Provided by: erlang-manpages_20.2.2+dfsg-1ubuntu2_all 
NAME
maps - Maps processing functions.
DESCRIPTION
This module contains functions for maps processing.
EXPORTS
filter(Pred, Map1) -> Map2
Types:
Pred = fun((Key, Value) -> boolean())
Key = Value = term()
Map1 = Map2 = map()
Returns a map Map2 for which predicate Pred holds true in Map1.
The call fails with a {badmap,Map} exception if Map1 is not a map, 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}
find(Key, Map) -> {ok, Value} | error
Types:
Key = term()
Map = map()
Value = 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, Map) -> Acc
Types:
Fun = fun((K, V, AccIn) -> AccOut)
Init = Acc = AccIn = AccOut = term()
Map = map()
K = V = term()
Calls F(K, V, AccIn) for every K to value V association in Map 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.
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
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:
Key = term()
Map = map()
Value = Default = 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"
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
keys(Map) -> Keys
Types:
Map = map()
Keys = [Key]
Key = term()
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, Map1) -> Map2
Types:
Fun = fun((K, V1) -> V2)
Map1 = Map2 = map()
K = V1 = V2 = term()
Produces a new map Map2 by calling function fun F(K, V1) for every K to value V1 association in
Map1 in any order. Function fun F/2 must return value V2 to be associated with key K for the new
map Map2.
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}
new() -> Map
Types:
Map = map()
Returns a new empty map.
Example:
> maps:new().
#{}
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:
Key = term()
Map1 = map()
Value = term()
Map2 = map()
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 = map()
Key = Value = term()
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:
Key = Value = term()
Map1 = Map2 = map()
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:
Key = term()
Map1 = Map2 = map()
Fun = fun((Value1 :: term()) -> Value2 :: term())
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:
Key = term()
Map1 = Map1
Map2 = Map2
Fun = fun((Value1 :: term()) -> Value2 :: term())
Init = term()
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 = map()
Values = [Value]
Value = term()
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 = Map2 = map()
K = term()
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"}