Add support for maps:merge_with/3

Signed-off-by: Paul Guyot <pguyot@kallisys.net>

Author: pguyot

CHANGELOG.md

@@ -21,6 +21,7 @@ also non string parameters (e.g. `Enum.join([1, 2], ",")`
 - Support for `code:ensure_loaded/1`
 - Support for Elixir `List.Chars` protocol
 - Support for `lists:last/1` and `lists:mapfoldl/3`
+- Support for `maps:merge_with/3`
 
 ### Changed
 

libs/estdlib/src/maps.erl

@@ -55,6 +55,7 @@
     from_keys/2,
     map/2,
     merge/2,
+    merge_with/3,
     remove/2,
     update/3
 ]).
@@ -439,6 +440,28 @@ merge(Map1, _Map2) when not is_map(Map1) ->
 merge(_Map1, Map2) when not is_map(Map2) ->
     error({badmap, Map2}).
 
+%%-----------------------------------------------------------------------------
+%% @param   Combiner  a function to merge values from Map1 and Map2 if a key exists in both maps
+%% @param   Map1  a map
+%% @param   Map2  a map
+%% @returns the result of merging entries from `Map1' and `Map2'.
+%% @doc Merge two maps to yield a new map.
+%%
+%% If `Map1' and `Map2' contain the same key, then the value from `Combiner(Key, Value1, Value2)' will be used.
+%%
+%% This function raises a `badmap' error if neither `Map1' nor `Map2' is a map.
+%% @end
+%%-----------------------------------------------------------------------------
+-spec merge_with(
+    Combiner :: fun((Key, Value, Value) -> Value), Map1 :: #{Key => Value}, Map2 :: #{Key => Value}
+) -> #{Key => Value}.
+merge_with(Combiner, Map1, Map2) when is_map(Map1) andalso is_map(Map2) ->
+    iterate_merge_with(Combiner, maps:next(maps:iterator(Map1)), Map2);
+merge_with(_Combiner, Map1, _Map2) when not is_map(Map1) ->
+    error({badmap, Map1});
+merge_with(_Combiner, _Map1, Map2) when not is_map(Map2) ->
+    error({badmap, Map2}).
+
 %%-----------------------------------------------------------------------------
 %% @param   Key     the key to remove
 %% @param   MapOrIterator     the map or map iterator from which to remove the key
@@ -545,6 +568,19 @@ iterate_map(Fun, {Key, Value, Iterator}, Accum) ->
     NewAccum = Accum#{Key => Fun(Key, Value)},
     iterate_map(Fun, maps:next(Iterator), NewAccum).
 
+%% @private
+iterate_merge_with(_Combiner, none, Accum) ->
+    Accum;
+iterate_merge_with(Combiner, {Key, Value1, Iterator}, Accum) ->
+    case Accum of
+        #{Key := Value2} ->
+            iterate_merge_with(Combiner, maps:next(Iterator), Accum#{
+                Key := Combiner(Key, Value1, Value2)
+            });
+        #{} ->
+            iterate_merge_with(Combiner, maps:next(Iterator), Accum#{Key => Value1})
+    end.
+
 %% @private
 iterate_merge(none, Accum) ->
     Accum;

tests/libs/estdlib/test_maps.erl

@@ -56,6 +56,7 @@ test() ->
     ok = test_foreach(),
     ok = test_map(),
     ok = test_merge(),
+    ok = test_merge_with(),
     ok = test_remove(),
     ok = test_update(),
     ok.
@@ -284,6 +285,43 @@ test_merge() ->
     ok = check_bad_map(fun() -> maps:merge(id(not_a_map), maps:new()) end),
     ok.
 
+test_merge_with() ->
+    ?ASSERT_EQUALS(maps:merge_with(fun(_K, V1, V2) -> V1 + V2 end, maps:new(), maps:new()), #{}),
+    ?ASSERT_EQUALS(
+        maps:merge_with(fun(_K, V1, V2) -> V1 + V2 end, #{a => 1, b => 2, c => 3}, maps:new()), #{
+            a => 1, b => 2, c => 3
+        }
+    ),
+    ?ASSERT_EQUALS(
+        maps:merge_with(fun(_K, V1, V2) -> V1 + V2 end, maps:new(), #{a => 1, b => 2, c => 3}), #{
+            a => 1, b => 2, c => 3
+        }
+    ),
+    ?ASSERT_EQUALS(
+        maps:merge_with(fun(_K, V1, V2) -> V1 + V2 end, #{a => 1, b => 2, d => 4}, #{
+            a => 1, b => 2, c => 3
+        }),
+        #{a => 2, b => 4, c => 3, d => 4}
+    ),
+    ?ASSERT_EQUALS(
+        maps:merge_with(fun(_K, V1, V2) -> {V1, V2} end, #{a => 1, b => 2, c => 3}, #{
+            b => z, d => 4
+        }),
+        #{
+            a => 1,
+            b => {2, z},
+            c => 3,
+            d => 4
+        }
+    ),
+    ok = check_bad_map(fun() ->
+        maps:merge_with(fun(_K, V1, V2) -> V1 + V2 end, maps:new(), id(not_a_map))
+    end),
+    ok = check_bad_map(fun() ->
+        maps:merge_with(fun(_K, V1, V2) -> V1 + V2 end, id(not_a_map), maps:new())
+    end),
+    ok.
+
 test_remove() ->
     ?ASSERT_EQUALS(maps:remove(foo, maps:new()), #{}),
     ?ASSERT_EQUALS(maps:remove(a, #{a => 1, b => 2, c => 3}), #{b => 2, c => 3}),