\HeaderA{getMethod}{Get or Test for the Definition of a Method}{getMethod}
\aliasA{existsMethod}{getMethod}{existsMethod}
\aliasA{findMethod}{getMethod}{findMethod}
\aliasA{getMethods}{getMethod}{getMethods}
\aliasA{hasMethod}{getMethod}{hasMethod}
\aliasA{MethodsListSelect}{getMethod}{MethodsListSelect}
\aliasA{selectMethod}{getMethod}{selectMethod}
\keyword{programming}{getMethod}
\keyword{classes}{getMethod}
\keyword{methods}{getMethod}
\begin{Description}\relax
The functions \code{getMethod} and \code{selectMethod} get the
definition of a particular method; the functions \code{existsMethod}
and \code{hasMethod} test for the existence of a method.  In both
cases the first function only gets direct definitions and the second
uses inheritance.
The function \code{findMethod} returns the package(s) in the search
list (or in the packages specified by the \code{where} argument) that
contain a method for this function and signature.

The other functions are support functions: see the details below.
\end{Description}
\begin{Usage}
\begin{verbatim}
getMethod(f, signature=character(), where, optional=FALSE, mlist, fdef)

findMethod(f, signature, where)

getMethods(f, where)

existsMethod(f, signature = character(), where)

hasMethod(f, signature=character(), where)

selectMethod(f, signature, optional = FALSE, useInherited = TRUE,
             mlist = (if (is.null(fdef)) NULL else
                      getMethodsForDispatch(f, fdef)),
             fdef = getGeneric(f, !optional))

MethodsListSelect(f, env, mlist, fEnv, finalDefault, evalArgs,
                  useInherited, fdef, resetAllowed)

\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{f}] The character-string name of the generic function.
\item[\code{signature}] the signature of classes to match to the arguments
of \code{f}.  See the details below.

For \code{selectMethod}, the signature can optionally be an
environment with classes assigned to the names of the corresponding
arguments.  Note:  the names correspond to the names of the classes, \emph{not}
to the objects supplied in a call to the generic function.  (You
are not likely to find this approach convenient, but it is used
internally and is marginally more efficient.)

\item[\code{where}] The position or environment in which to look for the
method(s):  by default, anywhere in the current search list.
\item[\code{optional}] If the selection does not produce a unique result,
an error is generated, unless this argument is \code{TRUE}.  In that
case, the value returned is either a \code{MethodsList} object, if
more than one method matches this signature, or \code{NULL} if no
method matches.
\item[\code{mlist}] Optionally, the list of methods in which to search.  By
default, the function finds the methods for the corresponding
generic function.  To restrict the search to a particular package
or environment, e.g., supply this argument as
\code{getMethodsMetaData(f,where)}.  For \code{selectMethod}, see
the discussion of argument \code{fdef}.

\item[\code{fdef}] Optionally, the generic function from which the method is 
to be retrieved.
(Unlikely to be used, except internally in the package source.)
\item[\code{env}] The environment in which argument evaluations are done in
\code{MethodsListSelect}.  Currently must be supplied, but should
usually be \code{sys.frame(sys.parent())} when calling the function
explicitly for debugging purposes.
\item[\code{fEnv, finalDefault, evalArgs, useInherited, resetAllowed}] Internal-use
arguments for the function's environment, the method to use as
the overall default, whether to evaluate arguments, which
arguments should use inheritance, and whether the cached methods
are allowed to be reset.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
The \code{signature} argument specifies classes, in an extended
sense, corresponding to formal arguments of the generic function.
As supplied, the argument may be a vector of strings identifying
classes, and may be named or not.  Names, if supplied, match the
names of those formal arguments included in the signature of the
generic.  That signature is normally all the arguments except
\dots.  However, generic functions can be specified with only a
subset of the arguments permitted, or with the signature taking
the arguments in a different order.

It's a good idea to name the arguments in the signature to avoid
confusion, if you're dealing with a generic that does something
special with its signature.   In any case, the elements of the
signature are matched to the formal signature by the same rules
used in matching arguments in function calls (see
\code{\LinkA{match.call}{match.call}}).

The strings in the signature may be class names, \code{"missing"}
or \code{"ANY"}.  See \LinkA{Methods}{Methods} for the meaning of these in
method selection.  Arguments not supplied in the signature
implicitly correspond to class \code{"ANY"}; in particular, giving
an empty signature means to look for the default method.

A call to \code{getMethod} returns the method for a particular
function and signature.  As with other \code{get} functions,
argument \code{where} controls where the function looks (by default
anywhere in the search list) and argument \code{optional} controls
whether the function returns \code{NULL} or generates an error if
the method is not found.  The search for the method makes no use of
inheritance.

The function \code{selectMethod} also looks for a method given the
function and signature, but makes full use of the method dispatch
mechanism; i.e., inherited methods and group generics are taken into
account just as they would be in dispatching a method for the
corresponding signature, with the one exception that conditional
inheritance is not used.  Like \code{getMethod}, \code{selectMethod}
returns \code{NULL} or generates an error if
the method is not found, depending on the argument \code{optional}.

The functions \code{existsMethod} and \code{hasMethod} return
\code{TRUE} or \code{FALSE} according to whether a method is found,
the first corresponding to \code{getMethod} (no inheritance) and the
second to \code{selectMethod}.

The function \code{getMethods} returns all the methods for a
particular generic (in the form of a generic function with the
methods information in its environment).  The function is called
from the evaluator to merge method information, and is not intended
to be called directly.  Note that it gets \emph{all} the visible
methods for the specified functions.  If you want only the methods
defined explicitly in a particular environment, use the function
\code{\LinkA{getMethodsMetaData}{getMethodsMetaData}} instead.

The function \code{MethodsListSelect} performs a full search
(including all inheritance and group generic information: see the
\LinkA{Methods}{Methods} documentation page for details on how this works).
The call returns a possibly revised methods list object,
incorporating any method found as part of the \code{allMethods}
slot.

Normally you won't call \code{MethodsListSelect} directly, but it is
possible to use it for debugging purposes (only for distinctly
advanced users!).

Note that the statement that \code{MethodsListSelect} corresponds to the
selection done by the evaluator is a fact, not an assertion, in the
sense that the evaluator code constructs and executes a call to
\code{MethodsListSelect} when it does not already have a cached method
for this generic function and signature.  (The value returned is
stored by the evaluator so that the search is not required next
time.)
\end{Details}
\begin{Value}
The call to \code{selectMethod} or \code{getMethod} returns a
\code{\LinkA{MethodDefinition-class}{MethodDefinition.Rdash.class}} object, the selected method, if
a unique selection exists.
(This class extends \code{function}, so you can use the result
directly as a function if that is what you want.)
Otherwise an error is thrown if \code{optional} is \code{FALSE}.  If
\code{optional} is \code{TRUE}, the value returned is \code{NULL} if
no method matched, or a \code{MethodsList} object if multiple
methods matched.

The call to \code{getMethods} returns the \code{MethodsList} object
containing all the methods requested.  If there are none,
\code{NULL} is returned: \code{getMethods} does not generate an
error in this case.
\end{Value}
\begin{References}\relax
The R package \pkg{methods} implements, with a few exceptions, the
programming interface for classes
and methods in the book \emph{Programming with Data} (John
M. Chambers, Springer, 1998), in particular sections 1.6, 2.7, 2.8,
and chapters 7 and 8.

While the programming interface for the \pkg{methods} package follows
the reference, the R software is an original implementation, so
details in the reference that reflect the S4 implementation may appear
differently in R.  Also, there are extensions to the programming
interface developed more recently than the reference.  For a
discussion of details see \code{?\LinkA{Methods}{Methods}}
and the links from that documentation.
\end{References}
\begin{SeeAlso}\relax
\code{\LinkA{GenericFunctions}{GenericFunctions}}
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
setGeneric("testFun", function(x)standardGeneric("testFun"))
setMethod("testFun", "numeric", function(x)x+1)
hasMethod("testFun", "numeric")
## Not run: [1] TRUE
hasMethod("testFun", "integer") #inherited
## Not run: [1] TRUE
existsMethod("testFun", "integer")
## Not run: [1] FALSE
hasMethod("testFun") # default method
## Not run: [1] FALSE
hasMethod("testFun", "ANY")
## Not run: [1] FALSE

\end{ExampleCode}
\end{Examples}

