            halt(1)
            ).


1100..44..11 PPaassssiinngg aa ppaatthh ttoo tthhee aapppplliiccaattiioonn

Suppose   the  system   administrator  has   installed  the   SWI-Prolog
runtime  environment  in  /usr/local/lib/rt-pl-3.2.0.     A  user  wants
to  install  gnat,   but  gnat  will  look  for  its   configuration  in
/usr/local/lib/rt-pl-3.2.0/gnat where the user cannot write.

The user  decides to install the  gnat runtime files in  /users/bob/lib/
gnat.  For one-time  usage, the user may decide to start gnat  using the
command:

% gnat -p gnatdir=/users/bob/lib/gnat


1100..55 TThhee RRuunnttiimmee EEnnvviirroonnmmeenntt


1100..55..11 TThhee RRuunnttiimmee EEmmuullaattoorr

The sources  may be  used to built  two versions  of the emulator.    By
default,  the _d_e_v_e_l_o_p_m_e_n_t _e_m_u_l_a_t_o_r  is built.    This emulator  contains
all features  for interactive development  of Prolog  applications.   If
the system is  configured using --enable-runtime, make(1) will  create a
_r_u_n_t_i_m_e _v_e_r_s_i_o_n  of the emulator.   This emulator  is equivalent to  the
development version, except for the following features:

  o _N_o _i_n_p_u_t _e_d_i_t_i_n_g
    The  GNU library -lreadline  that provides EMACS compatible  editing
    of input lines will not be linked to the system.

  o _N_o _t_r_a_c_e_r
    The  tracer and  all its options  are removed,  making the system  a
    little faster too.

  o _N_o _p_r_o_f_i_l_e_r
    profile/3 and friends are  not supported.  This saves some space and
    provides better performance.

  o _N_o _i_n_t_e_r_r_u_p_t
    Keyboard  interrupt (Control-C  normally)  is not  rebound and  will
    normally terminate the application.

  o _c_u_r_r_e_n_t___p_r_o_l_o_g___f_l_a_g_(_r_u_n_t_i_m_e_, _t_r_u_e_) _s_u_c_c_e_e_d_s
    This  may  be used  to verify  your application  is  running in  the
    runtime environment rather than the development environment.

  o _c_l_a_u_s_e_/_[_2_,_3
    do not work  on static predicates] This prolog-flag inhibits listing
    your program.  It is only a very limited protection however.

The  following  fragment   is  an  example  for  building  the   runtime
environment in  \env{HOME}/lib/rt-pl-3.2.0.   If  possible, the  shared-
library interface should  be configured to ensure  it can serve a  large
number of applications.

% cd pl-3.2.0
% mkdir runtime
% cd runtime
% ../src/configure --enable-runtime --prefix=$HOME
% make
% make rt-install

The  runtime directory  contains  the components  listed  below.    This
directory may be tar'ed and shipped with your application.

           __________________________________________________
           |_README.RT____|Info_on_the_runtime_environment___|
           |_bin/<_a_r_c_h>/pl|The_emulator_itself_______________|

           |_man/pl.1_____|Manual_page_for_pl________________|
           |_swipl________|pointer_to_the_home_directory_(.)_|
           | lib/         |directory for shared libraries    |
           |_lib/<_a_r_c_h>/__|machine-specific_shared_libraries_|


CChhaapptteerr 1111..  TTHHEE SSWWII--PPRROOLLOOGG LLIIBBRRAARRYY

This chapter documents  the SWI-Prolog library.  As  SWI-Prolog provides
auto-loading,  there is  little  difference between  library  predicates
and built-in predicates.   Part of  the library is therefore  documented
in the  rest of  the manual.   Library  predicates differ from  built-in
predicates in the following ways.

  o User-definition  of a  built-in leads to  a permission-error,  while
    using the name of a library predicate is allowed.

  o If  autoloading is disabled explicitely or because  trapping unknown
    predicates  is disabled  (see unknown/2  and current_prolog_flag/2),
    library predicates must be loaded explicitely.

  o Using  libraries reduce  the  footprint of  applications that  don't
    need them.

    _T_h_e  _d_o_c_u_m_e_n_t_a_t_i_o_n _o_f  _t_h_e _l_i_b_r_a_r_y _i_s  _j_u_s_t _s_t_a_r_t_e_d_.   _M_a_t_e_r_i_a_l
    _f_r_o_m  _t_h_e _s_t_a_n_d_a_r_d _p_a_c_k_a_g_e_s _s_h_o_u_l_d _b_e _m_o_v_e_d _h_e_r_e_, _s_o_m_e _m_a_t_e_r_i_a_l
    _f_r_o_m  _o_t_h_e_r _p_a_r_t_s _o_f _t_h_e _m_a_n_u_a_l _s_h_o_u_l_d _b_e _m_o_v_e_d _t_o_o _a_n_d _v_a_r_i_o_u_s
    _l_i_b_r_a_r_i_e_s _a_r_e _n_o_t _d_o_c_u_m_e_n_t_e_d _a_t _a_l_l_.


1111..11 lists::  LLiisstt MMaanniippuullaattiioonn

This  library  provides  commonly accepted  basic  predicates  for  list
manipulation  in   the  Prolog   community.      Some  additional   list
manipulations are built-in.  Their description is in section 4.28.


aappppeenndd((_?_L_i_s_t_1_, _?_L_i_s_t_2_, _?_L_i_s_t_3))
    Succeeds  when _L_i_s_t_3  unifies with  the concatenation  of _L_i_s_t_1  and
    _L_i_s_t_2.   The  predicate can be  used with any instantiation  pattern
    (even three variables).


mmeemmbbeerr((_?_E_l_e_m_, _?_L_i_s_t))
    Succeeds  when _E_l_e_m can be unified with one of the members  of _L_i_s_t.
    The predicate can be used with any instantiation pattern.


nneexxttttoo((_?_X_, _?_Y_, _?_L_i_s_t))
    Succeeds when _Y immediatly follows _X in _L_i_s_t.


ddeelleettee((_+_L_i_s_t_1_, _?_E_l_e_m_, _?_L_i_s_t_2))
    Delete all members  of _L_i_s_t_1 that simultaneously unify with _E_l_e_m and
    unify the result with _L_i_s_t_2.


sseelleecctt((_?_E_l_e_m_, _?_L_i_s_t_, _?_R_e_s_t))
    Select  _E_l_e_m  from _L_i_s_t  leaving  _R_e_s_t.    It behaves  as  member/2,
    returning  the  remaining  elements in  _R_e_s_t.    Note  that  besides
    selecting  elements  from a  list, it  can also  be  used to  insert
    elements.


nntthh00((_?_I_n_d_e_x_, _?_L_i_s_t_, _?_E_l_e_m))
    Succeeds  when  the  _I_n_d_e_x-th element  of  _L_i_s_t unifies  with  _E_l_e_m.
    Counting starts at 0.


nntthh11((_?_I_n_d_e_x_, _?_L_i_s_t_, _?_E_l_e_m))
    Succeeds  when  the  _I_n_d_e_x-th element  of  _L_i_s_t unifies  with  _E_l_e_m.
    Counting starts at 1.


llaasstt((_?_L_i_s_t_, _?_E_l_e_m))
    Succeeds  if _E_l_e_m unifies with  the last element of  _L_i_s_t.  If  _L_i_s_t
    is  a proper list last/2 is deterministic.   If _L_i_s_t has  an unbound
    tail, backtracking will cause _L_i_s_t to grow.


rreevveerrssee((_+_L_i_s_t_1_, _-_L_i_s_t_2))
    Reverse  the order  of the elements  in _L_i_s_t_1  and unify the  result
    with the elements of _L_i_s_t_2.


ppeerrmmuuttaattiioonn((_?_L_i_s_t_1_, _?_L_i_s_t_2))
    Permuation  is true  when  _L_i_s_t_1 is  a permutation  of _L_i_s_t_2.    The
    implementation  can  solve  for _L_i_s_t_2  given  _L_i_s_t_1 or  _L_i_s_t_1  given
    _L_i_s_t_2, or even enumerate _L_i_s_t_1 and _L_i_s_t_2 together.


ffllaatttteenn((_+_L_i_s_t_1_, _-_L_i_s_t_2))
    Transform  _L_i_s_t_1, possibly holding lists  as elements into a  `flat'
    list by replacing  each list with its elements (recursively).  Unify
    the resulting flat list with _L_i_s_t_2.  Example:

    ?- flatten([a, [b, [c, d], e]], X).

    X = [a, b, c, d, e]


ssuummlliisstt((_+_L_i_s_t_, _-_S_u_m))
    Unify  _S_u_m to the result of adding all elements in _L_i_s_t.   _L_i_s_t must
    be  a proper  list holding numbers.   See  number/1 and  is/2.   for
    details on arithmetic.


nnuummlliisstt((_+_L_o_w_, _+_H_i_g_h_, _-_L_i_s_t))
    If  _L_o_w and _H_i_g_h are integers with Low=< High, unify _L_i_s_t to a list
    [Low, Low +1, ...High].  See also between/3.


1111..11..11 SSeett MMaanniippuullaattiioonn

The set  predicates listed  in this  section work  on ordinary  unsorted
lists.     Note that  this  makes  many  of  the  operations order  N2.
For  larger sets  consider the  use of  ordered sets  as implemented  by
library ordsets.pl,  running most  these operations  in order  N.   See
section 11.2.


iiss__sseett((_+_S_e_t))
    Succeeds if _S_e_t is a list (see is_list/1) without duplicates.


lliisstt__ttoo__sseett((_+_L_i_s_t_, _-_S_e_t))
    Unifies  _S_e_t  with a  list  holding the  same  elements as  _L_i_s_t  in
    the  same order.   If  _l_i_s_t contains duplicates,  only the first  is
    retained.  See also sort/2.  Example:

    ?- list_to_set([a,b,a], X)

    X = [a,b]


iinntteerrsseeccttiioonn((_+_S_e_t_1_, _+_S_e_t_2_, _-_S_e_t_3))
    Succeeds  if _S_e_t_3 unifies  with the intersection  of _S_e_t_1 and  _S_e_t_2.
    _S_e_t_1  and _S_e_t_2  are  lists without  duplicates.   They  need not  be
    ordered.


ssuubbttrraacctt((_+_S_e_t_, _+_D_e_l_e_t_e_, _-_R_e_s_u_l_t))
    Delete  all  elements  of set  `Delete'  from  `Set' and  unify  the
    resulting set with `Result'.


uunniioonn((_+_S_e_t_1_, _+_S_e_t_2_, _-_S_e_t_3))
    Succeeds if _S_e_t_3 unifies  with the union of _S_e_t_1 and _S_e_t_2.  _S_e_t_1 and
    _S_e_t_2 are lists without duplicates.  They need not be ordered.


ssuubbsseett((_+_S_u_b_s_e_t_, _+_S_e_t))
    Succeeds if all elements of _S_u_b_s_e_t are elements of _S_e_t as well.


1111..22 ordsets::  OOrrddeerreedd SSeett MMaanniippuullaattiioonn

Ordered  sets are  lists with  unique elements  sorted  to the  standard
order of  terms (see  sort/2).   Exploiting  ordering, many  of the  set
operations can  be expressed  in order  N rather  than N2  when dealing
with  unordered sets  that  may contain  duplicates.    The  ordsets  is
available in  a number of  Prolog implementations.   Our predicates  are
designed to be compatible with common practice in  the Prolog community.
The implementation  is incomplete and  relies partly  on oset, an  older
ordered set library  distributed with SWI-Prolog.  New  applications are
advices to use ordsets.

Some  of   these  predicates  match   directly  to  corresponding   list
operations.   It is  adviced to  use the versions  from this library  to
make clear you are operating on ordered sets.


oorrdd__eemmppttyy((_?_S_e_t))
    True  if _S_e_t is an  empty ordered set.   _S_e_t unifies with the  empty
    list.


lliisstt__ttoo__oorrdd__sseett((_+_L_i_s_t_, _-_O_r_d_S_e_t))
    Convert a _L_i_s_t to an ordered set.  Same as sort/2.


oorrdd__aadddd__eelleemmeenntt((_+_S_e_t_, _+_E_l_e_m_e_n_t_, _-_N_e_w_S_e_t))
    Add  an element to  an ordered set.   _N_e_w_S_e_t is  the same as _S_e_t  if
    _E_l_e_m_e_n_t is already part of _S_e_t.


oorrdd__ddeell__eelleemmeenntt((_+_S_e_t_, _+_E_l_e_m_e_n_t_, _-_N_e_w_S_e_t))
    Delete _E_l_e_m_e_n_t from _S_e_t.   Succeeds without changing _S_e_t if _S_e_t does
    not contain _E_l_e_m_e_n_t.


oorrdd__iinntteerrsseecctt((_+_S_e_t_1_, _+_S_e_t_2))
    True if the intersection of _S_e_t_1 and _S_e_t_2 is non-empty.


oorrdd__iinntteerrsseeccttiioonn((_+_S_e_t_1_, _+_S_e_t_2_, _-_I_n_t_e_r_s_e_c_t_i_o_n))
    True if _I_n_t_e_r_s_e_c_t_i_o_n is the intersection of _S_e_t_1 and _S_e_t_2.


oorrdd__ddiissjjooiinntt((_+_S_e_t_1_, _+_S_e_t_2))
    True  if  _S_e_t_1  and  _S_e_t_2 have  no  common  element.    Negation  of
    ord_intersect/2.


oorrdd__ssuubbttrraacctt((_+_S_e_t_, _+_D_e_l_e_t_e_, _-_R_e_m_a_i_n_i_n_g))
    True  if _R_e_m_a_i_n_i_n_g contains the elements of _S_e_t that are not  in set
    _D_e_l_e_t_e.


oorrdd__uunniioonn((_+_S_e_t_1_, _+_S_e_t_2_, _-_U_n_i_o_n))
    True if _U_n_i_o_n contains all elements from _S_e_t_1 and _S_e_t_2


oorrdd__uunniioonn((_+_S_e_t_1_, _+_S_e_t_2_, _-_U_n_i_o_n_, _-_N_e_w))
    Defined  as  if  ord_union(_S_e_t_1_,  _S_e_t_2_,  _U_n_i_o_n),  ord_subtract(_S_e_t_2_,
    _S_e_t_1_, _N_e_w).


oorrdd__ssuubbsseett((_+_S_u_b_, _+_S_u_p_e_r))
    True if all elements of _S_u_b are in _S_u_p_e_r.


oorrdd__mmeemmbbeerrcchhkk((_+_E_l_e_m_e_n_t_, _+_S_e_t))
    True  if _E_l_e_m_e_n_t  appears in  _S_e_t.   Does not  backtrack.   Same  as
    memberchk/2.


1111..33 assoc::  AAssssoocciiaattiioonn lliissttss

      Authors:  _R_i_c_h_a_r_d _A_. _O_'_K_e_e_f_e_, _L_._D_a_m_a_s_, _V_._S_._C_o_s_t_a _a_n_d _M_a_r_k_u_s _T_r_i_s_k_a

Elements of an association  list have 2 components:  A (unique)  _k_e_y and
a _v_a_l_u_e.   Keys should be  ground, values need not  be.  An  association
list can be used  to fetch elements via their keys and to  enumerate its
elements in ascending  order of their keys.   The assoc module uses  AVL
trees to  implement association lists.   This makes inserting,  changing
and fetching a single  element an O(log(N)) (where N denotes  the number
of elements in the list) expected time (and worst-case time) operation.


aassssoocc__ttoo__lliisstt((_+_A_s_s_o_c_, _-_L_i_s_t))
    _L_i_s_t is a  list of Key-Value pairs corresponding to the associations
    in _A_s_s_o_c in ascending order of keys.


eemmppttyy__aassssoocc((_-_A_s_s_o_c))
    _A_s_s_o_c is unified with an empty association list.


ggeenn__aassssoocc((_?_K_e_y_, _+_A_s_s_o_c_, _?_V_a_l_u_e))
    Enumerate  matching elements  of _A_s_s_o_c in  ascending order of  their
    keys via backtracking.


ggeett__aassssoocc((_+_K_e_y_, _+_A_s_s_o_c_, _?_V_a_l_u_e))
    _V_a_l_u_e  is the  value  associated with  _K_e_y in  the association  list
    _A_s_s_o_c.


ggeett__aassssoocc((_+_K_e_y_, _+_A_s_s_o_c_, _?_O_l_d_, _?_N_e_w_A_s_s_o_c_, _?_N_e_w))
    _N_e_w_A_s_s_o_c  is an association list identical to _A_s_s_o_c except  that the
    value associated with _K_e_y is _N_e_w instead of _O_l_d.


lliisstt__ttoo__aassssoocc((_+_L_i_s_t_, _?_A_s_s_o_c))
    _A_s_s_o_c  is an association list  corresponding to the Key-Value  pairs
    in _L_i_s_t.


mmaapp__aassssoocc((_:_G_o_a_l_, _+_A_s_s_o_c))
    _G_o_a_l_(_V_) is true for every value V in _A_s_s_o_c.


mmaapp__aassssoocc((_:_G_o_a_l_, _+_A_s_s_o_c_I_n_, _?_A_s_s_o_c_O_u_t))
    _A_s_s_o_c_O_u_t is _A_s_s_o_c_I_n  with _G_o_a_l applied to all corresponding pairs of
    values.


mmaaxx__aassssoocc((_+_A_s_s_o_c_, _?_K_e_y_, _?_V_a_l_u_e))
    _K_e_y and _V_a_l_u_e are  key and value of the element with the largest key
    in _A_s_s_o_c.


mmiinn__aassssoocc((_+_A_s_s_o_c_, _?_K_e_y_, _?_V_a_l_u_e))
    _K_e_y  and _V_a_l_u_e are  key and value of  the element with the  smallest
    key in _A_s_s_o_c.


oorrdd__lliisstt__ttoo__aassssoocc((_+_L_i_s_t_, _?_A_s_s_o_c))
    _A_s_s_o_c  is an association list  correpsond to the Key-Value pairs  in
    _L_i_s_t, which must occur in ascending order of their keys.


ppuutt__aassssoocc((_+_K_e_y_, _+_A_s_s_o_c_, _+_V_a_l_u_e_, _?_N_e_w_A_s_s_o_c))
    _N_e_w_A_s_s_o_c  is an association list identical to _A_s_s_o_c except  that _K_e_y
    is  associated with _V_a_l_u_e.   This can be  used to insert and  change
    associations.


1111..44 ugraphs::  UUnnwweeiigghhtteedd GGrraapphhss

                          Authors:  _R_i_c_h_a_r_d _O_'_K_e_e_f_e _& _V_i_t_o_r _S_a_n_t_o_s _C_o_s_t_a

    _I_m_p_l_e_m_e_n_t_a_t_i_o_n  _a_n_d  _d_o_c_u_m_e_n_t_a_t_i_o_n _a_r_e  _c_o_p_i_e_d _f_r_o_m  _Y_A_P _5_._0_._1_.
    _T_h_e   ugraph  _l_i_b_r_a_r_y  _i_s  _b_a_s_e_d  _o_n  _c_o_d_e  _o_r_i_g_i_n_a_l_l_y  _w_r_i_t_t_e_n
    _b_y  _R_i_c_h_a_r_d  _O_'_K_e_e_f_e_.     _T_h_e  _c_o_d_e  _w_a_s  _t_h_e_n _e_x_t_e_n_d_e_d  _t_o  _b_e
    _c_o_m_p_a_t_i_b_l_e  _w_i_t_h _t_h_e _S_I_C_S_t_u_s _P_r_o_l_o_g _u_g_r_a_p_h_s  _l_i_b_r_a_r_y_.  _C_o_d_e _a_n_d
    _d_o_c_u_m_e_n_t_a_t_i_o_n  _h_a_v_e _b_e_e_n _c_l_e_a_n_e_d _a_n_d _s_t_y_l_e  _h_a_s _b_e_e_n _c_h_a_n_g_e_d _t_o
    _b_e _m_o_r_e _i_n _l_i_n_e _w_i_t_h _t_h_e _r_e_s_t _o_f _S_W_I_-_P_r_o_l_o_g_.

    _T_h_e  _u_g_r_a_p_h_s  _l_i_b_r_a_r_y  _w_a_s _o_r_i_g_i_n_a_l_l_y  _r_e_l_e_a_s_e_d  _i_n  _t_h_e _p_u_b_l_i_c
    _d_o_m_a_i_n_.   _Y_A_P _i_s  _c_o_n_v_e_r_e_d _b_y _t_h_e _P_e_r_l  _a_r_t_i_s_t_i_c _l_i_c_e_n_s_e_, _w_h_i_c_h
    _d_o_e_s  _n_o_t  _i_m_p_l_y _f_u_r_t_h_e_r  _r_e_s_t_r_i_c_t_i_o_n_s _o_n  _t_h_e  _S_W_I_-_P_r_o_l_o_g _L_G_P_L
    _l_i_c_e_n_s_e_.

The  routines   assume  directed  graphs,   undirected  graphs  may   be
implemented by using two edges.

Originally  graphs  where represented  in  two  formats.    The  SICStus
library and this  version of ugraphs.pl only uses the  _S_-_r_e_p_r_e_s_e_n_t_a_t_i_o_n.
The S-representation of  a graph is a list of  (vertex-neighbors) pairs,
where the pairs are  in standard order (as produced by keysort)  and the
neighbors of  each vertex  are also  in standard order  (as produced  by
sort).   This form  is convenient  for many calculations.   Each  vertex
appears in the S-representation, also if it has no neighbors.


vveerrttiicceess__eeddggeess__ttoo__uuggrraapphh((_+_V_e_r_t_i_c_e_s_, _+_E_d_g_e_s_, _-_G_r_a_p_h))
    Given  a graph  with a set  of _V_e_r_t_i_c_e_s  and a set  of _E_d_g_e_s,  _G_r_a_p_h
    must  unify with the corresponding S-representation.  Note  that the
    vertices  without edges will  appear in _V_e_r_t_i_c_e_s  but not in  _E_d_g_e_s.
    Moreover, it is sufficient for a vertice to appear in _E_d_g_e_s.

    ?- vertices_edges_to_ugraph([],[1-3,2-4,4-5,1-5], L).
    L = [1-[3,5], 2-[4], 3-[], 4-[5], 5-[]]

    In this case all  vertices are defined implicitly.  The next example
    shows three unconnected vertices:

    ?- vertices_edges_to_ugraph([6,7,8],[1-3,2-4,4-5,1-5], L).
    L = [1-[3,5], 2-[4], 3-[], 4-[5], 5-[], 6-[], 7-[], 8-[]] ?


vveerrttiicceess((_+_G_r_a_p_h_, _-_V_e_r_t_i_c_e_s))
    Unify   _V_e_r_t_i_c_e_s  with  all  vertices  appearing  in   graph  _G_r_a_p_h.
    Example:

    ?- vertices([1-[3,5],2-[4],3-[],4-[5],5-[]], L).
    L = [1, 2, 3, 4, 5]


eeddggeess((_+_G_r_a_p_h_, _-_E_d_g_e_s))
    Unify  _E_d_g_e_s  with all  edges  appearing  in _G_r_a_p_h.    In  the  next
    example:

    ?- edges([1-[3,5],2-[4],3-[],4-[5],5-[]], L).
    L = [1-3, 1-5, 2-4, 4-5]


aadddd__vveerrttiicceess((_+_G_r_a_p_h_, _+_V_e_r_t_i_c_e_s_, _-_N_e_w_G_r_a_p_h))
    Unify  _N_e_w_G_r_a_p_h with  a new  graph obtained  by adding  the list  of
    _V_e_r_t_i_c_e_s to the _G_r_a_p_h.  Example:

    ?- add_vertices([1-[3,5],2-[]], [0,1,2,9], NG).
    NG = [0-[], 1-[3,5], 2-[], 9-[]]


ddeell__vveerrttiicceess((_+_V_e_r_t_i_c_e_s_, _+_G_r_a_p_h_, _-_N_e_w_G_r_a_p_h))
    Unify  _N_e_w_G_r_a_p_h with a  new graph obtained  by deleting the list  of
    _V_e_r_t_i_c_e_s  and all the  edges that start  from or go  to a vertex  in
    _V_e_r_t_i_c_e_s to the _G_r_a_p_h.  Example:

    ?- del_vertices([2,1],
                    [1-[3,5],2-[4],3-[],4-[5],5-[],6-[],7-[2,6],8-[]],
                    NL).
    NL = [3-[],4-[5],5-[],6-[],7-[6],8-[]]


aadddd__eeddggeess((_+_G_r_a_p_h_, _+_E_d_g_e_s_, _-_N_e_w_G_r_a_p_h))
    Unify  _N_e_w_G_r_a_p_h with  a new  graph obtained  by adding  the list  of
    edges _E_d_g_e_s to the graph _G_r_a_p_h.  Example:

    ?- add_edges([1-[3,5],2-[4],3-[],4-[5],5-[],6-[],7-[],8-[]],
                 [1-6,2-3,3-2,5-7,3-2,4-5],
                 NL).
    NL = [1-[3,5,6], 2-[3,4], 3-[2], 4-[5], 5-[7], 6-[], 7-[], 8-[]]


ddeell__eeddggeess((_+_G_r_a_p_h_, _+_E_d_g_e_s_, _-_N_e_w_G_r_a_p_h))
    Unify  _N_e_w_G_r_a_p_h with a  new graph obtained  by removing the list  of
    _E_d_g_e_s from the _G_r_a_p_h.   Notice that no vertices are deleted.  In the
    next example:

    ?- del_edges([1-[3,5],2-[4],3-[],4-[5],5-[],6-[],7-[],8-[]],
                 [1-6,2-3,3-2,5-7,3-2,4-5,1-3],
                 NL).
    NL = [1-[5],2-[4],3-[],4-[],5-[],6-[],7-[],8-[]]


ttrraannssppoossee((_+_G_r_a_p_h_, _-_N_e_w_G_r_a_p_h))
    Unify  _N_e_w_G_r_a_p_h with a  new graph obtained  from _G_r_a_p_h by  replacing
    all  edges of the form V1-V2 by edges  of the form V2-V1.   The cost
    is  O(|V|2).  Notice  that an undirected graph is  its own transpose.
    Example:

    ?- transpose([1-[3,5],2-[4],3-[],4-[5],5-[],6-[],7-[],8-[]], NL).
    NL = [1-[],2-[],3-[1],4-[2],5-[1,4],6-[],7-[],8-[]]


nneeiigghhbboouurrss((_+_V_e_r_t_e_x_, _+_G_r_a_p_h_, _-_V_e_r_t_i_c_e_s))
    Unify  _V_e_r_t_i_c_e_s with  the  list of  neighbours of  vertex _V_e_r_t_e_x  in
    _G_r_a_p_h.  Example:

    ?- neighbours(4,[1-[3,5],2-[4],3-[],
                     4-[1,2,7,5],5-[],6-[],7-[],8-[]], NL).
    NL = [1,2,7,5]


nneeiigghhbboorrss((_+_V_e_r_t_e_x_, _+_G_r_a_p_h_, _-_V_e_r_t_i_c_e_s))
    American version of neighbours/3.


ccoommpplleemmeenntt((_+_G_r_a_p_h_, _-_N_e_w_G_r_a_p_h))
    Unify _N_e_w_G_r_a_p_h with the graph complementary to _G_r_a_p_h.  Example:

    ?- complement([1-[3,5],2-[4],3-[],
                   4-[1,2,7,5],5-[],6-[],7-[],8-[]], NL).
    NL = [1-[2,4,6,7,8],2-[1,3,5,6,7,8],3-[1,2,4,5,6,7,8],
          4-[3,5,6,8],5-[1,2,3,4,6,7,8],6-[1,2,3,4,5,7,8],
          7-[1,2,3,4,5,6,8],8-[1,2,3,4,5,6,7]]


ccoommppoossee((_+_L_e_f_t_G_r_a_p_h_, _+_R_i_g_h_t_G_r_a_p_h_, _-_N_e_w_G_r_a_p_h))
    Compose,  by connecting the  _d_r_a_i_n_s of _L_e_f_t_G_r_a_p_h  to the _s_o_u_r_c_e_s  of
    _R_i_g_h_t_G_r_a_p_h.  Example:

    ?- compose([1-[2],2-[3]],[2-[4],3-[1,2,4]],L).
    L = [1-[4], 2-[1,2,4], 3-[]]


uuggrraapphh__uunniioonn((_+_G_r_a_p_h_1_, _+_G_r_a_p_h_2_, _-_N_e_w_G_r_a_p_h))
    _N_e_w_G_r_a_p_h is the union of _G_r_a_p_h_1 and _G_r_a_p_h_2.  Example:

    ?- ugraph_union([1-[2],2-[3]],[2-[4],3-[1,2,4]],L).
    L = [1-[2], 2-[3,4], 3-[1,2,4]]


ttoopp__ssoorrtt((_+_G_r_a_p_h_, _-_S_o_r_t))
    Generate  the set of  nodes _S_o_r_t as  a topological sorting of  graph
    _G_r_a_p_h,  if  one is  possible.    A toplogical  sort  is possible  if
    the  graph is connected  and acyclic.   In the  example we show  how
    topological sorting works for a linear graph:

    ?- top_sort([1-[2], 2-[3], 3-[]], L).
    L = [1, 2, 3]


ttoopp__ssoorrtt((_+_G_r_a_p_h_, _-_S_o_r_t_0_, _-_S_o_r_t))
    Generate the difference  list Sort-Sort0 as a topological sorting of
    graph _G_r_a_p_h, if one is possible.


ttrraannssiittiivvee__cclloossuurree((_+_G_r_a_p_h_, _-_C_l_o_s_u_r_e))
    Generate  the  graph  Closure as  the  transitive closure  of  graph
    _G_r_a_p_h.  Example:

     ?- transitive_closure([1-[2,3],2-[4,5],4-[6]],L).
    L = [1-[2,3,4,5,6], 2-[4,5,6], 4-[6]]


rreeaacchhaabbllee((_+_V_e_r_t_e_x_, _+_G_r_a_p_h_, _-_V_e_r_t_i_c_e_s))
    Unify _V_e_r_t_i_c_e_s with the  set of all vertices in graph _G_r_a_p_h that are
    reachable from _V_e_r_t_e_x.  Example:

    ?- reachable(1,[1-[3,5],2-[4],3-[],4-[5],5-[]],V).
    V = [1, 3, 5]


1111..55 nbset::  NNoonn--bbaacckkttrraacckkaabbllee sseett

The  library  nb_set  defines  _n_o_n_-_b_a_c_k_t_r_a_c_k_a_b_l_e  _s_e_t_s,  implemented  as
binary  trees.     The  sets  are  represented  as  compound  terms  and
manipulated  using  nb_setarg/3.     Non-backtrackable  manipulation  of
datastructures  is   not  supported   by  a  large   number  of   Prolog
implementation,  but  it  it  has  several  advantages  over  using  the
database.    It produces  less garbage,  is thread-safe,  reentrant  and
deals with exceptions without leaking data.

Similar to the assoc library keys can be any Prolog term,  but it is not
allowed to instantiate or modify a term.

One of  the ways to  use this  library is to  generate unique values  on
backtracking  _w_i_t_h_o_u_t generating  _a_l_l solutions  first,  for example  to
act as  a filter between  a generator producing  many duplicates and  an
expensive test routine, as outlines below.

generate_and_test(Solution) :-
        empty_nb_set(Set),
        generate(Solution),
        add_nb_set(Solution, Set, true),
        test(Solution).


eemmppttyy__nnbb__sseett((_?_S_e_t))
    True if _S_e_t is a non-backtrackable emoty set.


aadddd__nnbb__sseett((_+_K_e_y_, _!_S_e_t))
    Add  _K_e_y to _S_e_t.   If _K_e_y  is already a member  of _S_e_t, add_nb_set/3
    succeeds without modifying _S_e_t.


aadddd__nnbb__sseett((_+_K_e_y_, _!_S_e_t_, _?_N_e_w))
    If  _K_e_y is not  in _S_e_t and _N_e_w  is unified to  true _K_e_y is added  to
    _S_e_t.  If _K_e_y is  in _S_e_t _N_e_w is unified to false.  It can be used for
    many purposes:

             add_nb_set(+, +, false) Test membership
             add_nb_set(+, +, true)  Succeed only if new member
             add_nb_set(+, +, Var)   Succeed, bindin _V_a_r


ggeenn__nnbb__sseett((_+_S_e_t_, _-_K_e_y))
    Generate  all members of _S_e_t  on backtracking in the standard  order
    of terms.  To test membership, use add_nb_set/3.


ssiizzee__nnbb__sseett((_+_S_e_t_, _-_S_i_z_e))
    Unify _S_i_z_e with the number of elements in _S_e_t.


nnbb__sseett__ttoo__lliisstt((_+_S_e_t_, _-_L_i_s_t))
    Unify _L_i_s_t with a  list of all elements in set in the standard order
    of terms (i.e. and _o_r_d_e_r_e_d _l_i_s_t).


1111..66 gensym::  GGeenneerraattee uunniiqquuee iiddeennttiiffiieerrss

Gensym  (GGeennerate SSyymmbols)  is  an  old library  for  generating  unique
symbols (atoms).    Such symbols are  generated from  a base atom  which
gets a sequence number  appended.  Of course there is no  guarantee that
`catch22'  is not  an already  defined atom  and therefore  one must  be
aware these atoms are only unique in an isolated context.

The SWI-Prolog gensym library is thread-safe.  The  sequence numbers are
global over  all threads and therefore  generated atoms are unique  over
all threads.


ggeennssyymm((_+_B_a_s_e_, _-_U_n_i_q_u_e))
    Generate  a unique  atom from base  _B_a_s_e and  unify it with  _U_n_i_q_u_e.
    _B_a_s_e  should be an  atom.  The  first call will  return <_b_a_s_e>1,  the
    next  <_b_a_s_e>2, etc.   Note that this is  no warrant that the atom  is
    unique in the system.


rreesseett__ggeennssyymm((_+_B_a_s_e))
    Restart  generation of identifiers  from _B_a_s_e  at <_B_a_s_e>1.   Used to
    make  sure a program produces  the same results on subsequent  runs.
    Use with care.


rreesseett__ggeennssyymm
    Reset  gensym for all registered keys.  This predicate  is available
    for  compatibility only.  New  code is strongly advice to avoid  the
    use  of reset_gensym or at least to reset only the keys  used by your
    program to avoid unexpected site-effects on other components.


1111..77 check::  EElleemmeennttaarryy ccoommpplleetteenneessss cchheecckkss

This library defines the predicate check/0 and a few  friends that allow
for a quick-and-dirty cross-referencing.


cchheecckk
    Performs  the three checking passes implemented by list_undefined/0,
    list_autoload/0 and  list_redefined/0.   Please check the  definition
    of these predicates for details.

    The  typical usage  of this  predicate is right  after loading  your
    program  to get a  quick overview on  the completeness and  possible
    conflicts in your program.


lliisstt__uunnddeeffiinneedd
    Scans  the  database for  predicates that  have no  definition.    A
    predicate  is  considered defined  if it  has clauses,  is  declared
    using  dynamic/1 or  multifile/1.   As a  program is compiled  calls
    are  translated to predicates.   If the called predicate is not  yet
    defined  it  is created  as a  predicate without  definition.    The
    same  happens with runtime  generated calls.   This predicate  lists
    all  such undefined predicates that  are referenced and not  defined
    in  the library.   See  also list_autoload/0.   Below is an  example
    from a real  program and an illustration how to edit the referencing
    predicate using edit/1.

    ?- list_undefined.
    Warning: The predicates below are not defined. If these are defined
    Warning: at runtime using assert/1, use :- dynamic Name/Arity.
    Warning:
    Warning: rdf_edit:rdfe_retract/4, which is referenced by
    Warning:         1-st clause of rdf_edit:undo/4
    Warning: rdf_edit:rdfe_retract/3, which is referenced by
    Warning:         1-st clause of rdf_edit:delete_object/1
    Warning:         1-st clause of rdf_edit:delete_subject/1
    Warning:         1-st clause of rdf_edit:delete_predicate/1

    ?- edit(rdf_edit:undo/4).


lliisstt__aauuttoollooaadd
    Lists  all undefined (see  list_undefined/0)  predicates that have  a
    definition  in the library along with the file from which  they will
    be autoloaded when accessed.  See also autoload/0.


lliisstt__rreeddeeffiinneedd
    Lists  predicates that  are  defined in  the global  module user  as
    well  as in a normal  module.  I.e.  predicates for which the  local
    definition overrules the global default definition.


1111..88 debug::  SSoommee rreeuussaabbllee ccooddee ttoo hheellpp ddeebbuuggggiinngg aapppplliiccaattiioonnss

This  library provides  an  structured  alternative for  putting  print-
statements into  your source-code  to trace  what is  going on.    Debug
messages are organised in _t_o_p_i_c_s that can be  activated and de-activated
without  changing  the  source.     In  addition,   if  the  application
is  compiled  with  the -O  flag  these  predicates  are  removed  using
goal_expansion/2.

Although  this library  can be  used through  the normal  demand-loading
mechanism it is adviced  to load it explicitely before code using  it to
profit from goal-expansion,  which removes these calls if  compiled with
optimisation on and records the topics from debug/3  and debugging/1 for
list_debug_topics/0.


ddeebbuugg((_+_T_o_p_i_c_, _+_F_o_r_m_a_t_, _+_A_r_g_s))
    If  _T_o_p_i_c is a selected debugging  topic (see debug/1) a message  is
    printed using  print_message/2with level informational.   _F_o_r_m_a_t and
    _A_r_g_s are interpreted by format/2.  Here is a typical example:

            ...,
            debug(init, 'Initialised ~w', [Module]),
            ...,

    _T_o_p_i_c  can be any Prolog term.   Compound terms can be used  to make
    categories of topics that can be activated using debug/1.


ddeebbuuggggiinngg((_+_T_o_p_i_c))
    Succeeds  if _T_o_p_i_c is  a selected debugging topic.   It is  intended
    to  execute  arbitrary  code  depending on  the  users  debug  topic
    selection.     The  construct  (debugging(Topic) -> Code ; true)  is
    removed if the code is compiled in optimise mode.


ddeebbuugg((_+_T_o_p_i_c))
    Select  all registered topics that  unify with _T_o_p_i_c for  debugging.
    This  call is normally  used from the  toplevel to activate a  topic
    for debugging.  Topics are de-activated using nodebug/1.


nnooddeebbuugg((_+_T_o_p_i_c))
    Deactivates topics for debugging.  See debug/1 for the arguments.


lliisstt__ddeebbuugg__ttooppiiccss
    List   the  current  status  of   registered  topics.     See   also
    debugging/0.


aasssseerrttiioonn((_:_G_o_a_l))
    This  predicate  is   to  be  compared  to  the  C-library  assert()
    function.   By inserting this goal you explicitely state  you expect
    _G_o_a_l  to succeed at  this place.   As assertion/1 calls are  removed
    when compiling in  optimized mode _G_o_a_l should not have side-effects.
    Typical  examples are type-tests  and validating invariants  defined
    by your application.

    If   _G_o_a_l  fails  the  system  prints  a  message,  followed   by  a
    stack-trace and starts the debugger.

    In  older  versions  of  this  library  this  predicate  was  called
    assume/1.    Code using  assume/1 is  automatically converted  while
    printing a warning on the first occurrence.


1111..99 readutil::  RReeaaddiinngg lliinneess,, ssttrreeaammss aanndd ffiilleess

This library contains  primitives to read lines, files,  multiple terms,
etc.   The package clib provides  a shared object (DLL) named  readutil.
If the  library can locate  this shared object it  will use the  foreign
implementation for  reading character codes.   Otherwise  it will use  a
Prolog implementation.    Distributed applications should  make sure  to
deliver the  readutil shared object if  performance of these  predicates
is critical.


rreeaadd__lliinnee__ttoo__ccooddeess((_+_S_t_r_e_a_m_, _-_C_o_d_e_s))
    Read  the  next line  of  input from  _S_t_r_e_a_m  and unify  the  result
    with  _C_o_d_e_s _a_f_t_e_r the  line has  been read.   A line  is ended by  a
    newline character or end-of-file.  Unlike  read_line_to_codes/3, this
    predicate removes trailing newline character.

    On  end-of-file  the  atom  end_of_file  is  returned.     See  also
    at_end_of_stream/[0,1].


rreeaadd__lliinnee__ttoo__ccooddeess((_+_S_t_r_e_a_m_, _-_C_o_d_e_s_, _?_T_a_i_l))
    Diference-list version to  read an input line to a list of character
    codes.   Reading stops at the newline or end-of-file  character, but
    unlike  read_line_to_codes/2, the newline is retained in  the output.
    This  predicate is especially  useful for readine  a block of  lines
    upto  some delimiter.   The following  example reads an HTTP  header
    ended by a blank line:

    read_header_data(Stream, Header) :-
            read_line_to_codes(Stream, Header, Tail),
            read_header_data(Header, Stream, Tail).

    read_header_data("\r\n", _, _) :- !.
    read_header_data("\n", _, _) :- !.
    read_header_data("", _, _) :- !.
    read_header_data(_, Stream, Tail) :-
            read_line_to_codes(Stream, Tail, NewTail),
            read_header_data(Tail, Stream, NewTail).


rreeaadd__ssttrreeaamm__ttoo__ccooddeess((_+_S_t_r_e_a_m_, _-_C_o_d_e_s))
    Read all input until end-of-file and unify the result to _C_o_d_e_s.


rreeaadd__ssttrreeaamm__ttoo__ccooddeess((_+_S_t_r_e_a_m_, _-_C_o_d_e_s_, _?_T_a_i_l))
    Difference-list version of read_stream_to_codes/2.


rreeaadd__ffiillee__ttoo__ccooddeess((_+_S_p_e_c_, _-_C_o_d_e_s_, _+_O_p_t_i_o_n_s))
    Read  a  file to  a  list of  character  codes.    _S_p_e_c is  a  file-
    specification  for  absolute_file_name/3.    _C_o_d_e_s is  the  resulting
    code-list.   _O_p_t_i_o_n_s  is a list of  options for absolute_file_name/3
    and open/4.   In addition, the option tail(_T_a_i_l) is defined, forming
    a difference-list.


rreeaadd__ffiillee__ttoo__tteerrmmss((_+_S_p_e_c_, _-_T_e_r_m_s_, _+_O_p_t_i_o_n_s))
    Read  a  file  to  a list  of  prolog  terms  (see read/1).     _S_p_e_c
    is  a  file-specification for  absolute_file_name/3.    _T_e_r_m_s is  the
    resulting  list of  Prolog  terms.   _O_p_t_i_o_n_s  is a  list of  options
    for  absolute_file_name/3 and  open/4.    In  addition,  the  option
    tail(_T_a_i_l) is defined, forming a difference-list.


1111..1100 netscape::  AAccttiivvaattiinngg yyoouurr WWeebb--bbrroowwsseerr

This library deals with the very system dependent task  of opening a web
page in a browser.  See also url and the HTTP package.


wwwwww__ooppeenn__uurrll((_+_U_R_L))
    Open  _U_R_L in  an external  web-browser.   The reason  to place  this
    in  the library  is  to centralise  the maintenance  on this  highly
    platform  and browser specific task.   It distinguishes between  the
    following cases:

      o  _M_S_-_W_i_n_d_o_w_s
         If it detects MS-Windows  it uses win_shell/2 to open the  _U_R_L.
         The behaviour  and browser  started depends on  the Window  and
         Windows-shell configuration,  but in general  it should be  the
         behaviour expected by the user.

      o  _O_t_h_e_r _p_l_a_t_f_o_r_m_s
         On  other platforms  it  tests  the environment  variable  (see
         getenv/2) named BROWSER  or uses netscape  if this variable  is
         not  set.    If the  browser  is  either mozilla  or  netscape,
         www_open_url/1 first tries to  open a new  window on a  running
         using the  -remote option of netscape.   If  this fails or  the
         browser is  not mozilla  or netscape the  system simply  passes
         the URL as first argument to the program.


1111..1111 registry::  MMaanniippuullaattiinngg tthhee WWiinnddoowwss rreeggiissttrryy

The registry is only available on the MS-Windows  version of SWI-Prolog.
It loads  the foreign extension  plregtry.dll, providing the  predicates
described below.   This  library only makes  the most common  operations
on the registry available  through the Prolog user.  The  underlying DLL
provides a  more complete coverage of  the Windows registry API.  Please
consult  the  sources  in  pl/src/win32/foreign/plregtry.c  for  further
details.

In  all these  predicates,  _P_a_t_h refers  to a  `/' separated  path  into
the registry.   This  is _n_o_t an atom  containing `/'-characters as  used
for  filenames, but  a term  using the  functor  //2.   Windows  defines
the  following  roots for  the  registry:   classes_root,  current_user,
local_machine and users


rreeggiissttrryy__ggeett__kkeeyy((_+_P_a_t_h_, _-_V_a_l_u_e))
    Get  the principal (default)  value associated to  this key.   Fails
    silently of the key does not exist.


rreeggiissttrryy__ggeett__kkeeyy((_+_P_a_t_h_, _+_N_a_m_e_, _-_V_a_l_u_e))
    Get a named value associated to this key.


rreeggiissttrryy__sseett__kkeeyy((_+_P_a_t_h_, _+_V_a_l_u_e))
    Set the principal (default)  value of this key.  Creates (a path to)
    the key if this does not already exist.


rreeggiissttrryy__sseett__kkeeyy((_+_P_a_t_h_, _+_N_a_m_e_, _+_V_a_l_u_e))
    Associated  a named value to this key.  Creates (a path  to) the key
    if this does not already exist.


rreeggiissttrryy__ddeelleettee__kkeeyy((_+_P_a_t_h))
    Delete the indicated key.


sshheellll__rreeggiisstteerr__ffiillee__ttyyppee((_+_E_x_t_, _+_T_y_p_e_, _+_N_a_m_e_, _+_O_p_e_n_A_c_t_i_o_n))
    Register  a file-type.   _E_x_t is  the extension to  associate.   _T_y_p_e
    is  the type name,  often something link prolog.type.   _N_a_m_e is  the
    name visible in  the Windows file-type browser.  Finally, _O_p_e_n_A_c_t_i_o_n
    defines  the action to  execute when a  file with this extension  is
    opened in the Windows explorer.


sshheellll__rreeggiisstteerr__ddddee((_+_T_y_p_e_, _+_A_c_t_i_o_n_, _+_S_e_r_v_i_c_e_, _+_T_o_p_i_c_, _+_C_o_m_m_a_n_d_, _+_I_f_N_o_t_R_u_n_n_i_n_g))

    Associate   DDE   actions  to   a  type.       _T_y_p_e   is  the   same
    type  as used  for the  2nd  argument of  shell_register_file_type/4,
    _A_c_t_i_o_n  is the a  action to perform,  _S_e_r_v_i_c_e and _T_o_p_i_c specify  the
    DDE  topic to address and _C_o_m_m_a_n_d is the command to execute  on this
    topic.   Finally, _I_f_N_o_t_R_u_n_n_i_n_g defines the command to execute if the
    required DDE server is not present.


sshheellll__rreeggiisstteerr__pprroolloogg((_+_E_x_t))
    Default  registration of  SWI-Prolog, which  is invoked  as part  of
    the  initialisation process on Windows systems.  As the  source also
    explains the above predicates, it is given as an example:

    shell_register_prolog(Ext) :-
            current_prolog_flag(argv, [Me|_]),
            concat_atom(['"', Me, '" "%1"'], OpenCommand),
            shell_register_file_type(Ext, 'prolog.type', 'Prolog Source',
                                     OpenCommand),
            shell_register_dde('prolog.type', consult,
                               prolog, control, 'consult(''%1'')', Me),
            shell_register_dde('prolog.type', edit,
                               prolog, control, 'edit(''%1'')', Me).


1111..1122 url::  AAnnaallyyssiinngg aanndd ccoonnssttrruuccttiinngg UURRLL

This  library  deals  with the  analysis  and  construction  of  a  URL,
UUniversal  RResource  LLocator.    URL  is  the  basis  for  communicating
locations of resources (data) on the web.  A URL  consists of a protocol
identifier  (e.g. HTTP,  FTP), and  a  protocol-specific syntax  further
defining the location.  URLs are standardized in RFC-1738.

The implementation in  this library covers only  a small portion of  the
defined protocols.  Though the initial  implementation followed RFC-1738
strictly, the current  is more relaxed to deal with  frequent violations
of the standard encountered in practical use.

This  library contains  code by  Jan Wielemaker  who  wrote the  initial
version and Lukas Faulstich who added various extensions.


ppaarrssee__uurrll((_?_U_R_L_, _?_P_a_r_t_s))
    Construct  or analyse  a _U_R_L.  _U_R_L is  an atom  holding a  URL or  a
    variable.  _P_a_r_t_s  is a list of components.  Each component is of the
    format Name(_V_a_l_u_e).  Defined components are:

    pprroottooccooll((_P_r_o_t_o_c_o_l))
         The used  protocol.    This is,  after  the optional  url:,  an
         identifier  separated  from the  remainder  of  the  URL  using
         :.   parse_url/2 assumes the  http protocol  if no protocol  is
         specified and  the  URL can  be  parsed as  a valid  HTTP  url.
         In addition  to  the RFC-1738  specified protocols,  the  file:
         protocol is supported as well.

    hhoosstt((_H_o_s_t))
         Host-name  or IP-address  on  which  the resource  is  located.
         Supported by all network-based protocols.

    ppoorrtt((_P_o_r_t))
         Integer port-number to access  on the _H_o_s_t.  This  only appears
         if  the port  is  explicitly  specified in  the  URL.  Implicit
         default  ports  (e.g.  80  for  HTTP)  do  _n_o_t  appear  in  the
         part-list.

    ppaatthh((_P_a_t_h))
         (File-) path addressed  by the URL.  This is supported for  the
         ftp, http and file protocols.  If no path  appears, the library
         generates the path /.

    sseeaarrcchh((_L_i_s_t_O_f_N_a_m_e_V_a_l_u_e))
         Search-specification of HTTP  URL. This is  the part after  the
         ?, normally used to transfer data from HTML forms  that use the
         `GET' protocol.   In the URL it consists of  a www-form-encoded
         list of _N_a_m_e=_V_a_l_u_e pairs.   This is mapped to a list  of Prolog
         _N_a_m_e=_V_a_l_u_e terms with decoded names and values.

    ffrraaggmmeenntt((_F_r_a_g_m_e_n_t))
         Fragment specification of HTTP URL. This is the  part after the
         # character.

    The example below illustrates the all this for an HTTP UTL.

    ?- parse_url('http://swi.psy.uva.nl/message.cgi?msg=Hello+World%21#x',
                 P).
    P = [ protocol(http),
          host('swi.psy.uva.nl'),
          fragment(x),
          search([ msg = 'Hello World!'
                 ]),
          path('/message.cgi')
        ].

    By  instantiating  the  parts-list this  predicate  can be  used  to
    create a URL.


ppaarrssee__uurrll((_?_U_R_L_, _+_B_a_s_e_U_R_L_, _?_P_a_r_t_s))
    Same  as parse_url/2,  but dealing  a url  that is  relative to  the
    given  _B_a_s_e_U_R_L. This is used to analyse or construct a URI  found in
    the document behind _B_a_s_e_U_R_L.


gglloobbaall__uurrll((_+_U_R_L_, _+_B_a_s_e_U_R_L_, _-_A_b_s_o_l_u_t_e_U_r_l))
    Transform a (possibly) relative URL into a global one.


hhttttpp__llooccaattiioonn((_?_P_a_r_t_s_, _?_L_o_c_a_t_i_o_n))
    Similar to  parse_url/2, but only deals with the  location part of an
    HTTP  URL. That is,  the path, search and  fragment specifiers.   In
    the HTTP protocol, the first line of a message is

         _A_c_t_i_o_n _L_o_c_a_t_i_o_n [HTTP/_H_t_t_p_V_e_r_s_i_o_n]

    _L_o_c_a_t_i_o_n is either an atom or a code-list.


wwwwww__ffoorrmm__eennccooddee((_?_V_a_l_u_e_, _?_W_w_w_F_o_r_m_E_n_c_o_d_e_d))
    Translate  between   a  string-literal  and  the  x-www-form-encoded
    representation  used in path and  search specifications of the  HTTP
    protocol.

    Encoding  implies  mapping space  to +,  preserving  alpha-numercial
    characters,  map newlines to %0D%0A  and anything else to %XX.  When
    decoding, newlines appear as a single newline (10) character.


1111..1133 clp/bounds::  IInntteeggeerr BBoouunnddss CCoonnssttrraaiinntt SSoollvveerr

                                     Author:  _T_o_m _S_c_h_r_i_j_v_e_r_s, K.U.Leuven

The  bounds  solver  is  a  rather  simple  integer  constraint  solver,
implemented with attributed  variables.  Its  syntax is a subset of  the
SICStus clp(FD) syntax.

The bounds library is  _n_o_t an _a_u_t_o_l_o_a_d library and therefore it  must be
loaded explicitly before using it via:

:- use_module(library(bounds)).


1111..1133..11 CCoonnssttrraaiinnttss

The following constraints are supported:


_-_V_a_r iinn _+_R_a_n_g_e
    Varibale  _V_a_r  is restricted  to be  in range  _R_a_n_g_e.    A range  is
    denoted by L..U where both _L and _U are integers.


_-_V_a_r_s iinn _+_R_a_n_g_e
    A list of variables _V_a_r_s are restriced to be in range _R_a_n_g_e.


ttuupplleess__iinn((_+_T_u_p_l_e_s_, _+_E_x_t_e_n_s_i_o_n))
    Where _T_u_p_l_e_s is  a list of tuples (lists) of variables and integers,
    each  of length N, and  _E_x_t_e_n_s_i_o_n is a list of  tuples of integers,
    each  of length N.   Each tuple  of _T_u_p_l_e_s is constrained  to be in
    the  relation defined  by _E_x_t_e_n_s_i_o_n.    See section  11.13.4 for  an
    example.


_?_E_x_p_r #> _?_E_x_p_r
    The  left-hand  expression is  constrained to  be  greater than  the
    right-hand expressions.


_?_E_x_p_r #< _?_E_x_p_r
    The  left-hand  expression is  constrained to  be  smaller than  the
    right-hand expressions.


_?_E_x_p_r #>= _?_E_x_p_r
    The left-hand expression  is constrained to be greater than or equal
    to the right-hand expressions.


_?_E_x_p_r #=< _?_E_x_p_r
    The left-hand expression  is constrained to be smaller than or equal
    to the right-hand expressions.


_?_E_x_p_r #= _?_E_x_p_r
    The  left-hand  expression   is  constrained  to  be  equal  to  the
    right-hand expressions.


_?_E_x_p_r #\= _?_E_x_p_r
    The  left-hand expression  is  constrained to  be not  equal to  the
    right-hand expressions.


ssuumm((_+_V_a_r_s_,_+_O_p_,_?_V_a_l_u_e))
    Here  _V_a_r_s is a  list of variables  and integers, _O_p  is one of  the
    binary constraint relation  symbols above and _V_a_l_u_e is an integer or
    variable.  It represents the constraint (SUM_V_a_r_s) _O_p _V_a_l_u_e.


lleexx__cchhaaiinn((_+_V_a_r_s_L_i_s_t_s))
    The  constraint enforces lexicographic ordering on the lists  in the
    argument.   The argument _V_a_r_s_L_i_s_t_s  is a list of lists of  variables
    and integers.   The current implementation was contributed by Markus
    Triska.


aallll__ddiiffffeerreenntt((_+_V_a_r_s))
    Constrains all variabls in the list _V_a_r_s to be pairwise not equal.


iinnddoommaaiinn((_+_V_a_r))
    Unify variable _V_a_r with  a value in its domain.  Backtracks over all
    possible  values from  lowest to  greatest.   Contributed by  Markus
    Triska.


llaabbeell((_+_V_a_r_s))
    Equivalent to labeling([], Vars).


llaabbeelliinngg((_+_O_p_t_i_o_n_s_, _+_V_a_r_s))
    All  variables  in the  list  _V_a_r_s are  assigned values  from  their
    respective  domains such  that no  constraint on  them is  violated.
    _O_p_t_i_o_n_s  is a list of options.   One set of options lets  you choose
    the variable selection strategy:

    lleeffttmmoosstt
         Label the variables in  the order they occur in _V_a_r_s  from left
         to right.  This is the default.

    ffff
         Label the  variable whose domain  contains the smallest  number
         of  elements  among  the  remaining  variables  next.      This
         is  called  "first-fail".    The  intention  here  is  twofold:
         First,  to detect  infeasibility  early by  always  considering
         the variable  most likely  to cause  failure;  second, to  give
         priority to variables that  are close to running out  of domain
         elements.

    mmiinn
         Label the variable  whose lower bound  is the lowest among  the
         remaining variables next.

    mmaaxx
         Label the variable whose  upper bound is the highest  among the
         remaining variables next.

    Another set of options lets you search for extrema:

    mmiinn((_E_x_p_r))

    mmaaxx((_E_x_p_r))
         Label  the  variables   such  that  _E_x_p_r  assumes  the   small-
         est/highest possible value.


sseerriiaalliizzeedd((_+_S_t_a_r_t_s_,_+_D_u_r_a_t_i_o_n_s))
    _S_t_a_r_t_s = [S1; :::;Sn] is a list of variables or integers, and _D_u_r_a_t_i_o_n_s
    =  [D1;:::; Dn]  is  a  list  of non-negative  integers.     _S_t_a_r_t_s are
    constrained  to denote the starting times of  non-overlapping tasks,
    i.e., Si+ Di=<Sj or Sj+ Dj=<Si for all 1=< i< j=< n.

Here _E_x_p_r can be one of

iinntteeggeerr  Any integer.

vvaarriiaabbllee  A variable.


_?_E_x_p_r + _?_E_x_p_r
    The sum of two expressions.


_?_E_x_p_r * _?_E_x_p_r
    The product of two expressions.


_?_E_x_p_r - _?_E_x_p_r
    The difference of two expressions.


mmaaxx((_?_E_x_p_r_,_?_E_x_p_r))
    The maximum of two expressions.


mmiinn((_?_E_x_p_r_,_?_E_x_p_r))
    The minimum of two expressions.


_?_E_x_p_r mmoodd _?_E_x_p_r
    The first expression modulo the second expression.


aabbss((_?_E_x_p_r))
    The absolute value of an expression.


1111..1133..22 CCoonnssttrraaiinntt IImmpplliiccaattiioonn aanndd RReeiiffiieedd CCoonnssttrraaiinnttss

The following constraint implication predicates are available:


_+_P #=> _+_Q
    _P implies _Q, where _P and _Q are reifyable constraints.


_+_Q #<= _+_P
    _P implies _Q, where _P and _Q are reifyable constraints.


  _+_P #<=> _+_Q
      _P and _Q are equivalent, where _P and _Q are reifyable constraints.

  In addition,  instead of  being a  reifyable constraint, either  _P or  _Q
  can be a boolean  variable that is the truth value of  the corresponding
  constraint.

  The  following constraints  are reifyable:   #=/2,  #\=/2,  #</2,  #>/2,
  #=</2, #>/2.

  For example,  to count the number of  occurrences of a particular  value
  in a list of constraint variables:

o _U_s_i_n_g _c_o_n_s_t_r_a_i_n_t _i_m_p_l_i_c_a_t_i_o_n
      occurrences(List,Value,Count) :-
              occurrences(List,Value,0,Count).

      occurrences([],_,Count,Count).
      occurrences([X|Xs],Value,Acc,Count) :-
              X #= Value  #=> NAcc #= Acc + 1,
              X #\= Value #=> NAcc #= Acc,
              occurrences(Xs,Value,NAcc,Count).

o _U_s_i_n_g _r_e_i_f_i_e_d _c_o_n_s_t_r_a_i_n_t_s
      occurrences(List,Value,Count) :-
              occurrences(List,Value,0,Count).

      occurrences([],_,Count,Count).
      occurrences([X|Xs],Value,Acc,Count) :-
              X #= Value #=> B,
              NAcc #= Acc + B,
              occurrences(Xs,Value,NAcc,Count).


1111..1133..33 EExxaammppllee 11::  SSeenndd++MMoorree==MMoonneeyy

The following  is an  implementation of the  classic alphametics  puzzle
SEND + MORE = MONEY:

:- use_module(library(bounds)).

send([[S,E,N,D], [M,O,R,E], [M,O,N,E,Y]])  :-
              Digits   =  [S,E,N,D,M,O,R,Y],
              Carries  =  [C1,C2,C3,C4],
              Digits  in  0..9,
              Carries in  0..1,

              M                #=              C4,
              O  +  10  *  C4  #=  M  +  S  +  C3,
              N  +  10  *  C3  #=  O  +  E  +  C2,
              E  +  10  *  C2  #=  R  +  N  +  C1,
              Y  +  10  *  C1  #=  E  +  D,

              M  #>=  1,
              S  #>=  1,
              all_different(Digits),
              label(Digits).


1111..1133..44 EExxaammppllee 22::  UUssiinngg ttuupplleess__iinn ffoorr aa ttrraaiinn sscchheedduullee

This example demonstrates tuples_in/2.  A train schedule  is represented
as a list  Ts of quadruples, denoting  departure and arrival places  and
times for each  train.  The _p_a_t_h_/_3  predicate given below constrains Ps
to a  feasible journey from A  to D via 3  trains that are part  of the
given schedule.

:- use_module(library(bounds)).

schedule(Ts) :-
        Ts = [[1,2,0,1],[2,3,4,5],[2,3,0,1],[3,4,5,6],[3,4,2,3],[3,4,8,9]].

path(A, D, Ps) :-
        schedule(Ts),
        Ps = [[A,B,_T0,T1],[B,C,T2,T3],[C,D,T4,_T5]],
        tuples_in(Ps, Ts),
        T2 #> T1,
        T4 #> T3.

An example query:

?- path(1, 4, Ps), flatten(Ps, Vars), label(Vars).

Ps = [[1, 2, 0, 1], [2, 3, 4, 5], [3, 4, 8, 9]]


1111..1133..55 SSIICCSSttuuss ccllpp((FFDD)) ccoommppaattiibbiilliittyy

Apart  from  the  limited syntax,  the  bounds  solver  differs  in  the
following ways from the SICStus clp(FD) solver:

  o inf _a_n_d sup
    The  smallest  lowerbound  and  greatest upperbound  in  bounds  are
    max_integer and min_integer + 1.


1111..1144 clpqr::  CCoonnssttrraaiinntt LLooggiicc PPrrooggrraammmmiinngg oovveerr RRaattiioonnaallss aanndd RReeaallss

    Author:  _L_e_s_l_i_e _D_e _K_o_n_i_n_c_k, K.U. Leuven

This CLP(Q,R) system is a port of the CLP(Q,R)  system of Sicstus Prolog
by Christian  Holzbaur:   Holzbaur C.:   OFAI  clp(q,r) Manual,  Edition
1.3.3, Austrian Research Institute for Artificial  Intelligence, Vienna,
TR-95-09,  1995.   This manual  is roughly  based on the  manual of  the
above mentioned CLP(Q,R) implementation.

The CLP(Q,R) system consists of two components:  the  CLP(Q) library for
handling constraints  over the rational numbers  and the CLP(R)  library
for handling  constraints over  the real numbers  (using floating  point
numbers as  representation).  Both  libraries offer the same  predicates
(with exception  of bb_inf/4 in  CLP(Q) and bb_inf/5  in CLP(R)). It  is
allowed to use both libraries in one program, but  using both CLP(Q) and
CLP(R) constraints on the same variable will result in an exception.

Please  note that  the clpqr  library  is _n_o_t  an _a_u_t_o_l_o_a_d  library  and
therefore this library must be loaded explicitely before using it:

:- use_module(library(clpq)).

or

:- use_module(library(clpr)).


1111..1144..11 SSoollvveerr pprreeddiiccaatteess

The following predicates are provided to work with constraints:


{}((_+_C_o_n_s_t_r_a_i_n_t_s))
    Adds the constraints given by _C_o_n_s_t_r_a_i_n_t_s to the constraint store.


eennttaaiilleedd((_+_C_o_n_s_t_r_a_i_n_t))
    Succeeds  if  _C_o_n_s_t_r_a_i_n_t  is  necessarily true  within  the  current
    constraint  store.    This means  that adding  the  negation of  the
    constraint to the store results in failure.


iinnff((_+_E_x_p_r_e_s_s_i_o_n_, _-_I_n_f))
    Computes  the infimum of _E_x_p_r_e_s_s_i_o_n within the current state  of the
    constraint  store and returns that infimum  in _I_n_f.  This  predicate
    does not change the constraint store.


ssuupp((_+_E_x_p_r_e_s_s_i_o_n_, _-_S_u_p))
    Computes the supremum  of _E_x_p_r_e_s_s_i_o_n within the current state of the
    constraint  store and returns that supremum in _S_u_p.   This predicate
    does not change the constraint store.


mmiinniimmiizzee((_+_E_x_p_r_e_s_s_i_o_n))
    Minimizes  _E_x_p_r_e_s_s_i_o_n within the current constraint store.   This is
    the  same as computing  the infimum and  equation the expression  to
    that infimum.


mmaaxxiimmiizzee((_+_E_x_p_r_e_s_s_i_o_n))
    Maximizes  _E_x_p_r_e_s_s_i_o_n within the current constraint store.   This is
    the  same as computing the  supremum and equating the expression  to
    that supremum.


bbbb__iinnff((_+_I_n_t_s_, _+_E_x_p_r_e_s_s_i_o_n_, _-_I_n_f_, _-_V_e_r_t_e_x_, _+_E_p_s))
    This  predicate  is  offered  in  CLP(R) only.     It  computes  the
    infimum of _E_x_p_r_e_s_s_i_o_n  within the current constraint store, with the
    additional  constraint that in that  infimum, all variables in  _I_n_t_s
    have  integral values.   _V_e_r_t_e_x will contain  the values of _I_n_t_s  in
    the  infimum.   _E_p_s  denotes how  much a  value may  differ from  an
    integer to be considered  an integer.  E.g. when _E_p_s = 0.001, then X
    =  4.999 will be  considered as an  integer (5 in this  case).   _E_p_s
    should be between 0 and 0.5.


bbbb__iinnff((_+_I_n_t_s_, _+_E_x_p_r_e_s_s_i_o_n_, _-_I_n_f_, _-_V_e_r_t_e_x))
    This  predicate is offered in CLP(Q) only.   It behaves the  same as
    bb_inf/5 but does not use an error margin.


bbbb__iinnff((_+_i_n_t_s_, _+_E_x_p_r_e_s_s_i_o_n_, _-_I_n_f))
    The  same as bb_inf/5 or  bb_inf/4 but without returning the  values
    of the integers.  In CLP(R), an error margin of 0.001 is used.


dduummpp((_+_T_a_r_g_e_t_, _+_N_e_w_v_a_r_s_, _-_C_o_d_e_d_A_n_s_w_e_r))
    Returns the constraints  on _T_a_r_g_e_t in the list _C_o_d_e_d_A_n_s_w_e_r where all
    variables  of _T_a_r_g_e_t have veen replaced by _N_e_w_V_a_r_s.   This operation
    does not change the constraint store.  E.g. in

    dump([X,Y,Z],[x,y,z],Cons)

    Cons  will  contain  the constraints  on  X,  Y and  Z  where  these
    variables have been replaced by atoms x, y and z.


1111..1144..22 SSyynnttaaxx ooff tthhee pprreeddiiccaattee aarrgguummeennttss

The arguments  of the  predicates defined  in the  subsection above  are
defined in table 11.1.  Failing to meet the syntax  rules will result in
an exception.
__________________________________________________________________________________________
| <_C_o_n_s_t_r_a_i_n_t_s>::=  <_C_o_n_s_t_r_a_i_n_t>                    |single constraint                   |
|                 | <_C_o_n_s_t_r_a_i_n_t> , <_C_o_n_s_t_r_a_i_n_t_s>    |conjunction                         |
|                 | <_C_o_n_s_t_r_a_i_n_t> ; <_C_o_n_s_t_r_a_i_n_t_s>    |disjunction                         |
| <_C_o_n_s_t_r_a_i_n_t> ::=  <_E_x_p_r_e_s_s_i_o_n> <  <_E_x_p_r_e_s_s_i_o_n>    |less than                           |

|                 | <_E_x_p_r_e_s_s_i_o_n> >  <_E_x_p_r_e_s_s_i_o_n>    |greater than                        |
|                 | <_E_x_p_r_e_s_s_i_o_n> =<  <_E_x_p_r_e_s_s_i_o_n>   |less or equal                       |
|                 | <=(<_E_x_p_r_e_s_s_i_o_n>, <_E_x_p_r_e_s_s_i_o_n>)  |less or equal                       |
|                 | <_E_x_p_r_e_s_s_i_o_n> >= <_E_x_p_r_e_s_s_i_o_n>    |greater or equal                    |
|                 | <_E_x_p_r_e_s_s_i_o_n> =\= <_E_x_p_r_e_s_s_i_o_n>   |not equal                           |
|                 | <_E_x_p_r_e_s_s_i_o_n> =:= <_E_x_p_r_e_s_s_i_o_n>   |equal                               |
|                 | <_E_x_p_r_e_s_s_i_o_n> = <_E_x_p_r_e_s_s_i_o_n>     |equal                               |
| <_E_x_p_r_e_s_s_i_o_n> ::=  <_V_a_r_i_a_b_l_e>                      |Prolog variable                     |

|                 | <_N_u_m_b_e_r>                        |Prolog number (float, integer)      |
|                 | +<_E_x_p_r_e_s_s_i_o_n>                   |unary plus                          |
|                 | -<_E_x_p_r_e_s_s_i_o_n>                   |unary minus                         |
|                 | <_E_x_p_r_e_s_s_i_o_n> + <_E_x_p_r_e_s_s_i_o_n>     |addition                            |
|                 | <_E_x_p_r_e_s_s_i_o_n> - <_E_x_p_r_e_s_s_i_o_n>     |substraction                        |
|                 | <_E_x_p_r_e_s_s_i_o_n> * <_E_x_p_r_e_s_s_i_o_n>     |multiplication                      |
|                 | <_E_x_p_r_e_s_s_i_o_n> / <_E_x_p_r_e_s_s_i_o_n>     |division                            |

|                 | abs(<_E_x_p_r_e_s_s_i_o_n>)               |absolute value                      |
|                 | sin(<_E_x_p_r_e_s_s_i_o_n>)               |sine                                |
|                 | cos(<_E_x_p_r_e_s_s_i_o_n>)               |cosine                              |
|                 | tan(<_E_x_p_r_e_s_s_i_o_n>)               |tangent                             |
|                 | exp(<_E_x_p_r_e_s_s_i_o_n>)               |exponent                            |
|                 | pow(<_E_x_p_r_e_s_s_i_o_n>)               |exponent                            |
|                 | <_E_x_p_r_e_s_s_i_o_n> ^ <_E_x_p_r_e_s_s_i_o_n>     |exponent                            |
|                 | min(<_E_x_p_r_e_s_s_i_o_n>, <_E_x_p_r_e_s_s_i_o_n>) |minimum                             |

|_________________|_max(<_E_x_p_r_e_s_s_i_o_n>,_<_E_x_p_r_e_s_s_i_o_n>)_|maximum_____________________________|_
                  Table 11.1:  CLP(Q,R) constraint BNF


  1111..1144..33 UUssee ooff uunniiffiiccaattiioonn

  Instead  of using  the {}/1  predicate, you  can also  use the  standard
  unification mechanism to store constraints.  The  following code samples
  are equivalent:

o _U_n_i_f_i_c_a_t_i_o_n _w_i_t_h _a _v_a_r_i_a_b_l_e
      {X =:= Y}
      {X = Y}
      X = Y

o _U_n_i_f_i_c_a_t_i_o_n _w_i_t_h _a _n_u_m_b_e_r
      {X =:= 5.0}
      {X = 5.0}
      X = 5.0


1111..1144..44 NNoonn--lliinneeaarr ccoonnssttrraaiinnttss

The CLP(Q,R)  system deals only  passively with non-linear  constraints.
They remain in  a passive state until certain conditions  are satisfied.
These conditions,  which are called the  isolation axioms, are given  in
table 11.2.
     ______________________________________________________________
     | A =B *C     |B or C is ground       |A = 5 *  C or A = B * |
     |             |                       |4                     |
     |              |A  and  (B or  C)  are|20 = 5  * C or 20 = B |
     |______________|ground________________|*_4___________________|_
     | A =B=C      |C is ground            |A = B / 3             |
     |______________|A_and_B_are_ground____|4_=_12_/_C____________|_

     | X =min(Y; Z) |Y and Z are ground    |X = min(4,3)          |
     | X =max(Y; Z) |Y and Z are ground    |X = max(4,3)          |
     |_X_=abs(Y_)___|Y_is_ground___________|X_=_abs(-7)___________|_
     | X =pow(Y; Z) |X and Y are ground    |8 = 2 ^ Z             |
     | X =exp(Y; Z) |X and Z are ground    |8 = Y ^ 3             |
     |_X_=Y__^_Z___|Y_and_Z_are_ground_____|X_=_2_^_3_____________|_
     | X =sin(Y )   X|is ground            |1 = sin(Y)            |

     | X =cos(Y )   Y|is ground            |X = sin(1.5707)       |
     |_X_=tan(Y_)___|______________________|______________________|_
                 Table 11.2:  CLP(Q,R) isolating axioms


1111..1155 clp/clp_distinct::  WWeeaakk aarrcc ccoonnssiisstteenntt ``aallll__ddiissttiinncctt'' ccoonnssttrraaiinntt

                                                  Author:  _M_a_r_k_u_s _T_r_i_s_k_a

The clp/clp_distinct module provides the following constraints:


aallll__ddiissttiinncctt((_+_V_a_r_s))
    The  variables  in _V_a_r_s  are constrained  to  be pairwise  distinct.
    All  variables must  already be assigned  domains (via  vars_in/2 or
    vars_in/3) when this constraint is posted.


vvaarrss__iinn((_+_V_a_r_s_, _+_D_o_m_a_i_n))
    Where  _V_a_r_s  is  a  list  of  variables and  _D_o_m_a_i_n  is  a  list  of
    non-negative  integers.  Each variable of _V_a_r_s is constrained  to be
    in _D_o_m_a_i_n.


vvaarrss__iinn((_+_V_a_r_s_, _+_F_r_o_m_, _+_T_o))
    Where  _V_a_r_s  is  a list  of  variables,  and 0<= From<= To.    Each
    variable  in _V_a_r_s  is  constrained to  be in  the discrete  interval
    [From,To].


1111..1155..11 EExxaammppllee 11

The all_distinct/1 constraint can detect various inconsistencies:

?- vars_in([X,Y,Z], [1,2]), all_distinct([X,Y,Z]).

No


1111..1155..22 EExxaammppllee 22

In this example, 3 is assigned to Z without labeling any variables:

?- vars_in([X,Y], [1,2]), vars_in([Z], [1,2,3]), all_distinct([X,Y,Z]).

X = _G180{1-2}
Y = _G183{1-2}
Z = 3 ;


1111..1155..33 EExxaammppllee 33

The clp_distinct module can be used  in conjunction with clp/bounds.  All
relevant variables must still be assigned domains via one of the _v_a_r_s___i_n
predicates before all_distinct/1 can be posted:

:- use_module(library(bounds)).
:- use_module(library(clp_distinct)).

?- [X,Y] in 1..2, vars_in([X,Y], [1,2]), all_distinct([X,Y]), label([X,Y]).

X = 1
Y = 2 ;

X = 2
Y = 1 ;


1111..1166 simplex::  SSoollvvee lliinneeaarr pprrooggrraammmmiinngg pprroobblleemmss

                                                  Author:  _M_a_r_k_u_s _T_r_i_s_k_a

A linear programming problem consists of a set  of (linear) constraints,
a number  of variables and  a linear  objective function.   The goal  is
to assign values  to the variables so  as to maximize (or minimize)  the
value of the objective function while satisfying all constraints.

Many optimization problems can be modeled in this way.   Consider having
a knapsack with fixed capacity C, and a number of  items with sizes s(i)
and values v(i).   The goal is to  put as many items as possible  in the
knapsack (not exceeding its capacity) while maximizing the  sum of their
values.

As another example,  suppose you are given  a set of coins with  certain
values, and you are to find the minimum number of  coins such that their
values  sum up  to a  fixed amount.    Instances of  these problems  are
solved below.

The simplex module provides the following predicates:


aassssiiggnnmmeenntt((_+_C_o_s_t_, _-_A_s_s_i_g_n_m_e_n_t))
    _C_o_s_t  is a  list of  lists representing the  quadratic cost  matrix,
    where  element  (i,j) denotes  the  cost of  assigning  entity i  to
    entity  j.  An assignment with minimal cost is  computed and unified
    with  _A_s_s_i_g_n_m_e_n_t  as a  list  of lists,  representing  an  adjacency
    matrix.


ccoonnssttrraaiinntt((_+_C_o_n_s_t_r_a_i_n_t_, _+_S_0_, _-_S))
    Adds  a  linear  or integrality  constraint  to the  linear  program
    corresponding  to state  _S_0.   A linear  constraint is  of the  form
    "Left  Op C", where "Left"  is a list of Coefficient*Variable  terms
    (variables  in  the  context of  linear  programs  can be  atoms  or
    compound terms) and C  is a non-negative numeric constant.  The list
    represents  the sum of its elements.   _O_p can be  =, =< or >=.   The
    coefficient  "1" can be  omitted.   An integrality constraint is  of
    the  form integral(Variable) and constrains Variable to  an integral
    value.


ccoonnssttrraaiinntt((_+_N_a_m_e_, _+_C_o_n_s_t_r_a_i_n_t_, _+_S_0_, _-_S))
    Like  constraint/3, and attaches the name _N_a_m_e (an atom  or compound
    term) to the new constraint.


ccoonnssttrraaiinntt__aadddd((_+_N_a_m_e_, _+_L_e_f_t_, _+_S_0_, _-_S))
    _L_e_f_t  is a list of Coefficient*Variable terms.  The terms  are added
    to  the left-hand side of the constraint  named _N_a_m_e.  _S  is unified
    with the resulting state.


ggeenn__ssttaattee((_-_S_t_a_t_e))
    Generates  an  initial   state  corresponding  to  an  empty  linear
    program.


mmaaxxiimmiizzee((_+_O_b_j_e_c_t_i_v_e_, _+_S_0_, _-_S))
    Maximizes   the   objective   function,   stated   as  a   list   of
    "Coefficient*Variable"   terms  that  represents  the  sum   of  its
    elements, with respect  to the linear program corresponding to state
    _S_0.   _S  is unified  with an internal  representation of the  solved
    instance.


mmiinniimmiizzee((_+_O_b_j_e_c_t_i_v_e_, _+_S_0_, _-_S))
    Analogous to maximize/3.


oobbjjeeccttiivvee((_+_S_t_a_t_e_, _-_O_b_j_e_c_t_i_v_e))
    Unifies  _O_b_j_e_c_t_i_v_e with the result of the objective function  at the
    obtained extremum.  _S_t_a_t_e must correspond to a solved instance.


sshhaaddooww__pprriiccee((_+_S_t_a_t_e_, _+_N_a_m_e_, _-_V_a_l_u_e))
    Unifies  _V_a_l_u_e with  the shadow  price corresponding  to the  linear
    constraint  whose name is _N_a_m_e.   _S_t_a_t_e must correspond to a  solved
    instance.


ttrraannssppoorrttaattiioonn((_+_S_u_p_p_l_i_e_s_, _+_D_e_m_a_n_d_s_, _+_C_o_s_t_s_, _-_T_r_a_n_s_p_o_r_t))
    _S_u_p_p_l_i_e_s  and _D_e_m_a_n_d_s  are both lists  of positive  numbers.   Their
    respective  sums  must  be  equal.     _C_o_s_t_s  is  a  list  of  lists
    representing the cost  matrix, where an entry (i,j) denotes the cost
    of transporting  one unit from i to j.  A transportation plan having
    minimum  cost is computed and unified with _T_r_a_n_s_p_o_r_t in the  form of
    a  list of lists  that represents  the transportation matrix,  where
    element (i,j) denotes how many units to ship from i to j.


vvaarriiaabbllee__vvaalluuee((_+_S_t_a_t_e_, _+_V_a_r_i_a_b_l_e_, _-_V_a_l_u_e))
    _V_a_l_u_e  is unified with the value obtained for _V_a_r_i_a_b_l_e.   _S_t_a_t_e must
    correspond to a solved instance.

All numeric  quantities are  converted to  rationals via  rationalize/1,
and rational arithmetic is used throughout solving linear programs.   In
the  current implementation,  all variables  are implicitly  constrained
to  be  non-negative.     This  may  change  in  future  versions,   and
non-negativity constraints should therefore be stated explicitly.


1111..1166..11 EExxaammppllee 11

This is  the "radiation  therapy" example, taken  from "Introduction  to
Operations Research" by Hillier and Lieberman.  DCG  notation is used to
implicitly thread the state through posting the constraints:

:- use_module(library(simplex)).

post_constraints -->
        constraint([0.3*x1, 0.1*x2] =< 2.7),
        constraint([0.5*x1, 0.5*x2] = 6),
        constraint([0.6*x1, 0.4*x2] >= 6),
        constraint([x1] >= 0),
        constraint([x2] >= 0).

radiation(S) :-
        gen_state(S0),
        post_constraints(S0, S1),
        minimize([0.4*x1, 0.5*x2], S1, S).

An example query:

?- radiation(S), variable_value(S, x1, Val1), variable_value(S, x2, Val2).

Val1 = 15 rdiv 2
Val2 = 9 rdiv 2 ;


1111..1166..22 EExxaammppllee 22

Here is  an instance of  the knapsack problem  described above, where  C
= 8,  and we have two types  of items:  One  item with value 7 and  size
6,  and 2  items each  having size  4 and  value 4.    We introduce  two
variables, x(1)  and x(2)  that denote how  many items  to take of  each
type.

knapsack_constrain(S) :-
        gen_state(S0),
        constraint([6*x(1), 4*x(2)] =< 8, S0, S1),
        constraint([x(1)] =< 1, S1, S2),
        constraint([x(2)] =< 2, S2, S).

knapsack(S) :-
        knapsack_constrain(S0),
        maximize([7*x(1), 4*x(2)], S0, S).

An example query yields:

?- knapsack(S), variable_value(S, x(1), X1), variable_value(S, x(2), X2).

X1 = 1
X2 = 1 rdiv 2 ;

That is, we are to take the one item of the first  type, and half of one
of the items of the  other type to maximize the total value of  items in
the knapsack.

If items can not be split, integrality constraints have to be imposed:

knapsack_integral(S) :-
        knapsack_constrain(S0),
        constraint(integral(x(1)), S0, S1),
        constraint(integral(x(2)), S1, S2),
        maximize([7*x(1), 4*x(2)], S2, S).

Now the result is different:

?- knapsack_integral(S), variable_value(S, x(1), X1), variable_value(S, x(2), X2).

X1 = 0
X2 = 2

That  is,  we are  to  take  only the  two  items of  the  second  type.
Notice in particular  that always choosing the remaining item  with best
performance (ratio  of value to  size) that still  fits in the  knapsack
does  not necessarily  yield  an optimal  solution  in the  presence  of
integrality constraints.


1111..1166..33 EExxaammppllee 33

We are given 3 coins  each worth 1, 20 coins each worth 5, and  10 coins
each worth 20 units of  money.  The task is to find a minimal  number of
these coins that amount  to 111 units of money.  We  introduce variables
c(1), c(5) and c(20)  denoting how many coins to take of  the respective
type:

coins -->
        constraint([c(1), 5*c(5), 20*c(20)] = 111),
        constraint([c(1)] =< 3),
        constraint([c(5)] =< 20),
        constraint([c(20)] =< 10),
        constraint([c(1)] >= 0),
        constraint([c(5)] >= 0),
        constraint([c(20)] >= 0),
        constraint(integral(c(1))),
        constraint(integral(c(5))),
        constraint(integral(c(20))),
        minimize([c(1), c(5), c(20)]).

coins(S) :-
        gen_state(S0),
        coins(S0, S).

An example query:

?- coins(S), variable_value(S, c(1), C1), variable_value(S, c(5), C5), variable_value(S, c(20), C20).

C1 = 1
C5 = 2
C20 = 5


1111..1177 prologxref::  CCrroossss--rreeffeerreennccee ddaattaa ccoolllleeccttiioonn lliibbrraarryy

This library collects information on defined and used  objects in Prolog
sourcefiles.  Typically these are predicates, but  we expect the library
to deal  with other  types of objects  in the  future.   The library  is
a building  block for tools doing  dependency tracking in  applications.
Dependency tracking  is useful  to reveal  the structure  of an  unknown
program  or detect  missing components  at  compile-time, but  also  for
program  transformation or  minimising  a  program saved-state  by  only
saving the reachable objects.

This section gives  a partial description of the library  API, providing
some insight  in how you  can use it  for analysing your  program.   The
library should be  further modularized, moving its knowledge  about -for
example- XPCE  into a different file  and allowing for adding  knowledge
about other  libraries such  as Logtalk.   PPlleeaassee  ddoo nnoott ccoonnssiiddeerr  tthhiiss
iinntteerrffaaccee rroocckk--ssoolliidd..

The  library is  exploited  by two  graphical  tools in  the  SWI-Prolog
environment.   The  XPCE frontend  started by gxref/0  and described  in
section 3.7 and  PceEmacs (section 3.4) which exploits this  library for
its syntax colouring.

For  all predicates  described  below,  _S_o_u_r_c_e  is the  source  that  is
processed.    This is  normally a  filename in  any notation  acceptable
to the  file loading  predicates (see  load_files/2).    Using the  hooks
defined  in  section  11.17.1  it can  be  anything  else  that  can  be
translated into a  Prolog stream holding Prolog  source text.   _C_a_l_l_a_b_l_e
is a callable  term (see callable/1).   Callables do not carry a  module
qualifier unless  the referred predicate  is not  in the module  defined
_S_o_u_r_c_e.


xxrreeff__ssoouurrccee((_+_S_o_u_r_c_e))
    Gather information on  _S_o_u_r_c_e.  If _S_o_u_r_c_e has already been processed
    and  is still up-to-date according to the file timestamp,  no action
    is  taken.     This  predicate  must be  called  on  a  file  before
    information can be gathered.


xxrreeff__ccuurrrreenntt__ssoouurrccee((_?_S_o_u_r_c_e))
    _S_o_u_r_c_e has been processed.


xxrreeff__cclleeaann((_+_S_o_u_r_c_e))
    Remove the information gathered for _S_o_u_r_c_e


xxrreeff__ddeeffiinneedd((_?_S_o_u_r_c_e_, _?_C_a_l_l_a_b_l_e_, _-_H_o_w))
    _C_a_l_l_a_b_l_e is defined in _S_o_u_r_c_e.  _H_o_w is one of

             dynamic(_L_i_n_e)       Declared dynamic at _L_i_n_e

             thread_local(_L_i_n_e)  Declared thread local at _L_i_n_e
             multifile(_L_i_n_e)     Declared multifile at _L_i_n_e
             local(_L_i_n_e)         First clause at _L_i_n_e
             foreign(_L_i_n_e)       Foreign library loaded at _L_i_n_e
             constraint(_L_i_n_e)    CHR Constraint at _L_i_n_e
             imported(_F_i_l_e)      Imported from _F_i_l_e


xxrreeff__ccaalllleedd((_?_S_o_u_r_c_e_, _?_C_a_l_l_a_b_l_e_, _?_B_y))
    _C_a_l_l_a_b_l_e is called in _S_o_u_r_c_e by _B_y.


xxrreeff__eexxppoorrtteedd((_?_S_o_u_r_c_e_, _?_C_a_l_l_a_b_l_e))
    _C_a_l_l_a_b_l_e is public (exported from the module).


xxrreeff__mmoodduullee((_?_S_o_u_r_c_e_, _?_M_o_d_u_l_e))
    _S_o_u_r_c_e is a module-file defining the given module.


xxrreeff__bbuuiilltt__iinn((_?_C_a_l_l_a_b_l_e))
    True  if  _C_a_l_l_a_b_l_e is  a  built-in predicate.    Currently  this  is
    assumed  for all predicates defined in the system module  and having
    the  property built_in.  Built-in  predicates are not registered  as
    `called'.


1111..1177..11 EExxtteennddiinngg tthhee lliibbrraarryy

The library provides hooks  for extending its rules it uses  for finding
predicates called by some programming construct.


pprroolloogg::ccaalllleedd__bbyy((_+_G_o_a_l_, _-_C_a_l_l_e_d))
    Where  _G_o_a_l  is   a  non-var  subgoal  appearing  in  called  object
    (typically a clause-body).   If it succeeds it must return a list of
    goals called by _G_o_a_l.  As a special construct, if a term Callable +N
    is  returned,  N variable  arguments are  added to  _C_a_l_l_a_b_l_e before
    further  processing.  For simple meta-calls a single  fact suffices.
    Complex  rules as  used in  the html_write library  provided by  the
    HTTP  package examine  the  arguments and  create a  list of  called
    objects.

    The  current  system   cannot  deal  with  the  same  name/arity  in
    different  modules that  behave differently with  respect to  called
    arguments.


1111..1188 broadcast::  BBrrooaaddccaasstt aanndd rreecceeiivvee eevveenntt nnoottiiffiiccaattiioonnss

The  broadcast  library   was  invented  to  realise  GUI   applications
consisting of  stand-alone components that use  the Prolog database  for
storing  the application  data.    Figure  ????  illustrates the  flow  of
information using this design

The  broadcasting service  provides two  services.    Using the  `shout'
service, an unknown number of agents may listen to  the message and act.
The broadcaster is not (directly) aware of the implications.   Using the
`request' service, listening  agents are asked for an  answer one-by-one
and the  broadcaster is allowed  to reject  answers using normal  Prolog
failure.

Shouting  is  often used  to  inform  about  changes made  to  a  common
database.  Other messages can be ``save yourself'' or ``show this''.

Requesting  is used  to get  information while  the  broadcaster is  not
aware who might  be able to answer the  question.  For example ``who  is
showing X?''.


bbrrooaaddccaasstt((_+_T_e_r_m))
    Broadcast  _T_e_r_m.   There are  no limitations to  _T_e_r_m, though  being
    a  global  service,  it  is  good  practice  to  use  a  descriptive
    and  unique principal  functor.   All associated  goals are  started
    and  regardless  of their  success  or failure,  broadcast/1  always
    succeeds.  Exceptions are passed.


bbrrooaaddccaasstt__rreeqquueesstt((_+_T_e_r_m))
    Unlike  broadcast/1,  this  predicate stops  if an  associated  goal
    succeeds.    Backtracking  causes it  to  try other  listeners.    A
    broadcast  request is used to fetch information without  knowing the
    identity  of the agent  providing it.   C.f. ``Is there someone  who
    knows the age of John?''  could be asked using

            ...,
            broadcast_request(age_of('John', Age)),

    If there is  an agent (_l_i_s_t_e_n_e_r) that registered an `age-of' service
    and knows about the age of `John' this question will be answered.


lliisstteenn((_+_T_e_m_p_l_a_t_e_, _:_G_o_a_l))
    Register  a  _l_i_s_t_e_n channel.    Whenever  a term  unifying  _T_e_m_p_l_a_t_e
    is  broadcasted,  call  _G_o_a_l.    The  following  example  traps  all
    broadcasted  messages as a variable unifies  to any message.  It  is
    commonly used to debug usage of the library.

    ?- listen(Term, (writeln(Term),fail)).
    ?- broadcast(hello(world)).
    hello(world)

    Yes


lliisstteenn((_+_L_i_s_t_e_n_e_r_, _+_T_e_m_p_l_a_t_e_, _:_G_o_a_l))
    Declare  _L_i_s_t_e_n_e_r as the  owner of  the channel.   Unlike a  channel
    opened  using listen/2,  channels that have  an owner can  terminate
    the  channel.  This  is commonly used if  an object is listening  to
    broadcast  messages.  In the  example below we define a  `name-item'
    displaying  the name of an  identifier represented by the  predicate
    name_of/2.

    :- pce_begin_class(name_item, text_item).

    variable(id,    any,    get, "Id visualised").

    initialise(NI, Id:any) :->
            name_of(Id, Name),
            send_super(NI, initialise, name, Name,
                       message(NI, set_name, @arg1)),
            send(NI, slot, id, Id),
            listen(NI, name_of(Id, Name),
                   send(NI, selection, Name)).

    unlink(NI) :->
            unlisten(NI),
            send_super(NI, unlink).

    set_name(NI, Name:name) :->
            get(NI, id, Id),
            retractall(name_of(Id, _)),
            assert(name_of(Id, Name)),
            broadcast(name_of(Id, Name)).

    :- pce_end_class.


uunnlliisstteenn((_+_L_i_s_t_e_n_e_r))
    Deregister all entries created with listen/3 whose _L_i_s_t_e_n_e_r unify.


uunnlliisstteenn((_+_L_i_s_t_e_n_e_r_, _+_T_e_m_p_l_a_t_e))
    Deregister  all entries  created  with listen/3  whose _L_i_s_t_e_n_e_r  and
    _T_e_m_p_l_a_t_e unify.


uunnlliisstteenn((_+_L_i_s_t_e_n_e_r_, _+_T_e_m_p_l_a_t_e_, _:_G_o_a_l))
    Deregister  all   entries  created  with  listen/3  whose  _L_i_s_t_e_n_e_r,
    _T_e_m_p_l_a_t_e and _G_o_a_l unify.


lliisstteenniinngg((_?_L_i_s_t_e_n_e_r_, _?_T_e_m_p_l_a_t_e_, _?_G_o_a_l))
    Examine  the  current  listeners.    This  predicate is  useful  for
    debugging purposes.


CChhaapptteerr 1122..  HHAACCKKEERRSS CCOORRNNEERR

This  appendix  describes  a  number  of  predicates  which  enable  the
Prolog  user  to  inspect the  Prolog  environment  and  manipulate  (or
even redefine)  the debugger.    They can be  used as  entry points  for
experiments with debugging  tools for Prolog.  The  predicates described
here should  be handled  with some  care as it  is easy  to corrupt  the
consistency of the Prolog system by misusing them.


1122..11 EExxaammiinniinngg tthhee EEnnvviirroonnmmeenntt SSttaacckk


pprroolloogg__ccuurrrreenntt__ffrraammee((_-_F_r_a_m_e))
    Unify   _F_r_a_m_e  with  an  integer   providing  a  reference  to   the
    parent  of  the  current  local stack  frame.    A  pointer  to  the
    current  local frame  cannot be provided  as the predicate  succeeds
    deterministically  and therefore its frame is  destroyed immediately
    after succeeding.


pprroolloogg__ffrraammee__aattttrriibbuuttee((_+_F_r_a_m_e_, _+_K_e_y_, _-_V_a_l_u_e))
    Obtain  information  about  the local  stack  frame  _F_r_a_m_e.    _F_r_a_m_e
    is  a  frame reference  as obtained  through prolog_current_frame/1,
    prolog_trace_interception/4 or this predicate.   The key values  are
    described below.

    aalltteerrnnaattiivvee
         _V_a_l_u_e is unified with  an integer reference to the  local stack
         frame in  which execution  is  resumed if  the goal  associated
         with _F_r_a_m_e  fails.    Fails  if the  frame has  no  alternative
         frame.

    hhaass__aalltteerrnnaattiivveess
         _V_a_l_u_e is unified  with true if _F_r_a_m_e  still is a candidate  for
         backtracking.  false otherwise.

    ggooaall
         _V_a_l_u_e is unified with the  goal associated with _F_r_a_m_e.   If the
         definition module of the active predicate is not  user the goal
         is represented as <_m_o_d_u_l_e>:<_g_o_a_l>.  Do not instantiate variables
         in this goal  unless you kknnooww  what you are  doing!  Note  that
         the  returned term  may contain  references  to the  frame  and
         should be discarded before the frame terminates.

    ppaarreenntt__ggooaall
         If _V_a_l_u_e  is  instantiated to  a callable  term,  find a  frame
         executing  the  predicate described  by  _V_a_l_u_e  and  unify  the
         arguments of _V_a_l_u_e  to the goal  arguments associated with  the
         frame.    This  is  intended  to check  the  current  execution
         context.  The user  must ensure the checked parent goal  is not
         removed from  the stack  due to last-call  optimisation and  be
         aware of the slow operation on deeply nested calls.

    ccllaauussee
         _V_a_l_u_e is  unified with  a  reference to  the currently  running
         clause.    Fails  if the  current  goal  is associated  with  a
         foreign  (C) defined  predicate.    See  also nth_clause/3  and
         clause_property/2.

    lleevveell
         _V_a_l_u_e is unified  with the recursion level  of _F_r_a_m_e.  The  top
         level frame is at level `0'.

    ppaarreenntt
         _V_a_l_u_e is unified with an integer reference to  the parent local
         stack frame of _F_r_a_m_e.  Fails if _F_r_a_m_e is the top frame.

    ccoonntteexxtt__mmoodduullee
         _V_a_l_u_e is  unified with the  name of the  context module of  the
         environment.

    ttoopp
         _V_a_l_u_e is  unified with  true if _F_r_a_m_e  is the  top Prolog  goal
         from a recursive  call back from the  foreign language.   false
         otherwise.

    hhiiddddeenn
         _V_a_l_u_e is  unified with  true if the  frame is  hidden from  the
         user, either  because a  parent has  the hide-childs  attribute
         (all  system  predicates),  or  the  system   has  no  trace-me
         attribute.

    ppcc
         _V_a_l_u_e is unified  with the program-pointer  saved on behalf  of
         the parent-goal if  the parent-goal is  not owned by a  foreign
         predicate.

    aarrgguummeenntt((_N))
         _V_a_l_u_e is  unified with the _N-th  slot of the  frame.   Argument
         1 is  the first  argument of  the goal.    Arguments above  the
         arity refer to local variables.  Fails silently if _N  is out of
         range.


pprroolloogg__cchhooiiccee__aattttrriibbuuttee((_+_C_h_o_i_c_e_P_o_i_n_t_, _+_K_e_y_, _-_V_a_l_u_e))
    Extract  attributes of a choice-point.   _C_h_o_i_c_e_P_o_i_n_t is a  reference
    to  a choice-point as  passed to  prolog_trace_interception/4on  the
    3-th argument.  _K_e_y specifies the requested information:

    ppaarreenntt
         Requests a reference to the first older choice-point.

    ffrraammee
         Requests a  reference to  the frame to  which the  choice-point
         refers.

    ttyyppee
         Requests  the type.     Defined  values are  clause  (the  goal
         has alternative  clauses),  foreign (non-deterministic  foreign
         predicate),  jump (clause  internal choice-point),  top  (first
         dummy choice-point), catch  (catch/3 to allow for undo),  debug
         (help the debugger), or none (has been deleted).

    This  predicate  is used  for  the graphical  debugger to  show  the
    choice-point stack.


ddeetteerrmmiinniissttiicc((_-_B_o_o_l_e_a_n))
    Unifies  its argument with  true if the  clause in which is  appears
    has  not created any choice-points since it was started.   There are
    few  realistic situations for using this  predicate.  It is used  by
    the  prolog/0 toplevel  to check  whether Prolog  should prompt  the
    user for alternatives.


1122..22 IInntteerrcceeppttiinngg tthhee TTrraacceerr


pprroolloogg__ttrraaccee__iinntteerrcceeppttiioonn((_+_P_o_r_t_, _+_F_r_a_m_e_, _+_C_h_o_i_c_e_, _-_A_c_t_i_o_n))
    Dynamic  predicate, normally not defined.  This predicate  is called
    from  the SWI-Prolog debugger just before it would show a port.   If
    this  predicate succeeds the debugger  assumes the trace action  has
    been  taken care of and continues execution as described  by _A_c_t_i_o_n.
    Otherwise the normal Prolog debugger actions are performed.

    _P_o_r_t  denotes  the reason  to  activate the  tracer (`port'  in  the
    4/5-port, but with some additions:

    ccaallll
         Normal entry through the call-port of the 4-port debugger.

    rreeddoo
         Normal entry  through  the call-port  of the  4-port  debugger.
         The  redo  port  signals  resuming  a  predicate   to  generate
         alternative solutions.

    uunniiffyy
         The unify-port represents the _n_e_c_k instruction,  signalling the
         end  of the  head-matching  process.    This port  is  normally
         invisible.  See leash/1 and visible/1.

    eexxiitt
         The exit-port signals the goal  is proved.  It is  possible for
         the goal to have alternative.   See prolog_frame_attribute/3 to
         examine the goal-stack.

    ffaaiill
         The fail-port signals final failure of the goal.

    eexxcceeppttiioonn((_E_x_c_e_p_t))
         An  exception is  raised  and still  pending.    This  port  is
         activated on  each parent  frame  of the  frame generating  the
         exception until the  exception is caught  or the user  restarts
         normal  computation  using  retry.     _E_x_c_e_p_t  is  the  pending
         exception-term.

    bbrreeaakk((_P_C))
         A break instruction is executed.  _P_C is program counter.   This
         port is used by the graphical debugger.

    ccuutt__ccaallll((_P_C))
         A cut is encountered at _P_C. This port is used  by the graphical
         debugger.  to visualise the effect of the cut.

    ccuutt__eexxiitt((_P_C))
         A  cut  has  been   executed.     See   cut_call(_P_C)  for   more
         information.

    _F_r_a_m_e  is  a  reference to  the  current local  stack  frame,  which
    can  be  examined  using  prolog_frame_attribute/3.     _C_h_o_i_c_e  is  a
    reference  to  the  last  choice-point and  can  be  examined  using
    prolog_choice_attribute/3.   _A_c_t_i_o_n  should be unified  with one  of
    the  atoms  continue (just  continue  execution), retry  (retry  the
    current goal) or fail  (force the current goal to fail).  Leaving it
    a variable is identical to continue.

    Together  with  the predicates  described in  section  4.38 and  the
    other  predicates of this chapter this predicate enables  the Prolog
    user  to define a complete new debugger in Prolog.  Besides  this it
    enables  the Prolog programmer monitor  the execution of a  program.
    The  example below records  all goals trapped  by the tracer in  the
    database.

    prolog_trace_interception(Port, Frame, _PC, continue) :-
            prolog_frame_attribute(Frame, goal, Goal),
            prolog_frame_attribute(Frame, level, Level),
            recordz(trace, trace(Port, Level, Goal)).

    To  trace the execution of `go' this way the following  query should
    be given:

    ?- trace, go, notrace.


pprroolloogg__sskkiipp__lleevveell((_-_O_l_d_, _+_N_e_w))
    Unify  _O_l_d with  the old  value of `skip  level' and  than set  this
    level  according to _N_e_w.   New  is an integer,  or the special  atom
    very_deep  (meaning don't  skip).    The `skip  level' is  a  global
    variable  of the  Prolog system  that disables the  debugger on  all
    recursion  levels deeper than  the level of the  variable.  Used  to
    implement  the trace options  `skip' (sets skip  level to the  level
    of  the frame) and `up' (sets skip level to the level of  the parent
    frame (i.e., the level of this frame minus 1).


1122..33 AAddddiinngg ccoonntteexxtt ttoo eerrrroorrss::  pprroolloogg__eexxcceeppttiioonn__hhooookk

The hook prolog_exception_hook/4 has been introduced in SWI-Prolog 5.6.5
to  provide  dedicated exception  handling  facilities  for  application
frameworks.   For example non-interactive server applications that  wish
to provide extensive context for exceptions for offline debugging.


pprroolloogg__eexxcceeppttiioonn__hhooookk((_+_E_x_c_e_p_t_i_o_n_I_n_, _-_E_x_c_e_p_t_i_o_n_O_u_t_, _+_F_r_a_m_e_, _+_C_a_t_c_h_e_r_F_r_a_m_e))

    This   hook  predicate,   if  defined   in  the   module  user,   is
    between  raising  an exception  and handling  it.    It is  intended
    to  allow a  program adding  additional context to  an exception  to
    simplify  diagnosing  the problem.    _E_x_c_e_p_t_i_o_n_I_n  is the  exception
    term  as  raised  by throw/1  or  one  of the  bullt-in  predicates.
    The  output  argument  _E_x_c_e_p_t_i_o_n_O_u_t  describes  the  exception  that
    is  actually  raised.      _F_r_a_m_e  is  the  innermost  frame.     See
    prolog_frame_attribute/3 and  the library  prolog_stack for  getting
    information  from this.   _C_a_t_c_h_e_r_F_r_a_m_e is  a reference to the  frame
    calling  the  matching  catch/3 or  none  of  the exception  is  not
    caught.

    The  hook is run in `nodebug' mode.  If it succeeds  _E_x_c_e_p_t_i_o_n_O_u_t is
    considered the current  exception.  If it fails, _E_x_c_e_p_t_i_o_n_I_n is used
    for further processing.   The hook is _n_e_v_e_r called recursively.  The
    hook  is _n_o_t allowed to modify  _E_x_c_e_p_t_i_o_n_O_u_t in such as way that  it
    no longer unifies with the catching frame.

    Typically,   prolog_exception_hook/4 is  used  to  fill  the  second
    argument  of error(_F_o_r_m_a_l_, _C_o_n_t_e_x_t) exceptions.   _F_o_r_m_a_l is  defined
    by  the  ISO  standard,   while  SWI-Prolog  defines  _C_o_n_t_e_x_t  as  a
    term  context(_L_o_c_a_t_i_o_n_,  _M_e_s_s_a_g_e).    _L_o_c_a_t_i_o_n is  bound to  a  term
    <_n_a_m_e>/<_a_r_i_t_y>  by the kernel.    This hook can  be used to  add more
    information on the calling context, such as a full stack trace.

    Applications  that use exceptions as part of normal  processing must
    do  a  quick  test  of the  environment  before  starting  expensive
    gathering information on the state of the program.

    The hook can call trace/0 to enter trace mode immediately.


1122..44 HHooookkss uussiinngg tthhee eexxcceeppttiioonn pprreeddiiccaattee

This section describes  the predicate exception/3, which can  be defined
by the  user in the module  user as a multifile  predicate.  Unlike  the
name suggests,  this is actually a _h_o_o_k  predicate that has no  relation
to  Prolog exceptions  as  defined by  the  ISO predicates  catch/3  and
throw/1.

The  predicate exception/3  is  called by  the  kernel  on a  couple  of
events, allowing  the user  to `fix' errors  just in time  events.   The
mechanism allows for _l_a_z_y creation of objects such as predicates.


eexxcceeppttiioonn((_+_E_x_c_e_p_t_i_o_n_, _+_C_o_n_t_e_x_t_, _-_A_c_t_i_o_n))
    Dynamic  predicate,  normally not  defined.   Called  by the  Prolog
    system  on run-time exceptions that can be repaired `just  in time'.
    The values for _E_x_c_e_p_t_i_o_n  are described below.  See also catch/3 and
    throw/1.

    If  this  hook predicate  succeeds it  must  instantiate the  _A_c_t_i_o_n
    argument  to the  atom  fail to  make the  operation fail  silently,
    retry  to tell Prolog  to retry the operation  or error to make  the
    system generate an exception.   The action retry only makes sense if
    this  hook modified the environment such that the operation  can now
    succeed without error.

    uunnddeeffiinneedd__pprreeddiiccaattee
         _C_o_n_t_e_x_t is instantiated to  a term _N_a_m_e/_A_r_i_t_y.  _N_a_m_e  refers to
         the name  and _A_r_i_t_y to  the arity  of the undefined  predicate.
         If the definition module of the predicate is  not _u_s_e_r, _C_o_n_t_e_x_t
         will be of  the form <_M_o_d_u_l_e>:<_N_a_m_e>/<_A_r_i_t_y>.   If the  predicate
         fails Prolog will generate an existence_error exception.

    uunnddeeffiinneedd__gglloobbaall__vvaarriiaabbllee
         _C_o_n_t_e_x_t  is instantiated  to the  name  of the  missing  global
         variable.  The hook  must call nb_setval/2 or b_setval/2 before
         returning with the action retry.


1122..55 HHooookkss ffoorr iinntteeggrraattiinngg lliibbrraarriieess

Some libraries  realise an entirely new  programming paradigm on top  of
Prolog.   An example is  XPCE which adds  an object-system to Prolog  as
well as an extensive  set of graphical primitives.   SWI-Prolog provides
several hooks to  improve the integration of  such libraries.  See  also
section 4.4  for editing hooks  and section 4.9.3  for hooking into  the
message system.


pprroolloogg__lliisstt__ggooaall((_:_G_o_a_l))
    Hook, normally not defined.   This hook is called by the 'L' command
    of  the  tracer in  the module  user to  list  the currently  called
    predicate.   This hook may be defined to list only  relevant clauses
    of  the indicated  _G_o_a_l  and/or show  the actual  source-code in  an
    editor.  See also portray/1 and multifile/1.


pprroolloogg::ddeebbuugg__ccoonnttrrooll__hhooookk((_:_A_c_t_i_o_n))
    Hook for the  debugger-control predicates that allows the creator of
    more  high-level programming languages  to use the common  front-end
    predicates  to control de  debugger.  For  example, XPCE uses  these
    hooks  to allow for spying methods  rather then predicates.   _A_c_t_i_o_n
    is one of:

    ssppyy((_S_p_e_c))
         Hook in  spy/1.  If  the hook succeeds  spy/1 takes no  further
         action.

    nnoossppyy((_S_p_e_c))
         Hook in nospy/1.   If the hook succeeds spy/1 takes  no further
         action.    If  spy/1  is  hooked,  it is  advised  to  place  a
         complementary hook for nospy/1.

    nnoossppyyaallll
         Hook in nospyall/0.   Should remove all spy-points.   This hook
         is called in a failure-driven loop.

    ddeebbuuggggiinngg
         Hook in  debugging/0.   It can  be used in  two ways.   It  can
         report the status of the additional debug-points  controlled by
         the above hooks  and fail to let  the system report the  others
         or it succeed, overruling the entire behaviour of debugging/0.


pprroolloogg::hheellpp__hhooookk((_+_A_c_t_i_o_n))
    Hook  into help/0 and help/1.   If  the hook succeeds, the  built-in
    actions are not  executed.  For example, ?- help(picture). is caught
    by  the XPCE help-hook to give help  on the class _p_i_c_t_u_r_e.   Defined
    actions are:

    hheellpp
         User entered plain  help/0 to give default  help.  The  default
         performs help(help/1), giving help on help.

    hheellpp((_W_h_a_t))
         Hook in help/1 on the topic _W_h_a_t.

    aapprrooppooss((_W_h_a_t))
         Hook in apropos/1 on the topic _W_h_a_t.


1122..66 HHooookkss ffoorr llooaaddiinngg ffiilleess

All  loading of  source-files is  achieved by  load_files/2.   The  hook
prolog_load_file/2 can be  used to  load Prolog code  from non-files  or
even load entirely different information, such as foreign files.


pprroolloogg__llooaadd__ffiillee((_+_S_p_e_c_, _+_O_p_t_i_o_n_s))
    Load  a single object.  If this  call succeeds, load_files/2 assumes
    the  action has been  taken care of.   This hook  is only called  if
    _O_p_t_i_o_n_s  does not contain the stream(_I_n_p_u_t)  option.  The hook  must
    be defined in the module user.

    The  http_load  provides an example, loading Prolog sources  directly
    from an HTTP server.


pprroolloogg::ccoommmmeenntt__hhooookk((_+_C_o_m_m_e_n_t_s_, _+_P_o_s_, _+_T_e_r_m))
    This   hook   allows   for  processing   ---structured---   comments
    encountered  by the  compiler.    The reader  collects all  comments
    found  from the current position  to the end of  the next term.   It
    calls  this hook providing a  list of _P_o_s_i_t_i_o_n-_C_o_m_m_e_n_t in  _C_o_m_m_e_n_t_s,
    the start-position of the  next term in _P_o_s and the next term itself
    in  _T_e_r_m.  All  positions are stream-position terms.   This hook  is
    exploited  by the documentation system.  See stream_position_data/3.
    See also read_term/3.


1122..77 RReeaaddlliinnee IInntteerraaccttiioonn

The       following       predicates       are        available       if
current_prolog_flag(readline, true) succeeds.      They  allow  for  di-
rect  interaction   with  the   GNU  readline   library.      See   also
readline(3)


rrll__rreeaadd__iinniitt__ffiillee((_+_F_i_l_e))
    Read  a readline  initialisation file.   Readline  by default  reads
    ~/.inputrc.     This  predicate  may be  used  to  read  alternative
    readline initialisation files.


rrll__aadddd__hhiissttoorryy((_+_L_i_n_e))
    Add  a  line  to  the  Control-P/Control-N  history  system  of  the
    readline library.


CChhaapptteerr 1133..  GGLLOOSSSSAARRYY OOFF TTEERRMMSS

aannoonnyymmoouuss [[vvaarriiaabbllee]]
    The  variable  _  is  called  the  _a_n_o_n_y_m_o_u_s  variable.     Multiple
    occurrences of _ in a single _t_e_r_m are not _s_h_a_r_e_d.

aarrgguummeennttss
    Arguments  are _t_e_r_m_s that appear in a _c_o_m_p_o_u_n_d _t_e_r_m.  _A_1  and _a_2 are
    the first and second argument of the term myterm(_A_1_, _a_2).

aarriittyy
    Argument count (is number of arguments) of a _c_o_m_p_o_u_n_d _t_e_r_m.

aasssseerrtt
    Add a _c_l_a_u_s_e to  a _p_r_e_d_i_c_a_t_e.  Clauses can be added at either end of
    the clause-list of a _p_r_e_d_i_c_a_t_e.  See assert/1 and assertz/1.

aattoomm
    Textual  constant.   Used as name for  _c_o_m_p_o_u_n_d terms, to  represent
    constants or text.

bbaacckkttrraacckkiinngg
    Searching  process used by Prolog.   If a predicate offers  multiple
    _c_l_a_u_s_e_s  to  solve  a _g_o_a_l,  they  are  tried one-by-one  until  one
    _s_u_c_c_e_e_d_s.    If  a subsequent  part of  the proof  is not  satisfied
    with  the resulting _v_a_r_i_a_b_l_e _b_i_n_d_i_n_g, it may ask for  an alternative
    _s_o_l_u_t_i_o_n (= _b_i_n_d_i_n_g  of the _v_a_r_i_a_b_l_e_s), causing Prolog to reject the
    previously chosen _c_l_a_u_s_e and try the next one.

bbiinnddiinngg [[ooff aa vvaarriiaabbllee]]
    Current value of the _v_a_r_i_a_b_l_e.  See also _b_a_c_k_t_r_a_c_k_i_n_g and _q_u_e_r_y.

bbuuiilltt--iinn [[pprreeddiiccaattee]]
    Predicate  that is part of the  Prolog system.  Built in  predicates
    cannot  be redefined  by the  user, unless this  is overruled  using
    redefine_system_predicate/1.

bbooddyy
    Part of a _c_l_a_u_s_e behind the _n_e_c_k operator (:-).

ccllaauussee
    `Sentence'  of a Prolog program.   A _c_l_a_u_s_e  consists of a _h_e_a_d  and
    _b_o_d_y  separated by  the _n_e_c_k operator  (:-) or it  is a _f_a_c_t.    For
    example:

    parent(X) :-
            father(X, _).

    Expressed  ``X is a parent if X is a father of someone''.   See also
    _v_a_r_i_a_b_l_e and _p_r_e_d_i_c_a_t_e.

ccoommppiillee
    Process  where  a Prolog  _p_r_o_g_r_a_m  is translated  to a  sequence  of
    instructions.   See  also _i_n_t_e_r_p_r_e_t_e_d.   SWI-Prolog always  compiles
    your program before executing it.

ccoommppoouunndd [[tteerrmm]]
    Also  called  _s_t_r_u_c_t_u_r_e.    It  consists of  a  name followed  by  _N
    _a_r_g_u_m_e_n_t_s,  each of which are _t_e_r_m_s.   _N is called the _a_r_i_t_y  of the
    term.

ccoonntteexxtt mmoodduullee
    If  a _t_e_r_m  is referring  to a _p_r_e_d_i_c_a_t_e  in a  _m_o_d_u_l_e, the  _c_o_n_t_e_x_t
    _m_o_d_u_l_e  is used to find the target module.  The context  module of a
    _g_o_a_l  is the module in which  the _p_r_e_d_i_c_a_t_e is defined, unless  this
    _p_r_e_d_i_c_a_t_e  is _m_o_d_u_l_e _t_r_a_n_s_p_a_r_e_n_t, in  which case the _c_o_n_t_e_x_t  _m_o_d_u_l_e
    is inherited from the parent _g_o_a_l.  See also module_transparent/1.

ddyynnaammiicc [[pprreeddiiccaattee]]
    A _d_y_n_a_m_i_c predicate  is a predicate to which _c_l_a_u_s_e_s may be _a_s_s_e_r_ted
    and  from  which  _c_l_a_u_s_e_s may  be  _r_e_t_r_a_c_ted  while the  program  is
    running.  See also _u_p_d_a_t_e _v_i_e_w.

eexxppoorrtteedd [[pprreeddiiccaattee]]
    A  _p_r_e_d_i_c_a_t_e is  said to  be _e_x_p_o_r_t_e_d from  a _m_o_d_u_l_e  if it  appears
    in  the  _p_u_b_l_i_c _l_i_s_t.     This implies  that  the predicate  can  be
    _i_m_p_o_r_t_e_d  into another module  to make it visible  there.  See  also
    use_module/[1,2].

ffaacctt
    _C_l_a_u_s_e  without a _b_o_d_y.   This is called a fact because  interpreted
    as logic, there is  no condition to be satisfied.  The example below
    states john is a person.

    person(john).

ffaaiill
    A _g_o_a_l is said to haved failed if it could not be _p_r_o_v_e_n.

ffllooaatt
    Computers crippled representation  of a real number.  Represented as
    `IEEE double'.

ffoorreeiiggnn
    Computer code expressed  in other languages than Prolog.  SWI-Prolog
    can only cooperate directly with the C and C++ computer languages.

ffuunnccttoorr
    Combination  of name and _a_r_i_t_y of a _c_o_m_p_o_u_n_d term.  The  term foo(_a_,
    _b_,  _c) is said to be a  term belonging to the functor foo/3.   foo/0
    is used to refer to the _a_t_o_m foo.

ggooaall
    Question  stated to the Prolog engine.  A _g_o_a_l is either  an _a_t_o_m or
    a  _c_o_m_p_o_u_n_d term.  A _g_o_a_l  succeeds, in which case the  _v_a_r_i_a_b_l_e_s in
    the _c_o_m_p_o_u_n_d terms have  a _b_i_n_d_i_n_g or _f_a_i_l_s if Prolog fails to prove
    the _g_o_a_l.

hhaasshhiinngg
    _I_n_d_e_x_i_n_g technique used for quick lookup.

hheeaadd
    Part  of a _c_l_a_u_s_e before the _n_e_c_k  instruction.  This is an  atom or
    _c_o_m_p_o_u_n_d term.

iimmppoorrtteedd [[pprreeddiiccaattee]]
    A  _p_r_e_d_i_c_a_t_e is said to be _i_m_p_o_r_t_e_d  into a _m_o_d_u_l_e if it  is defined
    in  another _m_o_d_u_l_e  and made  available in this  _m_o_d_u_l_e.   See  also
    chapter 5.

iinnddeexxiinngg
    Indexing  is a  technique used to  quickly select candidate  _c_l_a_u_s_e_s
    of  a  _p_r_e_d_i_c_a_t_e for  a specific  _g_o_a_l.    In most  Prolog  systems,
    including  SWI-Prolog, indexing  is done  on the  first _a_r_g_u_m_e_n_t  of
    the  _h_e_a_d.  If  this argument is  instantiated to an _a_t_o_m,  _i_n_t_e_g_e_r,
    _f_l_o_a_t or _c_o_m_p_o_u_n_d  term with _f_u_n_c_t_o_r, _h_a_s_h_i_n_g is used quickly select
    all  _c_l_a_u_s_e_s of which  the first argument  may _u_n_i_f_y with the  first
    argument of the _g_o_a_l.

iinntteeggeerr
    Whole number.   On all implementations of SWI-Prolog integers are at
    least  64-bit signed values.   When linked  to the GNU GMP  library,
    integer  arithmetic is unbounded.    See also current_prolog_flag/2,
    flags bounded, max_integer and min_integer.

iinntteerrpprreetteedd
    As  opposed  to  _c_o_m_p_i_l_e_d,   interpreted  means  the  Prolog  system
    attempts  to prove  a _g_o_a_l  by directly reading  the _c_l_a_u_s_e_s  rather
    than executing instructions  from an (abstract) instruction set that
    is not or only indirectly related to Prolog.

mmeettaa--pprreeddiiccaattee
    A  _p_r_e_d_i_c_a_t_e that reasons about other _p_r_e_d_i_c_a_t_e_s, either  by calling
    them, (re)defining them or querying _p_r_o_p_e_r_t_i_e_s.

mmoodduullee
    Collection  of predicates.    Each module defines  a name-space  for
    predicates.   _b_u_i_l_t_-_i_n predicates  are accessible from all  modules.
    Predicates  can be published (_e_x_p_o_r_t_e_d)  and _i_m_p_o_r_t_e_d to make  their
    definition available to other modules.

mmoodduullee ttrraannssppaarreenntt [[pprreeddiiccaattee]]
    A  _p_r_e_d_i_c_a_t_e that  does not change  the _c_o_n_t_e_x_t  _m_o_d_u_l_e.   Sometimes
    also called a _m_e_t_a_-_p_r_e_d_i_c_a_t_e.

mmuullttiiffiillee [[pprreeddiiccaattee]]
    Predicate  for which  the  definition is  distributed over  multiple
    source-files.  See multifile/1.

nneecckk
    Operator (:-) separating _h_e_a_d from _b_o_d_y in a _c_l_a_u_s_e.

ooppeerraattoorr
    Symbol (_a_t_o_m) that  may be placed before its _o_p_e_r_a_n_d (prefix), after
    its _o_p_e_r_a_n_d (postfix) or between its two _o_p_e_r_a_n_d_s (infix).

    In  Prolog, the expression a+b is exactly the same as  the canonical
    term +(a,b).

ooppeerraanndd
    _A_r_g_u_m_e_n_t of an _o_p_e_r_a_t_o_r.

pprreecceeddeennccee
    The  _p_r_i_o_r_i_t_y  of an  _o_p_e_r_a_t_o_r.    Operator  precedence is  used  to
    interpret a+b*c as +(a, *(b,c)).

pprreeddiiccaattee
    Collection  of _c_l_a_u_s_e_s  with the same  _f_u_n_c_t_o_r (name/_a_r_i_t_y).   If  a
    _g_o_a_l  is proved,  the system  looks for  a _p_r_e_d_i_c_a_t_e  with the  same
    functor,  then used  _i_n_d_e_x_i_n_g to select  candidate _c_l_a_u_s_e_s and  then
    tries these _c_l_a_u_s_e_s one-by-one.  See also _b_a_c_k_t_r_a_c_k_i_n_g.

pprriioorriittyy
    In the context of _o_p_e_r_a_t_o_r_s a synonym for _p_r_e_c_e_d_e_n_c_e.

pprrooggrraamm
    Collection of _p_r_e_d_i_c_a_t_e_s.

pprrooppeerrttyy
    Attribute  of  an object.    SWI-Prolog  defines  various _*___p_r_o_p_e_r_t_y
    predicates to query the status of predicates, clauses.  etc.

pprroovvee
    Process  where Prolog attempts to prove a _q_u_e_r_y using  the available
    _p_r_e_d_i_c_a_t_e_s.

ppuubblliicc lliisstt
    List of _p_r_e_d_i_c_a_t_e_s exported from a _m_o_d_u_l_e.

qquueerryy
    See _g_o_a_l.

rreettrraacctt
    Remove  a _c_l_a_u_s_e from a  _p_r_e_d_i_c_a_t_e.   See also _d_y_n_a_m_i_c, _u_p_d_a_t_e  _v_i_e_w
    and _a_s_s_e_r_t.

sshhaarreedd
    Two  _v_a_r_i_a_b_l_e_s  are called  _s_h_a_r_e_d after  they are  _u_n_i_f_i_e_d.    This
    implies  if either of them is _b_o_u_n_d, the other is bound to  the same
    value:

    ?- A = B, A = a.

    A = a,
    B = a

ssiinngglleettoonn [[vvaarriiaabbllee]]
    _V_a_r_i_a_b_l_e  appearing only one time in a _c_l_a_u_s_e.   SWI-Prolog normally
    warns  for  this to  avoid  you  making spelling  mistakes.    If  a
    variable  appears on  purpose only  once in  a clause,  write it  as
    _  (see _a_n_o_n_y_m_o_u_s).    Rules for naming  a variable  and avoiding  a
    warning are given in section 2.15.1.5.

ssoolluuttiioonn
    _B_i_n_d_i_n_g_s resulting from a successfully _p_r_o_v_en _g_o_a_l.

ssttrruuccttuurree
    Synonym for _c_o_m_p_o_u_n_d term.

ssttrriinngg
    Used  for the  following representations  of text:   a packed  array
    (see  section 4.23, SWI-Prolog specific), a list of  character codes
    or a list of one-character _a_t_o_m_s.

ssuucccceeeedd
    A _g_o_a_l is said to have _s_u_c_c_e_e_d_e_d if it has been _p_r_o_v_e_n.

tteerrmm
    Value in Prolog.   A _t_e_r_m is either a _v_a_r_i_a_b_l_e, _a_t_o_m, integer, float
    or  _c_o_m_p_o_u_n_d term.   In addition,  SWI-Prolog also defines the  type
    _s_t_r_i_n_g

ttrraannssppaarreenntt
    See _m_o_d_u_l_e _t_r_a_n_s_p_a_r_e_n_t.

uunniiffyy
    Prolog  process to make  two terms equal  by assigning variables  in
    one term to  values at the corresponding location of the other term.
    For example:

    ?- foo(a, B) = foo(A, b).

    A = a,
    B = b

    Unlike  assignment (which does not exist in Prolog),  unification is
    not directed.

uuppddaattee vviieeww
    How  Prolog behaves when a _d_y_n_a_m_i_c _p_r_e_d_i_c_a_t_e is changed while  it is
    running.   There are two models.   In most older Prolog  systems the
    change  becomes immediately visible to  the _g_o_a_l, in modern  systems
    including  SWI-Prolog, the running _g_o_a_l is  not affected.  Only  new
    _g_o_a_l_s `see' the new definition.

vvaarriiaabbllee
    A  Prolog  variable is  a value  that `is  not yet  bound'.    After
    _b_i_n_d_i_n_g a variable, it  cannot be modified.  _B_a_c_k_t_r_a_c_k_i_n_g to a point
    in  the execution before  the variable was  bound will turn it  back
    into a variable:

    ?- A = b, A = c.
    No
    ?- (A = b; true; A = c).
    A = b ;
    A = _G283 ;
    A = c ;
    No

    See also _u_n_i_f_y.


CChhaapptteerr 1144..  SSWWII--PPRROOLLOOGG LLIICCEENNSSEE CCOONNDDIITTIIOONNSS AANNDD TTOOOOLLSS

SWI-Prolog licensing aims at a large audience, combining  ideas from the
Free Software Foundation and the less principal  Open Source Initiative.
The license aims at:

  o Make SWI-Prolog itself and its libraries are `As free as possible'.

  o Allow for easy integration of contributions.  See section 14.2.

  o Free software can build on SWI-Prolog without limitations.

  o Non-free  (open  or  proprietary)  software can  be  produced  using
    SWI-Prolog,  although contributed pure  GPL-ed components cannot  be
    used.

To achieve this, different parts of the system  have different licenses.
SWI-Prolog  programs consists  of  a mixture  of `native'  code  (source
compiled to  machine instructions)  and `virtual  machine' code  (Prolog
source  compiled to  SWI-Prolog virtual  machine instructions,  covering
both compiled SWI-Prolog libraries and your compiled application).

For  maximal coherence  between free  licenses, we  start  with the  two
prime  licenses from  the  Free  Software Foundation,  the  GNU  General
Public License (GPL)  and the Lesser GNU General Public  License (LGPL),
after which we add a proven (used by the  GNU-C compiler runtime library
as  well as  the  GNU _C_l_a_s_s_P_a_t_h  project)  exception  to deal  with  the
specific nature of compiled virtual machine code in a saved state.


1144..11 TThhee SSWWII--PPrroolloogg kkeerrnneell aanndd ffoorreeiiggnn lliibbrraarriieess

The SWI-Prolog  kernel and our foreign  libraries are distributed  under
the  LLGGPPLL. A  Prolog executable  consists of  the  combination of  these
`native'  code  components  and  Prolog  virtual  machine  code.     The
SWI-Prolog  plrc  utility allows  for  disassembling  and  re-assembling
these parts, a process satisfying article 66bb of the LGPL.

Under  the LGPL  SWI-Prolog  can be  linked  to code  distributed  under
arbitrary licenses,  provided a number  of requirements are  fullfilled.
The most  important requirement is  that, if  an application replies  on
a _m_o_d_i_f_i_e_d  version of  SWI-Prolog, the  modified sources  must be  made
available.


1144..11..11 TThhee SSWWII--PPrroolloogg PPrroolloogg lliibbrraarriieess

Lacking a  satisfactory technical solution  to handle  article 66 of  the
LGPL, this  license cannot be  used for the Prolog  source code that  is
part of the  SWI-Prolog system (both libraries  and kernel code).   This
situation is  comparable to libgcc,  the runtime  library used with  the
GNU C-compiler.    Therefore, we use  the same  proven license terms  as
this library.    The libgcc  license is  the with  a special  exception.
Below we rephrased this exception adjusted to our needs:

    _A_s  _a _s_p_e_c_i_a_l  _e_x_c_e_p_t_i_o_n_, _i_f _y_o_u  _l_i_n_k _t_h_i_s  _l_i_b_r_a_r_y _w_i_t_h _o_t_h_e_r
    _f_i_l_e_s_,  _c_o_m_p_i_l_e_d  _w_i_t_h  _a _F_r_e_e  _S_o_f_t_w_a_r_e  _c_o_m_p_i_l_e_r_,  _t_o _p_r_o_d_u_c_e
    _a_n  _e_x_e_c_u_t_a_b_l_e_,  _t_h_i_s  _l_i_b_r_a_r_y  _d_o_e_s _n_o_t  _b_y  _i_t_s_e_l_f  _c_a_u_s_e _t_h_e
    _r_e_s_u_l_t_i_n_g  _e_x_e_c_u_t_a_b_l_e _t_o _b_e  _c_o_v_e_r_e_d _b_y _t_h_e  _G_N_U _G_e_n_e_r_a_l _P_u_b_l_i_c
    _L_i_c_e_n_s_e_.   _T_h_i_s _e_x_c_e_p_t_i_o_n _d_o_e_s _n_o_t _h_o_w_e_v_e_r _i_n_v_a_l_i_d_a_t_e _a_n_y _o_t_h_e_r
    _r_e_a_s_o_n_s  _w_h_y _t_h_e  _e_x_e_c_u_t_a_b_l_e _f_i_l_e _m_i_g_h_t  _b_e _c_o_v_e_r_e_d _b_y  _t_h_e _G_N_U
    _G_e_n_e_r_a_l _P_u_b_l_i_c _L_i_c_e_n_s_e_.


1144..22 CCoonnttrriibbuuttiinngg ttoo tthhee SSWWII--PPrroolloogg pprroojjeecctt

To  achieve maximal  coherence using  SWI-Prolog for  Free and  Non-Free
software we  advice the  use of  the LGPL for  contributed foreign  code
and the  use of the  GPL with SWI-Prolog exception  for Prolog code  for
contributed modules.

As a  rule of thumb  it is  advised to use  the above licenses  whenever
possible and only use a strict GPL compliant license  only if the module
contains other code under strict GPL compliant licenses.


1144..33 SSooffttwwaarree ssuuppppoorrtt ttoo kkeeeepp ttrraacckk ooff lliicceennssee ccoonnddiittiioonnss

Given the above, it is possible that SWI-Prolog  packages and extensions
will  rely  on the  GPL.  The  predicates below  allow  for  registering
license  requirements  for  Prolog files  and  foreign  modules.     The
predicate  eval_license/0 reports  which  components from  the  currenly
configured  system  are  distributed under  copy-left  and  open  source
enforcing  licenses (the  GPL)  and therefore  must be  replaced  before
distributing linked applications under non-free license conditions.


eevvaall__lliicceennssee
    Evaluate  the license conditions of all  loaded components.  If  the
    system  contains  one or  more components  that  are licenced  under
    GPL-like  restrictions the  system indicates this  program may  only
    be  distributed under the  GPL license as  well as which  components
    prohibit the use of other license conditions.


lliicceennssee((_+_L_i_c_e_n_s_e_I_d_, _+_C_o_m_p_o_n_e_n_t))
    Register  the fact  that _C_o_m_p_o_n_e_n_t  is distributed  under a  license
    identified by _L_i_c_e_n_s_e_I_d.  The most important _L_i_c_e_n_s_e_I_d's are:

    sswwiippll
         Indicates this  module  is distributed  under the  GNU  General
         Public License (GPL) with the SWI-Prolog exception:

             _A_s _a  _s_p_e_c_i_a_l _e_x_c_e_p_t_i_o_n_, _i_f _y_o_u _l_i_n_k _t_h_i_s _l_i_b_r_a_r_y _w_i_t_h
             _o_t_h_e_r  _f_i_l_e_s_, _c_o_m_p_i_l_e_d _w_i_t_h _S_W_I_-_P_r_o_l_o_g_,  _t_o _p_r_o_d_u_c_e _a_n
             _e_x_e_c_u_t_a_b_l_e_, _t_h_i_s  _l_i_b_r_a_r_y _d_o_e_s _n_o_t _b_y _i_t_s_e_l_f _c_a_u_s_e _t_h_e
             _r_e_s_u_l_t_i_n_g _e_x_e_c_u_t_a_b_l_e  _t_o _b_e _c_o_v_e_r_e_d _b_y _t_h_e _G_N_U _G_e_n_e_r_a_l
             _P_u_b_l_i_c  _L_i_c_e_n_s_e_.    _T_h_i_s  _e_x_c_e_p_t_i_o_n  _d_o_e_s  _n_o_t _h_o_w_e_v_e_r
             _i_n_v_a_l_i_d_a_t_e  _a_n_y _o_t_h_e_r _r_e_a_s_o_n_s _w_h_y  _t_h_e _e_x_e_c_u_t_a_b_l_e _f_i_l_e
             _m_i_g_h_t _b_e _c_o_v_e_r_e_d _b_y _t_h_e _G_N_U _G_e_n_e_r_a_l _P_u_b_l_i_c _L_i_c_e_n_s_e_.

         This should  be the  default  for software  contributed to  the
         SWI-Prolog  project  as it  allows  the  community  to  prosper
         both in  the free  and non-free  world.    Still, people  using
         SWI-Prolog  to create  non-free  applications  must  contribute
         sources to improvements they make to the community.

    llggppll
         This is the  default license for foreign-libraries linked  with
         SWI-Prolog.   Use PL_license() to  register the condition  from
         foreign code.

    ggppll
         Indicates  this  module   is  strictly  Free  Software,   which
         implies it  cannot be  used together  with any  module that  is
         incompatible to the GPL. Please only use  these conditions when
         forced by other code used in the component.

    Other  licenses known to the system are guile,  gnu_ada,  x11, expat,
    sml,  public_domain,  cryptix, bsd,  zlib,  constlgpl_compatible  and
    gpl_compatible.    New licenses  can be  defined  by adding  clauses
    for  the  multifile  predicate  license:license/3.     Below  is  an
    example.   The  second argument  is either gpl  or lgpl to  indicate
    compatibility to these  licenses.  Other values cause the license to
    interpreted  as _p_r_o_p_r_i_e_t_a_r_y.   Proprietary licenses are reported  by
    eval_license/0.  See the file boot/license.pl for details.

    :- multifile license:license/3.

    license:license(mylicense, lgpl,
                    [ comment('My personal license'),
                      url('http://www.mine.org/license.html')
                    ]).

    :- license(mylicense).


lliicceennssee((_+_L_i_c_e_n_s_e_I_d))
    Intented  as a  directive  in Prolog  source files.    It takes  the
    current filename and calls license/2.


void PPLL__lliicceennssee(_c_o_n_s_t _c_h_a_r _*_L_i_c_e_n_s_e_I_d_, _c_o_n_s_t _c_h_a_r _*_C_o_m_p_o_n_e_n_t)
    Intended  for the install()  procedure of foreign  libraries.   This
    call can be made _b_e_f_o_r_e PL_initialise().


1144..44 LLiibbrraarryy pprreeddiiccaatteess


1144..44..11 check

 check/0          Program completeness and consistency
 list_undefined/0 List undefined predicates

 list_autoload/0  List predicates that require autoload
 list_redefined/0 List locally redefined predicates


1144..44..22 lists

 append/3        Concatenate lists
 delete/3        Delete all matching members from a list

 flatten/2       Transform nested list into flat list
 intersection/3  Set intersection
 is_set/1        Type check for a set
 list_to_set/2   Remove duplicates
 member/2        Element is member of a list
 nextto/3        Y follows X in List
 nth0/3          N-th element of a list (0-based)
 nth1/3          N-th element of a list (1-based)

 numlist/3       Create list of integers in interval
 permutation/2   Test/generate permutations of a list
 reverse/2       Inverse the order of the elements in a list
 select/3        Select element of a list
 subset/2        Check subset relation for unordered sets
 subtract/3      Delete elements that do not satisfy condition
 sumlist/2       Add all numbers in a list

 union/3         Union of two sets


1144..44..33 ordsets

 ord_empty/1        Test empty ordered set
 list_to_ord_set/2  Create ordered set

 ord_add_element/3  Add element to ordered set
 ord_del_element/3  Delete element from ordered set
 ord_intersect/2    Test non-empty intersection
 ord_intersection/3 Compute intersection
 ord_disjoint/2     Test empty intersection
 ord_subtract/3     Delete set from set
 ord_union/3        Union of two ordered sets
 ord_union/4        Union and difference of two ordered sets

 ord_subset/2       Test subset
 ord_memberchk/2    Deterministically test membership


1144..44..44 ugraphs

 vertices_edges_to_ugraph/3Create unweighted graph
 vertices/2                Find vertices in graph
 edges/2                   Find edges in graph
 add_vertices/3            Add vertices to graph
 del_vertices/3            Delete vertices from graph
 add_edges/3               Add edges to graph
 del_edges/3               Delete edges from graph
 transpose/2               Invert the direction of all edges

 neighbors/3               Find neighbors of vertice
 neighbours/3              Find neighbors of vertice
 complement/2              Inverse presense of edges
 compose/3
 top_sort/2                Sort graph topologically
 top_sort/3                Sort graph topologically
 transitive_closure/2      Create transitive closure of graph

 reachable/3               Find all reachable vertices
 ugraph_union/3            Union of two graphs


1144..44..55 www_browser

 www_open_url/1 Open a web-page in a browser


1144..44..66 readutil

 read_line_to_codes/2  Read line from a stream
 read_line_to_codes/3  Read line from a stream

 read_stream_to_codes/2Read contents of stream
 read_stream_to_codes/3Read contents of stream
 read_file_to_codes/3  Read contents of file
 read_file_to_terms/3  Read contents of file to Prolog terms


1144..44..77 registry

This library is only available on Windows systems.

 registry_get_key/2        Get principal value of key
 registry_get_key/3        Get associated value of key
 registry_set_key/2        Set principal value of key
 registry_set_key/3        Set associated value of key
 registry_delete_key/1     Remove a key
 shell_register_file_type/4Register a file-type
 shell_register_dde/6      Register DDE action
 shell_register_prolog/1   Register Prolog


1144..44..88 url

 parse_url/2       Analyse or construct a URL
 parse_url/3       Analyse or construct a relative URL

 global_url/3      Make relative URL global
 http_location/2   Analyse or construct location
 www_form_encode/2 Encode or decode form-data


1144..44..99 clp/bounds

 in/2            Define interval for variable
 #>/2            Greater than constraint
 #</2            Less than constraint
 #>=/2           Greater or equal constraint
 #=</2           Less of equal constraint
 #\=/2           Non-equal constraint
 #=/2            Equality constraint
 #<=>/2          Constraint equivalence

 #<=/2           Constraint implication to the left
 #=>/2           Constraint implication to the right
 all_different/1 Constraint all values to be unique
 indomain/1      Enumerate values from domain
 label/1         Solve constraints for variables
 labeling/2      Solve constraints for variables
 lex_chain/1     Constraint on lexicographic ordering

 sum/3           Constraint sum of variables
 tuples_in/2     Symbolic constraints on tuples
 serialized/2    Constraint intervals to be serialized


1144..44..1100 clp/clp_distinct

 all_distinct/1 Demand distinct values
 vars_in/2      Declare domain of variable as set
 vars_in/3      Declare domain of variable as interval


1144..44..1111 clp/simplex

 assignment/2      Solve assignment problem
 constraint/3      Add linear constraint to state
 constraint/4      Add named linear constraint to state
 gen_state/1       Create empty linear program
 maximize/3        Maximize objective function in to linear constraints
 minimize/3        Minimize objective function in to linear constraints
 objective/2       Fetch value of objective function
 shadow_price/3    Fetch shadow price in solved state

 transportation/4  Solve transportation problem
 variable_value/3  Fetch value of variable in solved state


1144..44..1122 clpqr

 entailed/1  Check if constraint is entailed
 inf/2       Find the infimum of an expression
 sup/2       Find the supremum of an expression
 minimize/1  Minimizes an expression
 maximize/1  Maximizes an expression
 bb_inf/3    Infimum of expression for mixed-integer problems
 bb_inf/4    Infimum of expression for mixed-integer problems
 bb_inf/5    Infimum of expression for mixed-integer problems

 dump/3      Dump constraints on variables


1144..44..1133 prologxref

 prolog:called_by/2    (hook) Extend cross-referencer
 xref_built_in/1       Examine defined built-ins
 xref_called/3         Examine called predicates
 xref_clean/1          Remove analysis of source
 xref_current_source/1 Examine cross-referenced sources
 xref_defined/3        Examine defined predicates
 xref_exported/2       Examine exported predicates
 xref_module/2         Module defined by source

 xref_source/1         Cross-reference analysis of source


1144..44..1144 broadcast

 broadcast/1         Send event notification
 broadcast_request/1 Request all agents

 listen/2            Listen to event notifications
 listen/3            Listen to event notifications
 unlisten/1          Stop listening to event notifications
 unlisten/2          Stop listening to event notifications
 unlisten/3          Stop listening to event notifications
 listening/3         Who is listening to event notifications?


Bibliography

[Anjewierden & Wielemaker, 1989]  A.  Anjewierden  and  J.   Wielemaker.
                                  Extensible  objects.  ESPRIT   Project
                                  1098 Technical Report  UvA-C1-TR-006a,
                                  University of Amsterdam, March 1989.

[BIM, 1989]                       _B_I_M  _P_r_o_l_o_g  _r_e_l_e_a_s_e  _2_._4.   Everberg,
                                  Belgium, 1989.

[Bowen & Byrd, 1983]              D. L. Bowen and L. M. Byrd. A portable
                                  Prolog compiler.  In  L.  M.  Pereira,
                                  editor,  _P_r_o_c_e_e_d_i_n_g_s   _o_f  _t_h_e   _L_o_g_i_n
                                  _P_r_o_g_r_a_m_m_i_n_g  _W_o_r_k_s_h_o_p  _1_9_8_3,  Lisabon,
                                  Portugal, 1983.  Universidade nova  de
                                  Lisboa.

[Bratko, 1986]                    I. Bratko. _P_r_o_l_o_g _P_r_o_g_r_a_m_m_i_n_g _f_o_r  _A_r_-
                                  _t_i_f_i_c_i_a_l _I_n_t_e_l_l_i_g_e_n_c_e. Addison-Wesley,
                                  Reading, Massachusetts, 1986.

[Butenhof, 1997]                  David R.  Butenhof.  _P_r_o_g_r_a_m_m_i_n_g  _w_i_t_h
                                  _P_O_S_I_X _t_h_r_e_a_d_s.  Addison-Wesley,  Read-
                                  ing, MA, USA, 1997.

[Clocksin & Melish, 1987]         W.  F.  Clocksin  and  C.  S.  Melish.
                                  _P_r_o_g_r_a_m_m_i_n_g   _i_n  _P_r_o_l_o_g.    Springer-
                                  Verlag, New York,  Third, Revised  and
                                  Extended edition, 1987.

[Demoen, 2002]                    Bart Demoen. Dynamic attributes, their
                                  hProlog implementation,  and  a  first
                                  evaluation. Report CW 350,  Department
                                  of  Computer   Science,    K.U.Leuven,
                                  Leuven,  Belgium,  oct  2002.   URL  =
                                  http://www.cs.kuleuven.ac.be/publicaties/rapporten/cw/CW350.abs.html.

[Deransart _e_t _a_l_., 1996]          P.  Deransart,    A.  Ed-Dbali,    and
                                  L. Cervoni.  _P_r_o_l_o_g_:    _T_h_e  _S_t_a_n_d_a_r_d.
                                  Springer-Verlag, New York, 1996.

[Fr"uhwirth, ]                    T.   Fr"uhwirth.   Thom    Fruehwirth's
                                  constraint   handling    rules    web-
                                  site.       http://www.informatik.uni-
                                  ulm.de/pm/mitarbeiter/fruehwirth/chr-
                                  intro.html.

[Fr"uhwirth, 1998]                T.  Fr"uhwirth.  Theory  and   Practice
                                  of  Constraint  Handling   Rules.   In
                                  P. Stuckey  and K.  Marriot,  editors,
                                  _S_p_e_c_i_a_l  _I_s_s_u_e  _o_n  _C_o_n_s_t_r_a_i_n_t   _L_o_g_i_c
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[Graham _e_t _a_l_., 1982]             Susan L. Graham, Peter B. Kessler, and
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[Hodgson, 1998]                   Jonathan     Hodgson.       validation
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[O'Keefe, 1990]                   R. A.  O'Keefe. _T_h_e  _C_r_a_f_t _o_f  _P_r_o_l_o_g.
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                                  1997.

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                                  Massachusetts, 1986.

                                  1324


Index

'MANUAL' _l_i_b_r_a_r_y, 43                 unix/1, 21, 557
--disable-segv-handling, 1044        unknown/2, 63, 626, 681, 1074
-lpl _l_i_b_r_a_r_y, 1045                   unlisten/1, 1272
-lreadline _l_i_b_r_a_r_y, 1073             unlisten/2, 1273
.pl, 91                              unlisten/3, 1274
.pro, 91                             unload_foreign_library/1, 833
?=/2, 195                            unsetenv/1, 554, 555
=:=/2, 445                           upcase_atom/2, 414, 418
/\/2, 479                            update view, 260
=\=/2, 444                           URL, 550
|/2, 204                             url _l_i_b_r_a_r_y, 1170, 1181, 1317
#=/2, 1196                           use_module/1, 62, 93, 672
#>=/2, 1194                          use_module/2, 63, 673
#>/2, 1192                           use_module/[1
#=</2, 1195                             2], 53, 60, 106, 123,  124, 126,
#</2, 1193                                 671, 674, 676, 690, 1299
#\=/2, 1197                          user profile file, 39
,/2, 202                             UTF-8, 76
{}/1, 1221                           utf-8, 122
!/0, 201
!/1, 236                             var/1, 12, 166, 701, 863
/, 92                                variable_value/3, 1252
//2, 454                             vars_in/2, 1235, 1236
./2, 463                             vars_in/3, 1235, 1237
=/2, 185                             verbose, 41
==/2, 183                            vertices/2, 1124
#<=>/2, 1215                         vertices_edges_to_ugraph/3, 1123
>=/2, 443                            visible/1, 625, 1283
>/2, 440                             volatile/1, 278, 780, 1062
^/2, 494
///2, 457                            wait_for_input/3, 297, 316
->/2, 205                            when _l_i_b_r_a_r_y, 714
=</2, 442                            when/2, 75, 714
#<=/2, 1214                          wildcard_match/2, 665
<</2, 477                            win_exec/2, 546, 549, 550
</2, 441                             win_has_menu/0, 570
-/1, 450                             win_insert_menu/2, 570--572
-/2, 452, 1207                       win_insert_menu_item/4, 570, 572
\=/2, 187                            win_registry_get_value/3, 552
\/1, 481                             win_shell/2, 546, 550, 551, 1171
\==/2, 184                           win_window_pos/1, 569
\+/1, 207                            Window interface, 8
\//2, 478                            window_title/2, 568
+/2, 451, 1205                       Windows, 7
**/2, 493                            windows, 60
#=>/2, 1213                          with_mutex/2, 784, 785, 804
>>/2, 476                            with_output_to/2,   310,  314,   362,
;/2, 203                                   535, 565
*->/2, 206                           working_directory/2, 557, 585,  594,
=@=/2, 188                                 595
\=@=/2, 189                          write(), 962
@>=/2, 193                           write/1, 60,  75,  367,  371,  405,
@>/2, 192                                  410, 529, 534, 881, 919, 962
*/2, 453, 1206                       write/2, 368
@=</2, 191                           write_canonical/1, 365, 534
@</2, 190                            write_canonical/2, 366, 388
=../2, 386                           write_canonical/[1
\, 92                                   2], 17
_PL_get_arg(), 893                   write_term/2,  51,  60,  186,  363--
64-bits                                    365,  388,  422,   529,  534,
   platforms, 85                     write_706term/3, 60, 62, 364, 387

abolish/1, 17, 241, 242              write_term/[2
abolish/2, 242, 833                     3], 17, 60
abolish/[1                           writef/1, 528
   2], 60                            writef/2, 32, 71, 362, 529, 530
abort/0, 41, 51, 60, 286, 297, 598,  writef/[1
      739, 788, 1021, 1031              2], 525
abs/1, 459, 1211                     writeln/1, 527
absolute_file_name/2, 32,  124, 136, writeq/1, 369, 529, 534
      584, 586, 594, 1067            writeq/2, 370
absolute_file_name/3, 60,  62,  116, www_browser _l_i_b_r_a_r_y, 1314
      123,   133,  584,  585,  831,  www_form_encode/2, 1186
      1016, 1017, 1168, 1169         www_open_url/1, 1171
absolute_file_name/[2
   3], 60, 132, 585                  X-Windows, 7
access_file/2, 60, 574, 585          X11, 8
acos/1, 487                          xor/2, 480
acquire(), 959                       XPCE, 8
acyclic_term/1, 75, 179, 180         xref_built_in/1, 1264
add_edges/3, 1128                    xref_called/3, 1261
add_import_module/3, 692, 693        xref_clean/1, 1259
add_nb_set/2, 1142                   xref_current_source/1, 1258
add_nb_set/3, 1142--1144             xref_defined/3, 1260
add_vertices/3, 1126                 xref_exported/2, 1262
agent, 1267                          xref_module/2, 1263
all_different/1, 1200                xref_source/1, 1257
all_distinct/1, 1235, 1238, 1240
Alpha
   DEC, 16
AMD64, 86
append/1, 299, 303
append/3, 407, 1076
apply/2, 211
apropos/1, 45, 46, 62, 1292
arg/3, 385
arithmetic_function/1,   501,   502,
      997
asin/1, 486
assert/1, 123,  131, 240, 246--249,
      260,   264,  265,  278,  678,
      697, 716, 780, 1299
assert/2, 249--251, 258, 281
asserta/1, 41, 131, 247
asserta/2, 250, 258
assertion/1, 1162
assertions, 1162
assertz/1, 248, 1299
assertz/2, 251, 258
assignment/2, 1242
assoc _l_i_b_r_a_r_y, 1109, 1140
assoc_to_list/2, 1110
at_end_of_stream/0, 357
at_end_of_stream/1, 358, 361
at_end_of_stream/[0
   1], 291, 1164
at_halt/1,   143,  144,   599,  760,
      1033, 1042
at_initialization/1, 143, 145, 1038
atan/1, 488
atan/2, 489
atom/1, 173, 863
atom_chars/2,  60,  122,  314,  342,
      397, 399, 401, 415
atom_codes/2,   21,  60,  122,  314,
      397--399, 403, 415
atom_concat/3, 407, 408, 426
atom_length/2, 60, 410, 425
atom_number/2, 314, 403
atom_prefix/2, 411
atom_to_term/3, 406
atomic/1, 175
attach_console/0, 793--795, 1021
attr_portray_hook/2, 363, 706
attr_unify_hook/2, 700, 705
attvar/1, 701
autoload/0, 128, 1059, 1061, 1154

b_getval/2, 718, 720
b_setval/2, 716, 717, 1288
backcomp _l_i_b_r_a_r_y, 17, 21
bagof/3, 75, 141, 515--517
bb_inf/3, 1229
bb_inf/4, 1228
bb_inf/5, 1227
between/3, 436, 1088
bits
   64, 84
blackboard, 1267
block/3, 232, 233
BOM, 78
bounds _l_i_b_r_a_r_y, 1187
break/0, 41, 51, 597, 739, 1021
broadcast, 1267
broadcast _l_i_b_r_a_r_y, 1267, 1323
broadcast/1, 1268, 1269
broadcast_request/1, 1269
Byte Order Mark, 78
byte_count/2, 295, 317
call/1,  21,  115, 141,  177,  209,
      211,   219,  617,  640,  642,
      712, 975
call/2, 210
call/[2-6], 210
call_cleanup/2, 216, 217, 314
call_cleanup/3, 216, 784
call_cleanup/[2-3], 785
call_shared_object_function/2, 829
call_with_depth_limit/3, 215
callable/1, 177, 1256
catch/3,  17,  19,  218--221,  232,
      321,   378,  640,  754,  762,
      763, 1280, 1286--1288
ceil/1, 475
ceiling/1, 474, 475
char_code/2, 122, 400
char_conversion/2, 60, 432, 433
char_type/2, 73, 414--417
character set, 70
character_count/2, 295, 317, 318
chdir/1, 594, 595
check _l_i_b_r_a_r_y, 130, 278, 1151, 1310
check/0, 128, 1151, 1152
check_old_select/0, 22
checkselect _l_i_b_r_a_r_y, 22
chr _l_i_b_r_a_r_y, 736, 740
chr_constraint/1, 734, 747
chr_leash/1, 743
chr_notrace/0, 739, 742
chr_option/2, 730, 747
chr_show_store/1, 744
chr_trace/0, 739, 741
chr_type/1, 735
clause/2, 280, 281
clause/3, 249, 258, 281, 283, 1063
clause/[2
   3], 60
clause_property/2, 136, 283, 1279
clib
   package, 1042
close/1, 284, 289
close/2, 290
close_dde_conversation/1, 653
close_shared_object/1, 828
clp/bounds _l_i_b_r_a_r_y, 1187, 1318
clp/clp_distinct   _l_i_b_r_a_r_y,    1234,
      1319
clp/simplex _l_i_b_r_a_r_y, 1320
clpqr _l_i_b_r_a_r_y, 1219, 1321
code_type/2, 413, 415, 416
collate, 419
collation_key/2, 420, 421, 556
COM, 957
commandline
   arguments, 41
compare
   language-specific, 419
compare(), 961
compare/3, 75, 194, 511, 1007
compile_predicates/1, 264, 265
compiling/0, 146, 152
complement/2, 1133
completion
   TAB, 98
compose/3, 1134
compound/1, 176
concat_atom/2, 408, 409
concat_atom/3, 409
constraint/3, 1243, 1244
constraint/4, 1244
constraint_add/4, 1245
consult/1,  36,  39, 53,  62,  106,
      123--126,   151,   152,  266,
      376, 627
context_module/1, 688, 981
convert_time/2, 60
convert_time/[2
   8], 583
copy_stream_data/2, 360, 361
copy_stream_data/3, 359
copy_term/2, 75, 391, 396, 710, 718
copy_term_nat/2, 710
cos/1, 484
cputime/0, 497
ctype _l_i_b_r_a_r_y, 413
current_arithmetic_function/1, 503
current_atom/1, 271
current_blob/2, 272, 958
current_char_conversion/2, 432, 433
current_flag/1, 274
current_foreign_library/2, 834
current_format_predicate/2, 538
current_functor/2, 273
current_input/1, 137, 304, 311
current_key/1, 275
current_module/1, 686
current_module/2, 687
current_mutex/3, 789
current_op/3, 430
current_output/1, 305, 312
current_predicate/1, 277
current_predicate/2, 276, 277
current_prolog_flag/2, 21,  41,  59,
      60,  63,  67,  71, 122,  124,
      182,   221,  224,  362,  367,
      378,   601,  627,  658,  823,
      835,  956, 1036,  1045, 1074,
      1299
current_signal/3, 229, 230
current_stream/3, 292
current_thread/2,   756,  757,  760,
      763
cyclic terms, 75
cyclic_term/1, 75, 179, 180

daemon, 1267
date_time_stamp/2, 563
date_time_value/3, 559, 564
DCG, 123, 237
dde_current_connection/2, 661
dde_current_service/2, 660
dde_execute/2, 655
dde_poke/4, 656
dde_register_service/2, 658
dde_request/3, 654
dde_unregister_service/1, 659
debug _l_i_b_r_a_r_y, 118, 1156
debug/0,  51, 221,  616, 618,  619,
      796, 1021
debug/1, 1157, 1159, 1160
debug/3, 1156, 1157
debugging
   exceptions, 221
debugging/0,  62,  616, 620,  1161,
      1291
debugging/1, 1156, 1158
DEC
   Alpha, 16
default_module/2, 695
del_attr/2, 704, 707
del_edges/3, 1129
del_vertices/3, 1127
delete/3, 1079
delete_directory/1, 593
delete_file/1, 580
delete_import_module/2, 692--694
deterministic/1, 1281
Development environment, 88
dif _l_i_b_r_a_r_y, 715
dif/2, 75, 195, 714, 715
discontiguous/1, 263, 267
display/1, 529, 919
display/[1
   2], 17
displayq/[1
   2], 17
downcase_atom/2, 414, 417, 418
dump/3, 1230
dup/2, 21
dup_stream/2, 21
duplicate_term/2,   75,  391,   394,
      396, 719
dwim_match/2, 279, 663, 664
dwim_match/3, 664
dwim_predicate/2, 279
dynamic/1, 60,  240, 263--265, 278,
      684, 779, 780, 1153

e/0, 496
edges/2, 1125
edit/0, 155
edit/1, 23,  60, 62, 98,  101, 106,
      119,   130,   153--156,  158,
      1153
edit_source/1, 159
editor _c_l_a_s_s, 97, 102
Emacs, 42
emacs/[0
   1], 101
emacs/prolog_colour _l_i_b_r_a_r_y, 105
emacs/swi_prolog _l_i_b_r_a_r_y, 23
empty_assoc/1, 1111
empty_nb_set/1, 1141
encoding/1, 77, 129
ensure_loaded/1, 53, 123, 126, 672
entailed/1, 1222
erase/1, 249, 252, 258, 281
eval_license/0, 1304--1306
exception/3, 716, 1287, 1288
exceptions
   debugging, 221
exists_directory/1, 579
exists_file/1, 60, 575
exit/2, 233, 234
exp/1, 492
expand_answer/2, 602, 603
expand_file_name/2,  60,  124,  554,
      557, 584, 585, 588
expand_file_search_path/2, 133
expand_goal/2, 60, 139, 141, 142
expand_query/4, 602
expand_term/2, 139--141, 237
explain/1, 47
explain/2, 48
export/1, 681, 690
export_list/2, 691

fail/0, 198
fail/1, 235
file_base_name/2, 577
file_directory_name/2, 576
file_name_extension/3, 587
file_search_path/2, 39, 41,  60, 63,
      66,  93, 116, 124,  125, 132,
      134,  831, 1017,  1047, 1064,
      1067, 1070
fileerrors/2, 60, 321
findall/3, 75, 141, 197, 515, 697
flag/3, 259, 274, 394
flatten/2, 1086
float/1, 169, 467
float_fractional_part/1, 470
float_integer_part/1, 471
floor/1, 473
flush_output/0, 335
flush_output/1, 225, 336, 361
flush_output/[0
   1], 286, 337
forall/2, 141, 524
format/1, 225, 533
format/2, 534, 535, 1157
format/3, 225, 314,  362, 447, 448,
      534, 535
format/[1
   2], 60, 362, 525
format/[2
   3], 71
format_predicate/2, 537
format_time/3, 556, 565
freeze/2, 711, 712, 714
frozen/2, 713
FTP, 1181
functor/3, 6, 176, 177, 384, 724

garbage_collect/0, 645
garbage_collect_atoms/0, 646, 1034
garbage_collect_clauses/0, 148--150
gdebug/0, 112
gen_assoc/3, 1112
gen_nb_set/2, 1144
gen_state/1, 1246
gensym _l_i_b_r_a_r_y, 1147
gensym/2, 1148
get/1, 346
get/2, 347
get0/1, 286, 344, 345
get0/2, 345
get_assoc/3, 1113
get_assoc/5, 1114
get_attr/3, 703, 707
get_attrs/2, 708, 709
get_byte/1, 338, 344, 348
get_byte/2, 339, 343, 349
get_byte/[1
   2], 122
get_char/1, 340, 342, 352
get_char/2, 343, 353
get_char/[1
   2], 122
get_code/1, 340, 344, 350, 354, 356
get_code/2, 77,  341, 343, 345, 351,
      359, 361
get_code/[1
   2], 122
get_single_char/1, 41, 60, 356
get_time/1, 561, 583
getenv/2, 553, 554, 1171
global_url/3, 1184
GMP, 447
GNU-Emacs, 42
goal_expansion/2,   123,  139,  141,
      142, 1156
Graphics, 8
ground/1, 75, 178, 262, 867
gspy/1, 113
gtrace/0, 111, 794
GUI, 8
guitracer/0,  23,   108--110,  119,
      609, 613
gxref/0, 116, 1256

halt/0, 51, 599, 600
halt/1, 600, 1021
halt/[0
   1], 144
hash/1, 268, 269
hash_term/2, 75, 261, 262, 268, 269
help/0, 44, 62, 1065, 1292
help/1, 43--45, 60, 62, 1292
helpidx _l_i_b_r_a_r_y, 43
hooks, 62
html_write _l_i_b_r_a_r_y, 1266
HTTP, 1181
http_load _l_i_b_r_a_r_y, 124, 1294
http_location/2, 1185
IA32, 86
IDE, 88
ignore/1, 141, 214, 640, 754
immediate
   update view, 260
import/1, 671, 674, 681
import_module/2, 692, 693, 695
in/2, 1189, 1190
in_pce_thread/1, 815, 816
include/1, 123, 124, 127, 137
index/1, 261, 268, 269, 278
indomain/1, 1201
inf/2, 1223
infinite trees, 75
initialization/1,  145,  716,  759,
      824, 1063
integer
   unbounded, 447
integer/1, 168, 466
interactor/0, 297, 793
internationalization, 76
intersection/3, 1092
is/2, 446, 447, 467, 502, 747, 1087
is_absolute_file_name/1, 586
is_list/1, 505, 1090
is_set/1, 1090
is_stream/1, 293
iso latin 1, 70

Java, 957
join_threads/0, 792

keysort/2, 510, 511

label/1, 1202
labeling/2, 1203
last/2, 1083
leash/1, 51, 624, 625, 743, 1283
length/2, 507
lex_chain/1, 1199
library_directory/1,  63,  66,  125,
      131
license/1, 1307
license/2, 1306, 1307
limit_stack/2, 644, 647
line_count/2, 295, 297, 319, 541
line_position/2, 295, 297, 320, 541
list_autoload/0, 1152--1154
list_debug_topics/0, 1156, 1161
list_redefined/0, 1152, 1155
list_to_assoc/2, 1115
list_to_ord_set/2, 1098
list_to_set/2, 1091
list_undefined/0, 130, 1152--1154
listen/2, 1270, 1271
listen/3, 1271--1274
listening/3, 1275
listing/0, 162
listing/1, 51, 161, 162
lists _l_i_b_r_a_r_y, 504, 1075, 1311
load_files/2, 60,  62, 77, 124, 125,
      1256, 1293, 1294
load_foreign_library/1,  832,  1045,
      1064
load_foreign_library/2, 831, 832
load_foreign_library/[1
   2], 132, 826
locale, 419
locale_sort/2, 420, 421, 556
log/1, 490
log10/1, 491
logical
   update view, 260
lsb/1, 499

MacOS X, 7
make/0,  7, 63,  66, 99,  106, 119,
      124, 130, 148
make_directory/1, 592
make_library_index/1, 63, 64
make_library_index/2, 63, 65
make_library_index/[1
   2], 66
manpce/0, 80
map_assoc/2, 1116
map_assoc/3, 1117
maplist/2, 519
maplist/3, 520, 679, 1061
maplist/4, 521
max/2, 461, 462, 1208
max_assoc/3, 1118
maximize/1, 1226
maximize/3, 1247, 1248
member/2, 51,  216, 282,  506, 585,
      1077, 1080
memberchk/2, 506, 1108
memory
   layout, 81
merge/3, 512
merge_set/3, 513
message
   service, 1267
message_hook/3, 20, 223--226
message_queue_create/1,  768,   772,
      773
message_queue_destroy/1, 773
message_queue_size/2, 776
message_to_string/2, 224, 226, 227
meta_predicate/1, 685, 697, 698
min/2, 462, 1209
min_assoc/3, 1119
minimize/1, 1225
minimize/3, 1248
mod/2, 455, 1210
module/1, 696
module/2, 139, 428, 670, 683, 690
module_transparent/1,    278,   684,
      697, 981, 1299
msb/1, 497, 498
msort/2, 509, 510
multifile/1,  153,  263, 266,  278,
      1153, 1290, 1299
mutex_create/1, 782, 784, 785
mutex_destroy/1, 783
mutex_lock/1, 785, 786
mutex_statistics/0, 765
mutex_trylock/1, 786
mutex_unlock/1, 787
mutex_unlock_all/0, 788
name/2, 397, 404
name_of/2, 1271
nb_current/2, 722
nb_delete/1, 723
nb_getval/2, 720
nb_linkarg/3, 394--396
nb_linkval/2, 395, 396, 721, 724
nb_set _l_i_b_r_a_r_y, 1140
nb_set_to_list/2, 1146
nb_setarg/3, 393--396, 1140
nb_setval/2,  394,  396,  716,  719,
      721, 724, 1288
nbset _l_i_b_r_a_r_y, 1140
neighbors/3, 1132
neighbours/3, 1131, 1132
netscape _l_i_b_r_a_r_y, 1170
nextto/3, 1078
nl/0, 323
nl/1, 324
nl/[0
   1], 529
nodebug/0, 619
nodebug/1, 1159, 1160
noguitracer/0, 108, 110, 119, 614
nonvar/1, 167
noprofile/1, 640
noprotocol/0, 607
nospy/1, 51, 62, 622, 801, 1291
nospyall/0, 62, 623, 1291
not/1, 141, 212
notrace/0, 612, 741, 742
notrace/1, 617
nth0/3, 1081
nth1/3, 1082
nth_clause/3, 282, 283, 1279
number
   rational, 447
number/1, 172, 1087
number_chars/2, 21, 122, 401, 402
number_codes/2,  21, 122,  397, 402,
      403
numbervars/3, 363, 387, 388
numbervars/4, 365, 387, 388
numbervars/[3
   4], 75
numlist/3, 1088

objective/2, 1249
on_signal/3, 19, 229, 230
once/1,  141, 213,  214, 216,  229,
      314, 617, 631, 784, 986
op/3, 263, 363, 429, 430
open/3, 60, 284, 285, 287
open/4, 13, 77,  78, 122, 286, 287,
      291,  296,  297,  358,  1168,
      1169
open_dde_conversation/3, 652
open_null_stream/1, 288
open_resource/3,   18,  1058,  1065,
      1068
open_shared_object/2, 60,  823, 826,
      827
open_shared_object/3, 827
operator
   and modules, 428
ord_add_element/3, 1099
ord_del_element/3, 1100
ord_disjoint/2, 1103
ord_empty/1, 1097
ord_intersect/2, 1101, 1103
ord_intersection/3, 1102
ord_list_to_assoc/2, 1120
ord_memberchk/2, 1108
ord_subset/2, 1107
ord_subtract/3, 1104
ord_union/3, 1105
ord_union/4, 1106
ordsets _l_i_b_r_a_r_y, 1096, 1312
oset _l_i_b_r_a_r_y, 1096

package
   clib, 1042
parse_time/2, 566
parse_url/2, 1182, 1183, 1185
parse_url/3, 1183
pce_call/1, 816, 818
pce_dispatch/1, 816, 817
pce_xref _l_i_b_r_a_r_y, 115
peek_byte/1, 348
peek_byte/2, 349
peek_byte/[1
   2], 122
peek_char/1, 352
peek_char/2, 353
peek_char/[1
   2], 122
peek_code/1, 350
peek_code/2, 351
peek_code/[1
   2], 122
permutation/2, 1085
phrase/2, 237, 238
phrase/3, 237, 239
pi/0, 495
PL_abort_hook(), 1031
PL_abort_unhook(), 1032
PL_action(), 1021
PL_agc_hook(), 1034
PL_atom_chars(), 856
PL_atom_nchars(), 907
PL_atom_wchars(), 910
PL_blob_data(), 969
PL_BLOB_NOCOPY, 958
PL_BLOB_TEXT, 958
PL_BLOB_UNIQUE, 958
PL_call(), 986
PL_call_predicate(), 985
PL_chars_to_term(), 955
PL_cleanup(), 1042
PL_close_foreign_frame(), 989
PL_close_query(), 984
PL_compare(), 1007
PL_cons_functor(), 935
PL_cons_functor_v(), 936
PL_cons_list(), 937
PL_context(), 993
PL_copy_term_ref(), 840
PL_create_engine(), 811
PL_cut_query(), 983
PL_destroy_engine(), 812
PL_discard_foreign_frame(), 990
PL_dispatch_hook(), 1030
PL_erase(), 1012
PL_erase_external(), 1015
PL_exception(), 1000
PL_fail(), 847
PL_foreign_context(), 852
PL_foreign_context_address(), 853
PL_foreign_control(), 851
PL_free(), 1053
PL_functor_arity(), 859
PL_functor_name(), 858
PL_get_arg(), 892
PL_get_atom(), 878
PL_get_atom_chars(), 879
PL_get_atom_nchars(), 895
PL_get_blob(), 968
PL_get_bool(), 886
PL_get_chars(), 881
PL_get_file_name(), 1017
PL_get_float(), 888
PL_get_functor(), 889
PL_get_head(), 916
PL_get_int64(), 885
PL_get_integer(), 883
PL_get_list(), 915
PL_get_list_chars(), 882
PL_get_list_nchars(), 896
PL_get_long(), 884
PL_get_module(), 891
PL_get_mpq(), 972
PL_get_mpz(), 971
PL_get_name_arity(), 890
PL_get_nchars(), 897
PL_get_nil(), 918
PL_get_pointer(), 887
PL_get_string_chars(), 880
PL_get_tail(), 917
PL_get_wchars(), 911
PL_halt(), 1043
PL_handle_signals(), 1004
PL_initialise(), 1038
PL_install_readline(), 1040
PL_is_atom(), 868
PL_is_atomic(), 875
PL_is_blob(), 965
PL_is_compound(), 872
PL_is_float(), 871
PL_is_functor(), 873
PL_is_ground(), 867
PL_is_initialised(), 1039
PL_is_integer(), 870
PL_is_list(), 874
PL_is_number(), 876
PL_is_string(), 869
PL_is_variable(), 866
PL_license(), 1308
PL_malloc(), 1051
PL_module_name(), 995
PL_new_atom(), 855
PL_new_atom_nchars(), 906
PL_new_atom_wchars(), 909
PL_new_functor(), 857
PL_new_module(), 996
PL_new_term_ref(), 838
PL_new_term_refs(), 839
PL_next_solution(), 982
PL_on_halt(), 1033
PL_open_foreign_frame(), 988
PL_open_query(), 981
PL_pred(), 977
PL_predicate(), 978
PL_predicate_info(), 979
PL_put_atom(), 922
PL_put_atom_chars(), 923
PL_put_atom_nchars(), 898
PL_put_blob(), 967
PL_put_float(), 930
PL_put_functor(), 931
PL_put_int64(), 928
PL_put_integer(), 927
PL_put_list(), 932
PL_put_list_chars(), 926
PL_put_list_nchars(), 901
PL_put_list_ncodes(), 900
PL_put_nil(), 933
PL_put_pointer(), 929
PL_put_string_chars(), 924
PL_put_string_nchars(), 899, 925
PL_put_term(), 934
PL_put_variable(), 921
PL_query(), 1023
PL_quote(), 956
PL_raise(), 1003
PL_raise_exception(), 998
PL_realloc(), 1052
PL_record(), 1010
PL_record_external(), 1013
PL_recorded(), 1011
PL_recorded_external(), 1014
PL_register_atom(), 861
PL_register_extensions(), 1028
PL_register_extensions_in_module(),
      1027
PL_register_foreign(), 1026
PL_register_foreign_in_module(), 1025
PL_reset_term_refs(), 841
PL_retry(), 849
PL_retry_address(), 850
PL_rewind_foreign_frame(), 991
PL_same_compound(), 1008
PL_set_engine(), 813
PL_signal(), 1002
PL_strip_module(), 994
PL_succeed(), 846
PL_term_type(), 865
PL_thread_at_exit(), 809
PL_thread_attach_engine(), 807
PL_thread_destroy_engine(), 808
PL_thread_self(), 806
PL_throw(), 999
PL_toplevel(), 1041
PL_unify(), 939
PL_unify_arg(), 953
PL_unify_atom(), 940
PL_unify_atom_chars(), 942
PL_unify_atom_nchars(), 902
PL_unify_blob(), 966
PL_unify_chars(), 941
PL_unify_float(), 948
PL_unify_functor(), 950
PL_unify_int64(), 947
PL_unify_integer(), 946
PL_unify_list(), 951
PL_unify_list_chars(), 943
PL_unify_list_nchars(), 905
PL_unify_list_ncodes(), 904
PL_unify_mpq(), 974
PL_unify_mpz(), 973
PL_unify_nil(), 952
PL_unify_pointer(), 949
PL_unify_string_chars(), 944
PL_unify_string_nchars(), 903, 945
PL_unify_term(), 954
PL_unify_wchars(), 912
PL_unify_wchars_diff(), 913
PL_unregister_atom(), 862
PL_unregister_blob_type(), 963
PL_warning(), 1019
plus/3, 210, 438
PLVERSION, 1054
popcount/1, 500
portray/1, 51,  94, 362,  363, 371,
      373, 981, 1035, 1290
portray_clause/1, 161, 163, 164
portray_clause/2, 163, 164, 388
predicate_property/2, 269, 278, 691
predsort/3, 511
preprocessor/2, 147
print/1, 363,  371, 373,  529, 534,
      981
print/2, 372
print/[1
   2], 362
print_message/2,  20,  60, 62,  124,
      144,   223--226,   378,  754,
      756, 1157
print_message_lines/3, 20, 224--226
profile file, 39
profile/1, 634, 635, 802
profile/3, 635, 1073
profiler/2, 638
prolog/0,  41, 60,  298, 597,  601,
      602, 696, 1041, 1281
prolog:called_by/2, 1266
prolog:comment_hook/3, 1295
prolog:debug_control_hook/1, 1291
prolog:help_hook/1, 1292
prolog_choice_attribute/3,     1280,
      1283
prolog_current_frame/1, 1278, 1279
prolog_edit:edit_command/2, 159
prolog_edit:edit_source/1, 158
prolog_edit:load/0, 160
prolog_edit:locate/2, 157
prolog_edit:locate/3, 156
prolog_exception_hook/4, 1285, 1286
prolog_file_type/2, 123, 134, 585
prolog_frame_attribute/3, 283, 1279,
      1283, 1286
prolog_ide _c_l_a_s_s, 118
prolog_ide/1, 117, 118
prolog_list_goal/1, 1290
prolog_load_context/2, 137, 138
prolog_load_file/2, 124, 1293, 1294
prolog_server _l_i_b_r_a_r_y, 298
prolog_skip_level/2, 1284
prolog_stack _l_i_b_r_a_r_y, 1286
prolog_to_os_filename/2,   92,   586,
      589
prolog_trace_interception/4,    107,
      613, 1279, 1280, 1283
prolog_xref _l_i_b_r_a_r_y, 115
prologxref _l_i_b_r_a_r_y, 1256, 1322
prompt/2, 380--382
prompt1/1, 382
protocol/1, 605, 606, 608
protocola/1, 606, 608
protocolling/1, 608
put/1, 325, 326
put/2, 326
put_assoc/4, 1121
put_attr/3, 701, 702, 705, 707
put_attrs/2, 709
put_byte/1, 327
put_byte/2, 328
put_byte/[1
   2], 122
put_char/1, 325, 329, 331
put_char/2, 330
put_char/[1
   2], 122
put_code/1, 325, 331, 332
put_code/2, 77, 332, 359, 361
put_code/[1
   2], 122

qcompile/1, 124, 146, 151, 152, 749
qsave_program/1, 1060
qsave_program/2,  14,  58,  60,  94,
      117, 1058, 1059, 1065
qsave_program/[1
   2],  13,  18, 41,  57, 60,  145,
      278,  824, 1038,  1047, 1061,
      1063
quiet, 41, 224
quintus _l_i_b_r_a_r_y, 21, 685, 698

random/1, 464
rational
   number, 447
rational/1, 170, 447, 448, 468, 469
rational/3, 171, 447
rationalize/1, 447,  448, 468, 469,
      1252
RDF
   memory usage, 87
rdiv/2, 447, 448, 454, 458
reachable/3, 1139
read/1, 60, 82, 286, 335, 369, 374,
      376,   378,  381,  422,  627,
      724, 1169
read/2, 316, 375
read_clause/1, 376, 377, 627
read_clause/2, 377
read_file_to_codes/3, 1168
read_file_to_terms/3, 1169
read_history/6, 380
read_line_to_codes/2, 1164, 1165
read_line_to_codes/3, 1164, 1165
read_link/3, 590
read_pending_input/3, 361
read_stream_to_codes/2, 1166, 1167
read_stream_to_codes/3, 1167
read_term/2, 60, 376, 378--380, 406
read_term/3,  378,  379,  432,  602,
      1295
read_term/[2
   3], 378
readutil _l_i_b_r_a_r_y, 96, 1163, 1315
recorda/2, 253
recorda/3, 75, 252,  254, 258, 275,
      716, 1009, 1010, 1013
recorded/2, 257
recorded/3, 256, 258, 1063
recordz/2, 255
recordz/3, 75, 254
redefine_system_predicate/1,     11,
      243, 1299
registry, 80
registry _l_i_b_r_a_r_y, 1172, 1316
registry_delete_key/1, 1177
registry_get_key/2, 1173
registry_get_key/3, 1174
registry_set_key/2, 1175
registry_set_key/3, 1176
release(), 960
reload_library_index/0, 66
rem/2, 456
rename_file/2, 581
repeat/0, 197, 200, 215
require/1, 128, 1061
reset_gensym/0, 1150
reset_gensym/1, 1149
reset_profiler/0, 635, 639
resource/3,  18,  60,  1058,  1059,
      1065, 1067, 1068
retract/1, 123, 131, 240, 241, 244,
      260, 264, 265, 278, 697, 780
retractall/1, 240, 241, 245
reverse/2, 670, 1084
RFC-1738, 1181
rl_add_history/1, 1298
rl_read_init_file/1, 1297
rlimit _l_i_b_r_a_r_y, 25
round/1, 465, 466
same_file/2, 578
see/1, 20, 284, 286, 299--301
seeing/1, 299, 300, 304
seek/4, 291, 294, 296
seen/0, 306
select/3, 22, 1080
serialized/2, 1204
set_input/1, 297, 301, 309
set_output/1, 302, 310
set_prolog_flag/2, 21,  49, 59,  61,
      439
set_prolog_IO/3, 298
set_stream/2,  77,  286,  291,  297,
      298, 316
set_stream_position/2, 291, 294, 296
set_tty/2, 543
setarg/3, 391, 393,  394, 396, 702,
      724
setenv/2, 554
setlocale/3, 421, 556
setof/3, 75, 141, 517
sformat/3, 314
shadow_price/3, 1250
shell/0, 548, 557
shell/1, 92, 159, 547, 557
shell/2, 546
shell/[0-2], 554
shell/[1
   2], 546
shell_register_dde/6, 1179
shell_register_file_type/4,     1178,
      1179
shell_register_prolog/1, 1180
show_profile/1, 635, 637
show_profile/2, 636
sign/1, 460
silent, 224
simplex _l_i_b_r_a_r_y, 1241
sin/1, 483
size_file/2, 582
size_nb_set/2, 1145
skip/1, 354, 355
skip/2, 355
sleep/1, 666
socket _l_i_b_r_a_r_y, 316
Solaris, 761
sort/2, 508,  509, 511,  517, 1091,
      1096, 1098
source_file/1, 135
source_file/2, 136, 152, 278
source_file/[1
   2], 687
source_location/2, 137, 138
spy/1, 51,  60, 62, 108,  109, 119,
      621, 800, 801, 1291
sqrt/1, 482
stack
   memory management, 81
stack_parameter/4, 649
stamp_date_time/3, 559, 562, 563
startup file, 39
statistics _l_i_b_r_a_r_y, 632
statistics/0, 630
statistics/2, 497, 629, 764
stream_position_data/3,  137,   291,
      295, 378, 1295
stream_property/2,   78,  286,  291,
      294--297, 378
string/1, 174, 175, 534
string_concat/3, 407, 426
string_length/2, 425
string_to_atom/2, 423
string_to_list/2, 424
strip_module/3, 689
style_check/1, 74, 82, 83, 267, 627
sub_atom/5, 412, 427
sub_string/5, 427
sublist/3, 522
subset/2, 1095
subtract/3, 1093
succ/2, 437
sum/3, 1198
sumlist/2, 1087
sup/2, 1224
swi/pce_profile _l_i_b_r_a_r_y, 632
swi_edit _l_i_b_r_a_r_y, 160
swi_help _l_i_b_r_a_r_y, 43
swritef/2, 531
swritef/3, 314, 530

TAB
   completion, 98
tab/1, 333
tab/2, 334
tan/1, 485
tdebug/0, 797, 801
tdebug/1, 796, 797, 800
tell/1,  20,  284, 286,  299,  300,
      302, 303
telling/1, 299, 300, 305
term_expansion/2,   62,  123,  139--
      141, 146, 152, 602, 736
term_to_atom/2, 314, 405, 955
term_variables/2, 75, 378, 389, 390
term_variables/3, 389, 390
terms
   cyclic, 75
thread_at_exit/1, 760, 809
thread_create/3, 754, 757, 815, 817
thread_detach/1, 754, 757
thread_exit/1, 756, 758, 763
thread_get_message/1, 770, 774
thread_get_message/2, 774
thread_initialization/1, 716, 759
thread_join/2, 754, 756, 758, 763
thread_local/1, 264, 278, 779, 780
thread_peek_message/1, 770, 771, 775
thread_peek_message/2, 775
thread_self/1, 60, 755, 760, 769
thread_send_message/2, 769, 772
thread_setconcurrency/2, 761
thread_signal/2,   778,  788,   796,
      1003
thread_statistics/3, 764
threads/0, 791
throw/1, 17, 51, 75, 218--220, 232,
      598,   763,  777,  778,  998,
      1000, 1286--1288
time/1, 497, 631, 634, 635
time_file/2, 583
tmp_file/2, 591
tnodebug/0, 799
tnodebug/1, 798, 801
told/0, 307
top_sort/2, 1136
top_sort/3, 1137
tprofile/1, 802, 803
trace/0,  51,  60, 108,  109,  119,
      610,   616,  741,  742,  778,
      1021, 1286
trace/1, 60, 615
trace/2, 616
tracing/0, 611
transitive_closure/2, 1138
transportation/4, 1251
transpose/2, 1130
trim_stacks/0, 60, 644, 645, 648
true/0, 60, 199, 215
truncate/1, 472
tspy/1, 794, 801
tspy/2, 800
tty_get_capability/3, 540, 542, 544
tty_goto/2, 541, 543
tty_put/2, 542, 543
tty_size/2, 544
ttyflush/0, 337, 529
tuples_in/2, 1191, 1217

UCS, 76
ugraph _l_i_b_r_a_r_y, 1122
ugraph_union/3, 1135
ugraphs _l_i_b_r_a_r_y, 1122, 1313
ugraphs.pl _l_i_b_r_a_r_y, 1122
Unicode, 76
unifiable/3, 196
unify_with_occurs_check/2, 186
union/3, 1094
Unix, 7
unix, 60

                                  1325
