From df4ce1995436c4f3777baad83aba088967d5ddd4 Mon Sep 17 00:00:00 2001
From: Josh Berdine <jjb@fb.com>
Date: Thu, 1 Jul 2021 03:30:30 -0700
Subject: [PATCH] [sledge] Add List.foldi

Differential Revision: D29441163

fbshipit-source-id: e58b57072
---
 sledge/nonstdlib/list.ml  | 1 +
 sledge/nonstdlib/list.mli | 1 +
 2 files changed, 2 insertions(+)

diff --git a/sledge/nonstdlib/list.ml b/sledge/nonstdlib/list.ml
index 30e7378b2..2e86ce033 100644
--- a/sledge/nonstdlib/list.ml
+++ b/sledge/nonstdlib/list.ml
@@ -57,6 +57,7 @@ let group_join_by ~eq ~hash = group_join_by ~eq ~hash
 let fold xs init ~f = fold_left ~f:(fun s x -> f x s) ~init xs
 let fold_left xs init ~f = fold_left ~f ~init xs
 let fold_right xs init ~f = fold_right ~f ~init xs
+let foldi xs init ~f = foldi ~f:(fun s i x -> f i x s) ~init xs
 
 let reduce xs ~f =
   match xs with [] -> None | x :: xs -> Some (fold ~f xs x)
diff --git a/sledge/nonstdlib/list.mli b/sledge/nonstdlib/list.mli
index 947b2a13e..8ac55c503 100644
--- a/sledge/nonstdlib/list.mli
+++ b/sledge/nonstdlib/list.mli
@@ -94,6 +94,7 @@ val group_join_by :
 val fold : 'a list -> 's -> f:('a -> 's -> 's) -> 's
 val fold_left : 'a list -> 's -> f:('s -> 'a -> 's) -> 's
 val fold_right : 'a list -> 's -> f:('a -> 's -> 's) -> 's
+val foldi : 'a t -> 's -> f:(int -> 'a -> 's -> 's) -> 's
 val reduce : 'a t -> f:('a -> 'a -> 'a) -> 'a option
 val fold_diagonal : 'a list -> 's -> f:('a -> 'a -> 's -> 's) -> 's
 val fold_map : 'a t -> 's -> f:('a -> 's -> 'b * 's) -> 'b t * 's