Rename and document lists:zf/2 as lists:filtermap/2

This function is used all over the place in OTP itself and people
sometimes want that functionality, they may as well not reimplement it
themselves.

4 files changed, 45 insertions, 10 deletions

diff --git a/lib/et/doc/src/et_desc.xmlsrc b/lib/et/doc/src/et_desc.xmlsrc
index c02517ae01..36274f6316 100644
--- a/lib/et/doc/src/et_desc.xmlsrc
+++ b/lib/et/doc/src/et_desc.xmlsrc
@@ -150,7 +150,7 @@
          NewEvent = #event{}]]></code>
 
     <p>The interface of the filter function is the same as the the
-    filter functions for the good old <c>lists:zf/2</c>. If the filter
+    filter functions for the good old <c>lists:filtermap/2</c>. If the filter
     returns <c>false</c> it means that the trace data should silently
     be dropped. <c>true</c> means that the trace data data already is
     an <c>Event Record</c> and that it should be kept as it is.
diff --git a/lib/stdlib/doc/src/lists.xml b/lib/stdlib/doc/src/lists.xml
index b6c0fa4e05..1aff78f4fc 100644
--- a/lib/stdlib/doc/src/lists.xml
+++ b/lib/stdlib/doc/src/lists.xml
@@ -152,6 +152,31 @@
       </desc>
     </func>
     <func>
+      <name name="filtermap" arity="2"/>
+      <fsummary>Filter and map elements which satisfy a function</fsummary>
+      <desc>
+        <p>Calls <c><anno>Fun</anno>(<anno>Elem</anno>)</c> on successive elements <c>Elem</c>
+          of <c><anno>List1</anno></c>. <c><anno>Fun</anno>/2</c> must return either a boolean
+          or a tuple <c>{true, <anno>Value</anno>}</c>. The function returns the list of elements
+          for which <c><anno>Fun</anno></c> returns a new value, where a value of <c>true</c>
+          is synonymous with <c>{true, <anno>Elem</anno>}</c>.</p>
+        <p>That is, <c>filtermap</c> behaves as if it had been defined as follows:</p>
+        <code type="none">
+filtermap(Fun, List1) ->
+    lists:foldr(fun(Elem, Acc) ->
+                       case Fun(Elem) of
+                           false -> Acc;
+                           true -> [Elem|Acc];
+                           {true,Value} -> [Value|Acc]
+                       end,
+                end, [], List1).</code>
+        <p>Example:</p>
+        <pre>
+> <input>lists:filtermap(fun(X) -> case X rem 2 of 0 -> {true, X div 2}; _ -> false end end, [1,2,3,4,5]).</input>
+[1,2]</pre>
+      </desc>
+    </func>
+    <func>
       <name name="flatlength" arity="1"/>
       <fsummary>Length of flattened deep list</fsummary>
       <desc>
diff --git a/lib/stdlib/src/lists.erl b/lib/stdlib/src/lists.erl
index 961c060019..0c033acd88 100644
--- a/lib/stdlib/src/lists.erl
+++ b/lib/stdlib/src/lists.erl
@@ -36,7 +36,7 @@
 -export([merge/3, rmerge/3, sort/2, umerge/3, rumerge/3, usort/2]).
 
 -export([all/2,any/2,map/2,flatmap/2,foldl/3,foldr/3,filter/2,
-	 partition/2,zf/2,
+	 partition/2,zf/2,filtermap/2,
 	 mapfoldl/3,mapfoldr/3,foreach/2,takewhile/2,dropwhile/2,splitwith/2,
 	 split/2]).
 
@@ -1291,18 +1291,28 @@ partition(Pred, [H | T], As, Bs) ->
 partition(Pred, [], As, Bs) when is_function(Pred, 1) ->
     {reverse(As), reverse(Bs)}.
 
--spec zf(fun((T) -> boolean() | {'true', X}), [T]) -> [(T | X)].
+-spec filtermap(Fun, List1) -> List2 when
+      Fun :: fun((Elem) -> boolean() | {'true', Value}),
+      List1 :: [Elem],
+      List2 :: [Elem | Value],
+      Elem :: term(),
+      Value :: term().
 
-zf(F, [Hd|Tail]) ->
+filtermap(F, [Hd|Tail]) ->
     case F(Hd) of
 	true ->
-	    [Hd|zf(F, Tail)];
+	    [Hd|filtermap(F, Tail)];
 	{true,Val} ->
-	    [Val|zf(F, Tail)];
+	    [Val|filtermap(F, Tail)];
 	false ->
-	    zf(F, Tail)
+	    filtermap(F, Tail)
     end;
-zf(F, []) when is_function(F, 1) -> [].
+filtermap(F, []) when is_function(F, 1) -> [].
+
+-spec zf(fun((T) -> boolean() | {'true', X}), [T]) -> [(T | X)].
+
+zf(F, L) ->
+    filtermap(F, L).
 
 -spec foreach(Fun, List) -> ok when
       Fun :: fun((Elem :: T) -> term()),
diff --git a/lib/stdlib/test/lists_SUITE.erl b/lib/stdlib/test/lists_SUITE.erl
index b56f0b39d8..cd7210f8ec 100644
--- a/lib/stdlib/test/lists_SUITE.erl
+++ b/lib/stdlib/test/lists_SUITE.erl
@@ -2532,8 +2532,8 @@ otp_5939(Config) when is_list(Config) ->
     ?line [] = lists:filter(Pred, []),
     ?line {'EXIT', _} = (catch lists:partition(func, [])),
     ?line {[],[]} = lists:partition(Pred, []),
-    ?line {'EXIT', _} = (catch lists:zf(func, [])),
-    ?line [] = lists:zf(Fun1, []),
+    ?line {'EXIT', _} = (catch lists:filtermap(func, [])),
+    ?line [] = lists:filtermap(Fun1, []),
     ?line {'EXIT', _} = (catch lists:foreach(func, [])),
     ?line ok = lists:foreach(Fun1, []),
     ?line {'EXIT', _} = (catch lists:mapfoldl(func, [], [])),