[utils] add fold_left_i utility

Reviewed By: jeremydubreil

Differential Revision: D3851332

fbshipit-source-id: a5649e7

infer/src/backend/iList.ml

@@ -38,6 +38,11 @@ let fold_right f l a =
   let g x y = f y x in
   fold_left g a (rev l)
 
+(** fold_left with indices *)
+let fold_lefti (f : 'a -> int -> 'b -> 'a) a l =
+  fold_left (fun (i, acc) e -> i +1, f acc i e) (0, a) l
+  |> snd
+
 (** tail-recursive variant of List.combine *)
 let combine =
   let rec combine acc l1 l2 = match l1, l2 with

infer/src/backend/iList.mli

@@ -42,6 +42,9 @@ val length : 'a list -> int
 (** tail-recursive variant of List.fold_right *)
 val fold_right : ('a -> 'b -> 'b) -> 'a list -> 'b -> 'b
 
+(** fold_left with indices *)
+val fold_lefti : ('a -> int -> 'b -> 'a) -> 'a -> 'b list -> 'a
+
 (** tail-recursive variant of List.map *)
 val map : ('a -> 'b) -> 'a list -> 'b list