Add CCList.iteri2 and CCList.foldi2

src/core/CCList.ml

@@ -1059,6 +1059,16 @@ let iteri f l =
     | x::l' -> f i x; aux f (i+1) l'
   in aux f 0 l
 
+let iteri2 f l1 l2 =
+  let rec aux f i l1 l2 = match l1, l2 with
+    | [], [] -> ()
+    | [], _
+    | _, [] -> invalid_arg "iteri2"
+    | x1::l1', x2::l2' ->
+       f i x1 x2;
+       aux f (i+1) l1' l2'
+  in aux f 0 l1 l2
+
 let foldi f acc l =
   let rec foldi f acc i l = match l with
     | [] -> acc
@@ -1068,6 +1078,17 @@ let foldi f acc l =
   in
   foldi f acc 0 l
 
+let foldi2 f acc l1 l2 =
+  let rec foldi f acc i l1 l2 = match l1, l2 with
+    | [], [] -> acc
+    | [], _
+    | _, [] -> invalid_arg "foldi2"
+    | x1::l1', x2::l2' ->
+      let acc = f acc i x1 x2 in
+      foldi f acc (i+1) l1' l2'
+  in
+  foldi f acc 0 l1 l2
+
 let rec get_at_idx_rec i l = match l with
   | [] -> raise Not_found
   | x::_ when i=0 -> x

src/core/CCList.mli

@@ -334,9 +334,18 @@ val mapi : (int -> 'a -> 'b) -> 'a t -> 'b t
 
 val iteri : (int -> 'a -> unit) -> 'a t -> unit
 
+val iteri2 : (int -> 'a -> 'b -> unit) -> 'a t -> 'b t -> unit
+(** @raise Invalid_argument when lists do not have the same length
+    @since NEXT_RELEASE *)
+
 val foldi : ('b -> int -> 'a -> 'b) -> 'b -> 'a t -> 'b
 (** Fold on list, with index *)
 
+val foldi2 : ('c -> int -> 'a -> 'b -> 'c) -> 'c -> 'a t -> 'b t -> 'c
+(** Fold on two lists, with index
+    @raise Invalid_argument when lists do not have the same length
+    @since NEXT_RELEASE *)
+
 val get_at_idx : int -> 'a t -> 'a option
 (** Get by index in the list.
     If the index is negative, it will get element starting from the end