Provided by: erlang-manpages_24.3.4.1+dfsg-1_all 
NAME
maps - Maps processing functions.
DESCRIPTION
This module contains functions for maps processing. The Efficiency Guide contains a
chapter that describes how to use maps efficiently.
DATA TYPES
iterator(Key, Value)
An iterator representing the associations in a map with keys of type Key and values
of type Value.
Created using maps:iterator/1.
Consumed by maps:next/1, maps:filter/2, maps:fold/3 and maps:map/2.
iterator() = iterator(term(), term())
EXPORTS
filter(Pred, MapOrIter) -> Map
Types:
Pred = fun((Key, Value) -> boolean())
MapOrIter = #{Key => Value} | iterator(Key, Value)
Map = #{Key => Value}
Returns a map Map for which predicate Pred holds true in MapOrIter.
The call fails with a {badmap,Map} exception if MapOrIter is not a map or valid
iterator, or with badarg if Pred is not a function of arity 2.
Example:
> M = #{a => 2, b => 3, c=> 4, "a" => 1, "b" => 2, "c" => 4},
Pred = fun(K,V) -> is_atom(K) andalso (V rem 2) =:= 0 end,
maps:filter(Pred,M).
#{a => 2,c => 4}
filtermap(Fun, MapOrIter) -> Map
Types:
Fun = fun((Key, Value1) -> boolean() | {true, Value2})
MapOrIter = #{Key => Value1} | iterator(Key, Value1)
Map = #{Key => Value1 | Value2}
Returns a map Map that is the result of calling Fun(Key, Value1) for every Key to
value Value1 association in MapOrIter in any order.
If Fun(Key, Value1) returns true, the association is copied to the result map. If
it returns false, the association is not copied. If it returns {true, NewValue},
the value for Key is replaced with NewValueat this position is replaced in the
result map.
The call fails with a {badmap,Map} exception if MapOrIter is not a map or valid
iterator, or with badarg if Fun is not a function of arity 2.
Example:
> Fun = fun(K,V) when is_atom(K) -> {true, V*2}; (_,V) -> (V rem 2) =:= 0 end,
Map = #{k1 => 1, "k2" => 2, "k3" => 3},
maps:filtermap(Fun,Map).
#{k1 => 2,"k2" => 2}
find(Key, Map) -> {ok, Value} | error
Types:
Map = #{Key => Value, term() => term()}
Returns a tuple {ok, Value}, where Value is the value associated with Key, or error
if no value is associated with Key in Map.
The call fails with a {badmap,Map} exception if Map is not a map.
Example:
> Map = #{"hi" => 42},
Key = "hi",
maps:find(Key,Map).
{ok,42}
fold(Fun, Init, MapOrIter) -> Acc
Types:
Fun = fun((Key, Value, AccIn) -> AccOut)
Init = term()
Acc = AccOut
AccIn = Init | AccOut
MapOrIter = #{Key => Value} | iterator(Key, Value)
Calls F(Key, Value, AccIn) for every Key to value Value association in MapOrIter in
any order. Function fun F/3 must return a new accumulator, which is passed to the
next successive call. This function returns the final value of the accumulator. The
initial accumulator value Init is returned if the map is empty.
The call fails with a {badmap,Map} exception if MapOrIter is not a map or valid
iterator, or with badarg if Fun is not a function of arity 3.
Example:
> Fun = fun(K,V,AccIn) when is_list(K) -> AccIn + V end,
Map = #{"k1" => 1, "k2" => 2, "k3" => 3},
maps:fold(Fun,0,Map).
6
foreach(Fun, MapOrIter) -> ok
Types:
Fun = fun((Key, Value) -> term())
MapOrIter = #{Key => Value} | iterator(Key, Value)
Calls fun F(Key, Value) for every Key to value Value association in MapOrIter in
any order.
The call fails with a {badmap,Map} exception if MapOrIter is not a map or valid
iterator, or with badarg if Fun is not a function of arity 2.
from_keys(Keys, Value) -> Map
Types:
Keys = list()
Value = term()
Map = map()
Takes a list of keys and a value and builds a map where all keys point to the same
value. The key can be in any order, and keys and value can be of any term.
Example:
> Keys = ["a", "b", "c"], maps:from_keys(Keys, ok).
#{"a" => ok,"b" => ok,"c" => ok}
from_list(List) -> Map
Types:
List = [{Key, Value}]
Key = Value = term()
Map = map()
Takes a list of key-value tuples elements and builds a map. The associations can be
in any order, and both keys and values in the association can be of any term. If
the same key appears more than once, the latter (right-most) value is used and the
previous values are ignored.
Example:
> List = [{"a",ignored},{1337,"value two"},{42,value_three},{"a",1}],
maps:from_list(List).
#{42 => value_three,1337 => "value two","a" => 1}
get(Key, Map) -> Value
Types:
Key = term()
Map = map()
Value = term()
Returns value Value associated with Key if Map contains Key.
The call fails with a {badmap,Map} exception if Map is not a map, or with a
{badkey,Key} exception if no value is associated with Key.
Example:
> Key = 1337,
Map = #{42 => value_two,1337 => "value one","a" => 1},
maps:get(Key,Map).
"value one"
get(Key, Map, Default) -> Value | Default
Types:
Map = #{Key => Value, term() => term()}
Returns value Value associated with Key if Map contains Key. If no value is
associated with Key, Default is returned.
The call fails with a {badmap,Map} exception if Map is not a map.
Example:
> Map = #{ key1 => val1, key2 => val2 }.
#{key1 => val1,key2 => val2}
> maps:get(key1, Map, "Default value").
val1
> maps:get(key3, Map, "Default value").
"Default value"
intersect(Map1, Map2) -> Map3
Types:
Map1 = #{Key => term()}
Map2 = #{term() => Value2}
Map3 = #{Key => Value2}
Intersects two maps into a single map Map3. If a key exists in both maps, the value
in Map1 is superseded by the value in Map2.
The call fails with a {badmap,Map} exception if Map1 or Map2 is not a map.
Example:
> Map1 = #{a => "value_one", b => "value_two"},
Map2 = #{a => 1, c => 2},
maps:intersect(Map1,Map2).
#{a => 1}
intersect_with(Combiner, Map1, Map2) -> Map3
Types:
Map1 = #{Key => Value1}
Map2 = #{term() => Value2}
Combiner = fun((Key, Value1, Value2) -> CombineResult)
Map3 = #{Key => CombineResult}
Intersects two maps into a single map Map3. If a key exists in both maps, the value
in Map1 is combined with the value in Map2 by the Combiner fun. When Combiner is
applied the key that exists in both maps is the first parameter, the value from
Map1 is the second parameter, and the value from Map2 is the third parameter.
The call fails with a {badmap,Map} exception if Map1 or Map2 is not a map. The call
fails with a badarg exception if Combiner is not a fun that takes three arguments.
Example:
> Map1 = #{a => "value_one", b => "value_two"},
Map2 = #{a => 1, c => 2},
maps:intersect_with(fun(_Key, Value1, Value2) -> {Value1, Value2} end, Map1, Map2).
#{a => {"value_one",1}}
is_key(Key, Map) -> boolean()
Types:
Key = term()
Map = map()
Returns true if map Map contains Key and returns false if it does not contain the
Key.
The call fails with a {badmap,Map} exception if Map is not a map.
Example:
> Map = #{"42" => value}.
#{"42" => value}
> maps:is_key("42",Map).
true
> maps:is_key(value,Map).
false
iterator(Map) -> Iterator
Types:
Map = #{Key => Value}
Iterator = iterator(Key, Value)
Returns a map iterator Iterator that can be used by maps:next/1 to traverse the
key-value associations in a map. When iterating over a map, the memory usage is
guaranteed to be bounded no matter the size of the map.
The call fails with a {badmap,Map} exception if Map is not a map.
Example:
> M = #{ a => 1, b => 2 }.
#{a => 1,b => 2}
> I = maps:iterator(M), ok.
ok
> {K1, V1, I2} = maps:next(I), {K1, V1}.
{a,1}
> {K2, V2, I3} = maps:next(I2),{K2, V2}.
{b,2}
> maps:next(I3).
none
keys(Map) -> Keys
Types:
Map = #{Key => term()}
Keys = [Key]
Returns a complete list of keys, in any order, which resides within Map.
The call fails with a {badmap,Map} exception if Map is not a map.
Example:
> Map = #{42 => value_three,1337 => "value two","a" => 1},
maps:keys(Map).
[42,1337,"a"]
map(Fun, MapOrIter) -> Map
Types:
Fun = fun((Key, Value1) -> Value2)
MapOrIter = #{Key => Value1} | iterator(Key, Value1)
Map = #{Key => Value2}
Produces a new map Map by calling function fun F(Key, Value1) for every Key to
value Value1 association in MapOrIter in any order. Function fun Fun/2 must return
value Value2 to be associated with key Key for the new map Map.
The call fails with a {badmap,Map} exception if MapOrIter is not a map or valid
iterator, or with badarg if Fun is not a function of arity 2.
Example:
> Fun = fun(K,V1) when is_list(K) -> V1*2 end,
Map = #{"k1" => 1, "k2" => 2, "k3" => 3},
maps:map(Fun,Map).
#{"k1" => 2,"k2" => 4,"k3" => 6}
merge(Map1, Map2) -> Map3
Types:
Map1 = Map2 = Map3 = map()
Merges two maps into a single map Map3. If two keys exist in both maps, the value
in Map1 is superseded by the value in Map2.
The call fails with a {badmap,Map} exception if Map1 or Map2 is not a map.
Example:
> Map1 = #{a => "value_one", b => "value_two"},
Map2 = #{a => 1, c => 2},
maps:merge(Map1,Map2).
#{a => 1,b => "value_two",c => 2}
merge_with(Combiner, Map1, Map2) -> Map3
Types:
Map1 = #{Key1 => Value1}
Map2 = #{Key2 => Value2}
Combiner = fun((Key1, Value1, Value2) -> CombineResult)
Map3 = #{Key1 => CombineResult, Key1 => Value1, Key2 => Value2}
Merges two maps into a single map Map3. If a key exists in both maps, the value in
Map1 is combined with the value in Map2 by the Combiner fun. When Combiner is
applied the key that exists in both maps is the first parameter, the value from
Map1 is the second parameter, and the value from Map2 is the third parameter.
The call fails with a {badmap,Map} exception if Map1 or Map2 is not a map. The call
fails with a badarg exception if Combiner is not a fun that takes three arguments.
Example:
> Map1 = #{a => "value_one", b => "value_two"},
Map2 = #{a => 1, c => 2},
maps:merge_with(fun(_Key, Value1, Value2) -> {Value1, Value2} end, Map1, Map2).
#{a => {"value_one",1},b => "value_two",c => 2}
new() -> Map
Types:
Map = #{}
Returns a new empty map.
Example:
> maps:new().
#{}
next(Iterator) -> {Key, Value, NextIterator} | none
Types:
Iterator = NextIterator = iterator(Key, Value)
Returns the next key-value association in Iterator and a new iterator for the
remaining associations in the iterator.
If there are no more associations in the iterator, none is returned.
Example:
> Map = #{a => 1, b => 2, c => 3}.
#{a => 1,b => 2,c => 3}
> I = maps:iterator(Map), ok.
ok
> {K1, V1, I1} = maps:next(I), {K1, V1}.
{a,1}
> {K2, V2, I2} = maps:next(I1), {K2, V2}.
{b,2}
> {K3, V3, I3} = maps:next(I2), {K3, V3}.
{c,3}
> maps:next(I3).
none
put(Key, Value, Map1) -> Map2
Types:
Key = Value = term()
Map1 = Map2 = map()
Associates Key with value Value and inserts the association into map Map2. If key
Key already exists in map Map1, the old associated value is replaced by value
Value. The function returns a new map Map2 containing the new association and the
old associations in Map1.
The call fails with a {badmap,Map} exception if Map1 is not a map.
Example:
> Map = #{"a" => 1}.
#{"a" => 1}
> maps:put("a", 42, Map).
#{"a" => 42}
> maps:put("b", 1337, Map).
#{"a" => 1,"b" => 1337}
remove(Key, Map1) -> Map2
Types:
Key = term()
Map1 = Map2 = map()
Removes the Key, if it exists, and its associated value from Map1 and returns a new
map Map2 without key Key.
The call fails with a {badmap,Map} exception if Map1 is not a map.
Example:
> Map = #{"a" => 1}.
#{"a" => 1}
> maps:remove("a",Map).
#{}
> maps:remove("b",Map).
#{"a" => 1}
size(Map) -> integer() >= 0
Types:
Map = map()
Returns the number of key-value associations in Map. This operation occurs in
constant time.
Example:
> Map = #{42 => value_two,1337 => "value one","a" => 1},
maps:size(Map).
3
take(Key, Map1) -> {Value, Map2} | error
Types:
Map1 = #{Key => Value, term() => term()}
Map2 = #{term() => term()}
The function removes the Key, if it exists, and its associated value from Map1 and
returns a tuple with the removed Value and the new map Map2 without key Key. If the
key does not exist error is returned.
The call will fail with a {badmap,Map} exception if Map1 is not a map.
Example:
> Map = #{"a" => "hello", "b" => "world"}.
#{"a" => "hello", "b" => "world"}
> maps:take("a",Map).
{"hello",#{"b" => "world"}}
> maps:take("does not exist",Map).
error
to_list(Map) -> [{Key, Value}]
Types:
Map = #{Key => Value}
Returns a list of pairs representing the key-value associations of Map, where the
pairs [{K1,V1}, ..., {Kn,Vn}] are returned in arbitrary order.
The call fails with a {badmap,Map} exception if Map is not a map.
Example:
> Map = #{42 => value_three,1337 => "value two","a" => 1},
maps:to_list(Map).
[{42,value_three},{1337,"value two"},{"a",1}]
update(Key, Value, Map1) -> Map2
Types:
Map1 = #{Key := term(), term() => term()}
Map2 = #{Key := Value, term() => term()}
If Key exists in Map1, the old associated value is replaced by value Value. The
function returns a new map Map2 containing the new associated value.
The call fails with a {badmap,Map} exception if Map1 is not a map, or with a
{badkey,Key} exception if no value is associated with Key.
Example:
> Map = #{"a" => 1}.
#{"a" => 1}
> maps:update("a", 42, Map).
#{"a" => 42}
update_with(Key, Fun, Map1) -> Map2
Types:
Map1 = #{Key := Value1, term() => term()}
Map2 = #{Key := Value2, term() => term()}
Fun = fun((Value1) -> Value2)
Update a value in a Map1 associated with Key by calling Fun on the old value to get
a new value. An exception {badkey,Key} is generated if Key is not present in the
map.
Example:
> Map = #{"counter" => 1},
Fun = fun(V) -> V + 1 end,
maps:update_with("counter",Fun,Map).
#{"counter" => 2}
update_with(Key, Fun, Init, Map1) -> Map2
Types:
Map1 = #{Key => Value1, term() => term()}
Map2 = #{Key := Value2 | Init, term() => term()}
Fun = fun((Value1) -> Value2)
Update a value in a Map1 associated with Key by calling Fun on the old value to get
a new value. If Key is not present in Map1 then Init will be associated with Key.
Example:
> Map = #{"counter" => 1},
Fun = fun(V) -> V + 1 end,
maps:update_with("new counter",Fun,42,Map).
#{"counter" => 1,"new counter" => 42}
values(Map) -> Values
Types:
Map = #{term() => Value}
Values = [Value]
Returns a complete list of values, in arbitrary order, contained in map Map.
The call fails with a {badmap,Map} exception if Map is not a map.
Example:
> Map = #{42 => value_three,1337 => "value two","a" => 1},
maps:values(Map).
[value_three,"value two",1]
with(Ks, Map1) -> Map2
Types:
Ks = [K]
Map1 = #{K => V, term() => term()}
Map2 = #{K => V}
Returns a new map Map2 with the keys K1 through Kn and their associated values from
map Map1. Any key in Ks that does not exist in Map1 is ignored.
Example:
> Map = #{42 => value_three,1337 => "value two","a" => 1},
Ks = ["a",42,"other key"],
maps:with(Ks,Map).
#{42 => value_three,"a" => 1}
without(Ks, Map1) -> Map2
Types:
Ks = [K]
Map1 = Map2 = map()
K = term()
Returns a new map Map2 without keys K1 through Kn and their associated values from
map Map1. Any key in Ks that does not exist in Map1 is ignored
Example:
> Map = #{42 => value_three,1337 => "value two","a" => 1},
Ks = ["a",42,"other key"],
maps:without(Ks,Map).
#{1337 => "value two"}