Make Seq.fold_lefti documentation and implementation consistent with respect to argument order

Changes

@@ -215,10 +215,11 @@ OCaml 4.14.0
   (Nicolás Ojeda Bär, review by John Whitington, Daniel Bünzli, David Allsopp
   and Xavier Leroy)
 
-* #10583: Add over 40 new functions in Seq.
+* #10583, #10998: Add over 40 new functions in Seq.
   (François Pottier and Simon Cruanes, review by Nicolás Ojeda Bär,
-   Daniel Bünzli, Naëla Courant, Craig Ferguson, Wiktor Kuchta, Xavier
-   Leroy, Guillaume Munch-Maccagnoni, Raphaël Proust, and Gabriel Scherer)
+  Daniel Bünzli, Naëla Courant, Craig Ferguson, Wiktor Kuchta,
+  Xavier Leroy, Guillaume Munch-Maccagnoni, Raphaël Proust, Gabriel Scherer
+  and Thierry Martinez)
 
 - #10596, #10978: Add with_open_bin, with_open_text and with_open_gen to
   In_channel and Out_channel. Also, add In_channel.input_all.

stdlib/seq.ml

@@ -118,16 +118,16 @@ let rec iteri_aux f i xs =
 let[@inline] iteri f xs =
   iteri_aux f 0 xs
 
-let rec fold_lefti_aux f i accu xs =
+let rec fold_lefti_aux f accu i xs =
   match xs() with
   | Nil ->
       accu
   | Cons (x, xs) ->
-      let accu = f i accu x in
-      fold_lefti_aux f (i+1) accu xs
+      let accu = f accu i x in
+      fold_lefti_aux f accu (i+1) xs
 
 let[@inline] fold_lefti f accu xs =
-  fold_lefti_aux f 0 accu xs
+  fold_lefti_aux f accu 0 xs
 
 let rec for_all p xs =
   match xs() with

stdlib/seq.mli

@@ -187,11 +187,11 @@ val iteri : (int -> 'a -> unit) -> 'a t -> unit
 
     @since 4.14 *)
 
-val fold_lefti : (int -> 'b -> 'a -> 'b) -> 'b -> 'a t -> 'b
+val fold_lefti : ('b -> int -> 'a -> 'b) -> 'b -> 'a t -> 'b
 (** [fold_lefti f _ xs] invokes [f _ i x] successively
     for every element [x] located at index [i] of the sequence [xs].
 
-    An accumulator of type ['a] is threaded through the calls to [f].
+    An accumulator of type ['b] is threaded through the calls to [f].
 
     It terminates only if the sequence [xs] is finite.
 

testsuite/tests/lib-seq/test.ml

@@ -89,7 +89,7 @@ let () =
 let () =
   let xs = !?["a"; "b"] in
   assert (
-    Seq.fold_lefti (fun i acc x -> (i, x) :: acc) [] xs = [ 1, "b"; 0, "a" ]
+    Seq.fold_lefti (fun acc i x -> (i, x) :: acc) [] xs = [ 1, "b"; 0, "a" ]
   )
 
 (* [scan] *)