proplists(3)

MODULE
proplists - Support functions for property lists

DESCRIPTION

Property lists are ordinary lists containing entries in the form of either tuples, whose first elements are keys used for lookup and insertion, or atoms, which work as shorthand for tuples {Atom, true}. (Other terms are allowed in the lists, but are ignored by this module.) If there is more than one entry in a list for a certain key, the first occurrence normally overrides any later (irrespective of the arity of the tuples).

Property lists are useful for representing inherited properties, such as options passed to a function where a user may specify options overriding the default settings, object properties, annotations, etc.

EXPORTS

append_values(Key, List) -> List

    Types
        Key = term()
        List = [term()]

    Similar to get_all_values/2, but each value is wrapped in a list unless it is already itself a list, and the resulting list of lists is concatenated. This is often useful for "incremental" options; e.g., append_values(a, [{a, [1, 2]}, {b, 0}, {a, 3}, {c, -1}, {a, [4]}]) will return the list [1, 2, 3, 4].

compact(List) -> List

    Types
        List = [term()]

    Minimizes the representation of all entries in the list. This is equivalent to [property(P) || P <- List].

    See also: property/1, unfold/1.

delete(Key, List) -> List

    Types
        Key = term()
        List = [term()]

    Deletes all entries associated with Key from List.

expand(Expansions, List) -> List

    Types
        Key = term()
        Expansions = [{Property, [term()]}]
        Property = atom() | tuple()

    Expands particular properties to corresponding sets of properties (or other terms). For each pair {Property, Expansion} in Expansions, if E is the first entry in List with the same key as Property, and E and Property have equivalent normal forms, then E is replaced with the terms in Expansion, and any following entries with the same key are deleted from List.

    For example, the following expressions all return [fie, bar, baz, fum]:
              expand([{foo, [bar, baz]}],
              [fie, foo, fum])
              expand([{{foo, true}, [bar, baz]}],
              [fie, foo, fum])
              expand([{{foo, false}, [bar, baz]}],
              [fie, {foo, false}, fum])

    However, no expansion is done in the following call:
              expand([{{foo, true}, [bar, baz]}],
              [{foo, false}, fie, foo, fum])

    because {foo, false} shadows foo.

    Note that if the original property term is to be preserved in the result when expanded, it must be included in the expansion list. The inserted terms are not expanded recursively. If Expansions contains more than one property with the same key, only the first occurrance is used.

    See also: normalize/2.

get_all_values(Key, List) -> [term()]

    Types
        Key = term()
        List = [term()]

    Similar to get_value/2, but returns the list of values for all entries {Key, Value} in List. If no such entry exists, the result is the empty list.

    See also: get_value/2.

get_bool(Key, List) -> bool()

    Types
        Key = term()
        List = [term()]

    Returns the value of a boolean key/value option. If lookup(Key, List) would yield {Key, true}, this function returns true; otherwise false is returned.

    See also: get_value/2, lookup/2.

get_keys(List) -> [term()]

    Types
        List = [term()]

    Returns an unordered list of the keys used in List, not containing duplicates.

get_value(Key, List) -> term()

    Types
        Key = term()
        List = [term()]

    Equivalent to get_value(Key, List, undefined).

get_value(Key, List, Default) -> term()

    Types
        Key = term()
        Default = term()
        List = [term()]

    Returns the value of a simple key/value property in List. If lookup(Key, List) would yield {Key, Value}, this function returns the corresponding Value, otherwise Default is returned.

    See also: get_all_values/2, get_bool/2, get_value/1, lookup/2.

is_defined(Key, List) -> bool()

    Types
        Key = term()
        List = [term()]

    Returns true if List contains at least one entry associated with Key, otherwise false is returned.

lookup(Key, List) -> none | tuple()

    Types
        Key = term()
        List = [term()]

    Returns the first entry associated with Key in List, if one exists, otherwise returns none. For an atom A in the list, the tuple {A, true} is the entry associated with A.

    See also: get_bool/2, get_value/2, lookup_all/2.

lookup_all(Key, List) -> [tuple()]

    Types
        Key = term()
        List = [term()]

    Returns the list of all entries associated with Key in List. If no such entry exists, the result is the empty list.

    See also: lookup/2.

normalize(List, Stages) -> List

    Types
        List = [term()]
        Stages = [Operation]
        Operation = {aliases, Aliases} | {negations, Negations} | {expand, Expansions}
        Aliases = [{Key, Key}]
        Negations = [{Key, Key}]
        Key = term()
        Expansions = [{Property, [term()]}]
        Property = atom() | tuple()

    Passes List through a sequence of substitution/expansion stages. For an aliases operation, the function substitute_aliases/2 is applied using the given list of aliases; for a negations operation, substitute_negations/2 is applied using the given negation list; for an expand operation, the function expand/2 is applied using the given list of expansions. The final result is automatically compacted (cf. compact/1).

    Typically you want to substitute negations first, then aliases, then perform one or more expansions (sometimes you want to pre-expand particular entries before doing the main expansion). You might want to substitute negations and/or aliases repeatedly, to allow such forms in the right-hand side of aliases and expansion lists.

    See also: compact/1, expand/2, substitute_aliases/2, substitute_negations/2.

property(Property) -> Property

    Types
        Property = atom() | tuple()

    Creates a normal form (minimal) representation of a property. If Property is {Key, true} where Key is an atom, this returns Key, otherwise the whole term Property is returned.

    See also: property/2.

property(Key, Value) -> Property

    Types
        Key = term()
        Value = term()
        Property = atom() | tuple()

    Creates a normal form (minimal) representation of a simple key/value property. Returns Key if Value is true and Key is an atom, otherwise a tuple {Key, Value} is returned.

    See also: property/1.

split(List, Keys) -> {Lists, Rest}

    Types
        List = [term()]
        Keys = [term()]
        Lists = [[term()]]
        Rest = [term()]

    Partitions List into a list of sublists and a remainder. Lists contains one sublist for each key in Keys, in the corresponding order. The relative order of the elements in each sublist is preserved from the original List. Rest contains the elements in List that are not associated with any of the given keys, also with their original relative order preserved.

    Example: split([{c, 2}, {e, 1}, a, {c, 3, 4}, d, {b, 5}, b], [a, b, c])

    returns

    {[[a], [{b, 5}, b],[{c, 2}, {c, 3, 4}]], [{e, 1}, d]}

substitute_aliases(Aliases, List) -> List

    Types
        Aliases = [{Key, Key}]
        Key = term()
        List = [term()]

    Substitutes keys of properties. For each entry in List, if it is associated with some key K1 such that {K1, K2} occurs in Aliases, the key of the entry is changed to Key2. If the same K1 occurs more than once in Aliases, only the first occurrence is used.

    Example: substitute_aliases([{color, colour}], L) will replace all tuples {color, ...} in L with {colour, ...}, and all atoms color with colour.

    See also: normalize/2, substitute_negations/2.

substitute_negations(Negations, List) -> List

    Types
        Negations = [{Key, Key}]
        Key = term()
        List = [term()]

    Substitutes keys of boolean-valued properties and simultaneously negates their values. For each entry in List, if it is associated with some key K1 such that {K1, K2} occurs in Negations, then if the entry was {K1, true} it will be replaced with {K2, false}, otherwise it will be replaced with {K2, true}, thus changing the name of the option and simultaneously negating the value given by get_bool(List). If the same K1 occurs more than once in Negations, only the first occurrence is used.

    Example: substitute_negations([{no_foo, foo}], L) will replace any atom no_foo or tuple {no_foo, true} in L with {foo, false}, and any other tuple {no_foo, ...} with {foo, true}.

    See also: get_bool/2, normalize/2, substitute_aliases/2.

unfold(List) -> List

    Types
        List = [term()]

    Unfolds all occurences of atoms in List to tuples {Atom, true}.