add CCList.fold_filter_map

src/core/CCList.ml

@@ -174,6 +174,21 @@ let fold_map2 f acc l1 l2 =
    with Invalid_argument _ -> true)
 *)
 
+let fold_filter_map f acc l =
+  let rec aux f acc map_acc l = match l with
+    | [] -> acc, List.rev map_acc
+    | x :: l' ->
+        let acc, y = f acc x in
+        aux f acc (cons_maybe y map_acc) l'
+  in
+  aux f acc [] l
+
+(*$= & ~printer:Q.Print.(pair int (list int))
+  (List.fold_left (+) 0 (1--10), [2;4;6;8;10]) \
+  (fold_filter_map (fun acc x -> acc+x, if x mod 2 = 0 then Some x else None) \
+    0 (1--10))
+*)
+
 let fold_flat_map f acc l =
   let rec aux f acc map_acc l = match l with
     | [] -> acc, List.rev map_acc

src/core/CCList.mli

@@ -53,6 +53,11 @@ val fold_map2 : ('acc -> 'a -> 'b -> 'acc * 'c) -> 'acc -> 'a list -> 'b list ->
     @raise Invalid_argument if the lists do not have the same length
     @since 0.16 *)
 
+val fold_filter_map : ('acc -> 'a -> 'acc * 'b option) -> 'acc -> 'a list -> 'acc * 'b list
+(** [fold_filter_map f acc l] is a [fold_left]-like function, but also
+    generates a list of output in a way similar to {!filter_map}
+    @since NEXT_RELEASE *)
+
 val fold_flat_map : ('acc -> 'a -> 'acc * 'b list) -> 'acc -> 'a list -> 'acc * 'b list
 (** [fold_flat_map f acc l] is a [fold_left]-like function, but it also maps the
     list to a list of lists that is then [flatten]'d..