[sledge] Add List.fold_diagonal

Reviewed By: ngorogiannis

Differential Revision: D28907802

fbshipit-source-id: 57b7b81c0

sledge/nonstdlib/list.ml

@@ -61,6 +61,15 @@ let fold_right xs init ~f = fold_right ~f ~init xs
 let reduce xs ~f =
   match xs with [] -> None | x :: xs -> Some (fold ~f xs x)
 
+let fold_diagonal xs init ~f =
+  let rec fold_diagonal_ z = function
+    | [] -> z
+    | x :: ys ->
+        let z = fold ~f:(fun y z -> f x y z) ys z in
+        fold_diagonal_ z ys
+  in
+  fold_diagonal_ init xs
+
 let fold_map xs init ~f =
   Pair.swap (fold_map ~f:(fun s x -> Pair.swap (f x s)) ~init xs)
 

sledge/nonstdlib/list.mli

@@ -95,6 +95,7 @@ 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 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
 val fold2_exn : 'a t -> 'b t -> 's -> f:('a -> 'b -> 's -> 's) -> 's