proplists
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.
Functions
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.
Example:
append_values(a, [{a, [1,2]}, {b, 0}, {a, 3}, {c, 1}, {a, [4]}])
returns:
[1,2,3,4]
compact(ListIn) > ListOut
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
Key = term()
List = [term()]
Deletes all entries associated with
from
.
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 {
in
: if
E
is the first entry in
with the
same key as
, and E
and
have equivalent normal forms, then
E
is replaced with the terms in
,
and any following entries with the same key are deleted from
.
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
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
. 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(
would yield
{
, 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
, not containing duplicates.
get_value(Key, List) > term()
Key = term()
List = [term()]
Equivalent to
get_value(
.
get_value(Key, List, Default) > term()
Key = term()
List = [term()]
Default = term()
Returns the value of a simple key/value property in
. If lookup(
would yield {
,
this function returns the corresponding Value
, otherwise
.
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
contains at least
one entry associated with
, otherwise
false
.
lookup(Key, List) > none  tuple()
Key = term()
List = [term()]
Returns the first entry associated with
in
, 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
in
. 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
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, functionexpand/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 preexpand particular entries before doing the main expansion). You might want to substitute negations and/or aliases repeatedly, to allow such forms in the righthand side of aliases and expansion lists.
See also
substitute_negations/2
.
property(PropertyIn) > PropertyOut
PropertyIn = PropertyOut = property()
Creates a normal form (minimal) representation of a property. If
is {Key, true}
, where
Key
is an atom, Key
is returned, otherwise
the whole term
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
if
is true
and
is an atom, otherwise a
tuple {
is returned.
See also
property/1
.
split(List, Keys) > {Lists, Rest}
List = Keys = [term()]
Lists = [[term()]]
Rest = [term()]
Partitions
into a list of sublists and a
remainder.
contains one sublist for each key
in
, in the corresponding order. The relative
order of the elements in each sublist is preserved from the original
.
contains the
elements in
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
Aliases = [{Key, Key}]
Key = term()
ListIn = ListOut = [term()]
Substitutes keys of properties. For each entry in
, if it is associated with some key
K1
such that {K1, K2}
occurs in
, the
key of the entry is changed to K2
. If the same
K1
occurs more than once in
, 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 booleanvalued properties and
simultaneously negates their values. For each entry in
, if it is associated with some key
K1
such that {K1, K2}
occurs in
: 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,
.
If the same K1
occurs more than once in
, 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
to
tuples {Atom, true}
.