proplists(3)

NAME
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.

Two keys are considered equal if they match (=:=). In other words, numbers are compared literally rather than by value, so that, for instance, 1 and 1.0 are different keys.

DATA TYPES
property() = atom() | tuple()

EXPORTS

append_values(Key, ListIn) -> ListOut

    Types:
        Key = term()
        ListIn = ListOut = [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(ListIn) -> ListOut

    Types:
        ListIn = ListOut = [property()]

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

    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, ListIn) -> ListOut

    Types:
        Expansions = [{Property :: property(), Expansion :: [term()]}]
        ListIn = ListOut = [term()]

    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 ListIn 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 ListIn.

    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 occurrence 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) -> boolean()

    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()
        List = [term()]
        Default = 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/2, lookup/2.

is_defined(Key, List) -> boolean()

    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(ListIn, Stages) -> ListOut

    Types:
        ListIn = [term()]
        Stages = [Operation]
        Operation =
            {aliases, Aliases} |
            {negations, Negations} |
            {expand, Expansions}
        Aliases = Negations = [{Key, Key}]
        Expansions = [{Property :: property(), Expansion :: [term()]}]
        ListOut = [term()]

    Passes ListIn 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(PropertyIn) -> PropertyOut

    Types:
        PropertyIn = PropertyOut = property()

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

    See also: property/2.

property(Key, Value) -> Property

    Types:
        Key = Value = term()
        Property = atom() | {term(), term()}

    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 = 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, ListIn) -> ListOut

    Types:
        Aliases = [{Key, Key}]
        Key = term()
        ListIn = ListOut = [term()]

    Substitutes keys of properties. For each entry in ListIn, if it is associated with some key K1 such that {K1, K2} occurs in Aliases, the key of the entry is changed to K2. 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, ListIn) -> ListOut

    Types:
        Negations = [{Key, Key}]
        Key = term()
        ListIn = ListOut = [term()]

    Substitutes keys of boolean-valued properties and simultaneously negates their values. For each entry in ListIn, 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(ListIn). 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(ListIn) -> ListOut

    Types:
        ListIn = ListOut = [term()]

    Unfolds all occurrences of atoms in ListIn to tuples {Atom, true}.