add CCList.partition_map

src/core/CCList.ml

@@ -206,6 +206,29 @@ let diagonal l =
   in
   gen [] l
 
+let partition_map f l =
+  let rec iter f l1 l2 l = match l with
+  | [] -> List.rev l1, List.rev l2
+  | x :: tl ->
+    match f x with
+    | `Left y -> iter f (y :: l1) l2 tl
+    | `Right y -> iter f l1 (y :: l2) tl
+    | `Drop -> iter f l1 l2 tl
+  in
+  iter f [] [] l
+
+(*$R
+  let l1, l2 =
+    partition_map (function
+      | n when n = 0 -> `Drop
+      | n when n mod 2 = 0 -> `Left n
+      | n -> `Right n
+    ) [0;1;2;3;4]
+  in
+  assert_equal [2;4] l1;
+  assert_equal [1;3] l2
+*)
+
 let return x = [x]
 
 let (>>=) l f = flat_map f l

src/core/CCList.mli

@@ -77,6 +77,14 @@ val diagonal : 'a t -> ('a * 'a) t
 (** All pairs of distinct positions of the list. [list_diagonal l] will
     return the list of [List.nth i l, List.nth j l] if [i < j]. *)
 
+val partition_map : ('a -> [<`Left of 'b | `Right of 'c | `Drop]) ->
+                    'a list -> 'b list * 'c list
+(** [partition_map f l] maps [f] on [l] and gather results in lists:
+    - if [f x = `Left y], adds [y] to the first list
+    - if [f x = `Right z], adds [z] to the second list
+    - if [f x = `Drop], ignores [x]
+    @since NEXT_RELEASE *)
+
 val pure : 'a -> 'a t
 
 val (<*>) : ('a -> 'b) t -> 'a t -> 'b t