Support functions for property lists.

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.


property() = atom() | tuple()


append_values(Key, ListIn) -> ListOut

  • 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.


append_values(a, [{a, [1,2]}, {b, 0}, {a, 3}, {c, -1}, {a, [4]}])



compact(ListIn) -> ListOut

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

  • Key = term()
  • List = [term()]

Deletes all entries associated with Key from List.

expand(Expansions, ListIn) -> ListOut

  • 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()]

  • 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()

  • 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()]

  • List = [term()]

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

get_value(Key, List) -> term()

  • Key = term()
  • List = [term()]

Equivalent to get_value(Key, List, undefined).

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

  • 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()

  • Key = term()
  • List = [term()]

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

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

  • 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()]

  • 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

  • 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

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

  • 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}

  • 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.


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


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

substitute_aliases(Aliases, ListIn) -> ListOut

  • 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

  • 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

  • ListIn = ListOut = [term()]

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