let comp f g x = f (g x) ;;

type 'a list = Nil | Cons of 'a * 'a list
;;

(* rev :: list -> list *)
let rev l =
  (* walk :: list -> (list -> list) *)
  let rec walk xs =
    match xs with
    | Nil -> (fun x -> x)
    | Cons(x,xs') ->
       comp (walk xs')
	 (fun ys -> Cons(x,ys))
  in walk l Nil
;;
()