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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 can specify options overriding the default settings, object
properties, annotations, and so on.
Two keys are considered equal if they match (=:=). That is, numbers are compared literally
rather than by value, so that, for example, 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. The resulting list of lists is concatenated. This is often
useful for "incremental" options.
Example:
append_values(a, [{a, [1,2]}, {b, 0}, {a, 3}, {c, -1}, {a, [4]}])
returns:
[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 because {foo, false} shadows
foo:
expand([{{foo, true}, [bar, baz]}], [{foo, false}, fie, foo, fum])
Notice 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.
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.
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.
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.
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, function substitute_aliases/2 is applied using the specified list of
aliases:
* For a negations operation, substitute_negations/2 is applied using the
specified negation list.
* For an expand operation, function expand/2 is applied using the specified list
of expansions.
The final result is automatically compacted (compare 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 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, Key is returned, 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 specified 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.
For example, substitute_aliases([{color, colour}], L) replaces 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 = [{Key1, Key2}]
Key1 = Key2 = 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: if the entry was {K1, true}, it is replaced with {K2,
false}, otherwise with {K2, true}, thus changing the name of the option and
simultaneously negating the value specified by get_bool(Key, ListIn). If the same
K1 occurs more than once in Negations, only the first occurrence is used.
For example, substitute_negations([{no_foo, foo}], L) replaces 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}.