add CCList.fold_map

src/core/CCList.ml

@@ -152,6 +152,25 @@ let rec fold_while f acc = function
   fold_while (fun acc b -> if b then acc+1, `Continue else acc, `Stop) 0 [true;true;false;true] = 2
 *)
 
+let fold_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 (y :: map_acc) l'
+  in
+  aux f acc [] l
+
+(*$=
+  (6, ["1"; "2"; "3"]) \
+    (fold_map (fun acc x->acc+x, string_of_int x) 0 [1;2;3])
+*)
+
+(*$Q
+  Q.(list int) (fun l -> \
+    fold_map (fun acc x -> x::acc, x) [] l = (List.rev l, l))
+*)
+
 let init len f =
   let rec init_rec acc i f =
     if i=0 then f i :: acc

src/core/CCList.mli

@@ -66,6 +66,11 @@ val fold_while : ('a -> 'b -> 'a * [`Stop | `Continue]) -> 'a -> 'b t -> 'a
     indicated by the accumulator
     @since 0.8 *)
 
+val fold_map : ('acc -> 'a -> 'acc * 'b) -> 'acc -> 'a list -> 'acc * 'b list
+(** [fold_map f acc l] is a [fold_left]-like function, but it also maps the
+    list to another list.
+    @since NEXT_RELEASE *)
+
 val init : int -> (int -> 'a) -> 'a t
 (** Similar to {!Array.init}
     @since 0.6 *)