-spec filter(Pred, MapOrIter) -> Map
                when
                    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}

-spec filtermap(Fun, MapOrIter) -> Map
                   when
                       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 NewValue 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}

-spec find(Key, Map) -> {ok, Value} | error when Map :: #{Key => Value, _ => _}.

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}

-spec fold(Fun, Init, MapOrIter) -> Acc
              when
                  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

-spec foreach(Fun, MapOrIter) -> ok
                 when
                     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.

-spec from_keys(Keys, Value) -> Map when 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}

-spec from_list(List) -> Map when List :: [{Key, Value}], Key :: term(), 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}

-spec get(Key, Map) -> Value when 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"

-spec get(Key, Map, Default) -> Value | Default when Map :: #{Key => Value, _ => _}.

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"

-spec groups_from_list(KeyFun, List) -> GroupsMap
                          when
                              KeyFun :: fun((Elem) -> Key),
                              GroupsMap :: #{Key => Group},
                              Key :: term(),
                              List :: [Elem],
                              Group :: [Elem],
                              Elem :: term().

Partitions the given List into a map of groups.

The result is a map where each key is given by KeyFun and each value is a list of elements from the given List for which KeyFun returned the same key.

The order of elements within each group list is preserved from the original list.

Examples:

> EvenOdd = fun(X) -> case X rem 2 of 0 -> even; 1 -> odd end end,
maps:groups_from_list(EvenOdd, [1, 2, 3]).
#{even => [2], odd => [1, 3]}
> maps:groups_from_list(fun erlang:length/1, ["ant", "buffalo", "cat", "dingo"]).
#{3 => ["ant", "cat"], 5 => ["dingo"], 7 => ["buffalo"]}

-spec groups_from_list(KeyFun, ValueFun, List) -> GroupsMap
                          when
                              KeyFun :: fun((Elem) -> Key),
                              ValueFun :: fun((Elem) -> Value),
                              GroupsMap :: #{Key := Group},
                              Key :: term(),
                              Value :: term(),
                              List :: [Elem],
                              Group :: [Value],
                              Elem :: term().

Partitions the given List into a map of groups.

The result is a map where each key is given by KeyFun and each value is a list of elements from the given List, mapped via ValueFun, for which KeyFun returned the same key.

The order of elements within each group list is preserved from the original list.

Examples:

> EvenOdd = fun(X) -> case X rem 2 of 0 -> even; 1 -> odd end end,
> Square = fun(X) -> X * X end,
> maps:groups_from_list(EvenOdd, Square, [1, 2, 3]).
#{even => [4], odd => [1, 9]}
> maps:groups_from_list(
    fun erlang:length/1,
    fun lists:reverse/1,
    ["ant", "buffalo", "cat", "dingo"]).
#{3 => ["tna", "tac"],5 => ["ognid"],7 => ["olaffub"]}

-spec intersect(Map1, Map2) -> Map3
                   when 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}

-spec intersect_with(Combiner, Map1, Map2) -> Map3
                        when
                            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}}

-spec is_key(Key, Map) -> boolean() when 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

-spec iterator(Map) -> Iterator when 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

-spec iterator(Map, Order) -> Iterator
                  when
                      Map :: #{Key => Value},
                      Order :: iterator_order(Key),
                      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 sorted by key using the given Order.

The call fails with a {badmap,Map} exception if Map is not a map or if Order is invalid.

Example (when Order is ordered):

> M = #{ a => 1, b => 2 }.
#{a => 1,b => 2}
> OrdI = maps:iterator(M, ordered), ok.
ok
> {K1, V1, OrdI2} = maps:next(OrdI), {K1, V1}.
{a,1}
> {K2, V2, OrdI3} = maps:next(OrdI2),{K2, V2}.
{b,2}
> maps:next(OrdI3).
none

Example (when Order is reversed):

> M = #{ a => 1, b => 2 }.
#{a => 1,b => 2}
> RevI = maps:iterator(M, reversed), ok.
ok
> {K2, V2, RevI2} = maps:next(RevI), {K2, V2}.
{b,2}
> {K1, V1, RevI3} = maps:next(RevI2),{K1, V1}.
{a,1}
> maps:next(RevI3).
none

Example (when Order is an arithmetic sorting function):

> M = #{ -1 => a, -1.0 => b, 0 => c, 0.0 => d }.
#{-1 => a,0 => c,-1.0 => b,0.0 => d}
> ArithOrdI = maps:iterator(M, fun(A, B) -> A =< B end), ok.
ok
> maps:to_list(ArithOrdI).
[{-1,a},{-1.0,b},{0,c},{0.0,d}]
> ArithRevI = maps:iterator(M, fun(A, B) -> B < A end), ok.
ok
> maps:to_list(ArithRevI).
[{0.0,d},{0,c},{-1.0,b},{-1,a}]

-spec keys(Map) -> Keys when Map :: #{Key => _}, 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"]

-spec map(Fun, MapOrIter) -> Map
             when
                 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}

-spec merge(Map1, Map2) -> Map3 when Map1 :: map(), Map2 :: map(), 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}

-spec merge_with(Combiner, Map1, Map2) -> Map3
                    when
                        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}

-spec new() -> Map when Map :: #{}.

Returns a new empty map.

Example:

> maps:new().
#{}

-spec next(Iterator) -> {Key, Value, NextIterator} | none
              when Iterator :: iterator(Key, Value), 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

-spec put(Key, Value, Map1) -> Map2 when Key :: term(), Value :: term(), Map1 :: map(), 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}

-spec remove(Key, Map1) -> Map2 when Key :: term(), Map1 :: map(), 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}

-spec size(Map) -> non_neg_integer() when 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

-spec take(Key, Map1) -> {Value, Map2} | error when Map1 :: #{Key => Value, _ => _}, Map2 :: #{_ => _}.

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

-spec to_list(MapOrIterator) -> [{Key, Value}]
                 when MapOrIterator :: #{Key => Value} | iterator(Key, Value).

Returns a list of pairs representing the key-value associations of MapOrIterator, where the pairs [{K1,V1}, ..., {Kn,Vn}] are returned in arbitrary order.

The call fails with a {badmap,Map} exception if MapOrIterator is not a map or an iterator obtained by a call to iterator/1 or iterator/2.

Example:

> Map = #{42 => value_three,1337 => "value two","a" => 1},
  maps:to_list(Map).
[{42,value_three},{1337,"value two"},{"a",1}]

Example (using iterator/2):

> Map = #{ z => 1, y => 2, x => 3 }.
#{x => 3,y => 2,z => 1}
> maps:to_list(maps:iterator(Map, ordered)).
[{x,3},{y,2},{z,1}]

-spec update(Key, Value, Map1) -> Map2 when Map1 :: #{Key := _, _ => _}, Map2 :: #{Key := Value, _ => _}.

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}

-spec update_with(Key, Fun, Map1) -> Map2
                     when
                         Map1 :: #{Key := Value1, _ => _},
                         Map2 :: #{Key := Value2, _ => _},
                         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}

-spec update_with(Key, Fun, Init, Map1) -> Map2
                     when
                         Map1 :: #{Key => Value1, _ => _},
                         Map2 :: #{Key := Value2 | Init, _ => _},
                         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}

-spec values(Map) -> Values when Map :: #{_ => 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]

-spec with(Ks, Map1) -> Map2 when Ks :: [K], Map1 :: #{K => V, _ => _}, 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}

-spec without(Ks, Map1) -> Map2 when Ks :: [K], Map1 :: map(), 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"}