Merge branch 'egil/maps-filter/OTP-12745'

* egil/maps-filter/OTP-12745:
  stdlib: Use lc to implement maps:map/2
  stdlib: Test maps:filter/2
  stdlib: Document maps:filter/2
  stdlib: Add maps:filter/2

3 files changed, 50 insertions, 7 deletions

diff --git a/lib/stdlib/doc/src/maps.xml b/lib/stdlib/doc/src/maps.xml
index e46068230a..7345a9357a 100644
--- a/lib/stdlib/doc/src/maps.xml
+++ b/lib/stdlib/doc/src/maps.xml
@@ -33,6 +33,28 @@
 	<funcs>
 
 		<func>
+			<name name="filter" arity="2"/>
+			<fsummary>Choose pairs which satisfy a predicate</fsummary>
+			<desc>
+				<p>
+                    Returns a map <c><anno>Map2</anno></c> for which predicate
+                    <c><anno>Pred</anno></c> holds true in <c><anno>Map1</anno></c>.
+				</p>
+				<p>
+                    The call will fail with a <c>{badmap,Map}</c> exception if
+                    <c><anno>Map1</anno></c> is not a map or with <c>badarg</c> if
+                    <c><anno>Pred</anno></c> is not a function of arity 2.
+				</p>
+				<p>Example:</p>
+				<code type="none">
+> 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} </code>
+			</desc>
+		</func>
+
+		<func>
 			<name name="find" arity="2"/>
 			<fsummary></fsummary>
 			<desc>
diff --git a/lib/stdlib/src/maps.erl b/lib/stdlib/src/maps.erl
index 3877c150ec..533ff08726 100644
--- a/lib/stdlib/src/maps.erl
+++ b/lib/stdlib/src/maps.erl
@@ -19,7 +19,8 @@
 
 -module(maps).
 
--export([get/3,fold/3, map/2, size/1,
+-export([get/3,filter/2,fold/3, map/2,
+         size/1,
          without/2, with/2]).
 
 
@@ -145,6 +146,19 @@ get(Key,Map,Default) ->
     erlang:error({badmap,Map},[Key,Map,Default]).
 
 
+-spec filter(Pred,Map1) -> Map2 when
+      Pred :: fun((Key, Value) -> boolean()),
+      Key  :: term(),
+      Value :: term(),
+      Map1 :: map(),
+      Map2 :: map().
+
+filter(Pred,Map) when is_function(Pred,2), is_map(Map) ->
+    maps:from_list([{K,V}||{K,V}<-maps:to_list(Map),Pred(K,V)]);
+filter(Pred,Map) ->
+    erlang:error(error_type(Map),[Pred,Map]).
+
+
 -spec fold(Fun,Init,Map) -> Acc when
     Fun :: fun((K, V, AccIn) -> AccOut),
     Init :: term(),
@@ -169,10 +183,7 @@ fold(Fun,Init,Map) ->
     V2 :: term().
 
 map(Fun,Map) when is_function(Fun, 2), is_map(Map) ->
-    maps:from_list(lists:map(fun
-		({K,V}) ->
-		    {K,Fun(K,V)}
-	    end,maps:to_list(Map)));
+    maps:from_list([{K,Fun(K,V)}||{K,V}<-maps:to_list(Map)]);
 map(Fun,Map) ->
     erlang:error(error_type(Map),[Fun,Map]).
 
diff --git a/lib/stdlib/test/maps_SUITE.erl b/lib/stdlib/test/maps_SUITE.erl
index 1d9c041a74..21e146ae3d 100644
--- a/lib/stdlib/test/maps_SUITE.erl
+++ b/lib/stdlib/test/maps_SUITE.erl
@@ -34,7 +34,7 @@
 -export([init_per_testcase/2]).
 -export([end_per_testcase/2]).
 
--export([t_get_3/1,
+-export([t_get_3/1, t_filter_2/1,
          t_fold_3/1,t_map_2/1,t_size_1/1,
          t_with_2/1,t_without_2/1]).
 
@@ -45,7 +45,7 @@ suite() ->
     [{ct_hooks, [ts_install_cth]}].
 
 all() ->
-    [t_get_3,
+    [t_get_3,t_filter_2,
      t_fold_3,t_map_2,t_size_1,
      t_with_2,t_without_2].
 
@@ -99,6 +99,16 @@ t_with_2(_Config) ->
     ?badarg(with,[a,#{}]) = (catch maps:with(a,#{})),
     ok.
 
+t_filter_2(Config) when is_list(Config) ->
+    M = #{a => 2, b => 3, c=> 4, "a" => 1, "b" => 2, "c" => 4},
+    Pred1 = fun(K,V) -> is_atom(K) andalso (V rem 2) =:= 0 end,
+    Pred2 = fun(K,V) -> is_list(K) andalso (V rem 2) =:= 0 end,
+    #{a := 2,c := 4} = maps:filter(Pred1,M),
+    #{"b" := 2,"c" := 4} = maps:filter(Pred2,M),
+    %% error case
+    ?badmap(a,filter,[_,a]) = (catch maps:filter(fun(_,_) -> ok end,id(a))),
+    ?badarg(filter,[<<>>,#{}]) = (catch maps:filter(id(<<>>),#{})),
+    ok.
 
 t_fold_3(Config) when is_list(Config) ->
     Vs = lists:seq(1,200),