main.ml

module type List = sig
  type a
  type t
  val nil : t
  val cons : a -> t -> t
  val switch : (unit -> 'b) -> (a -> t -> 'b) -> t -> 'b
end

let to_string (type e) (type s) (module Impl : List with type a = e and type t = s) string_of_item l =
  let rec inner ys =
    Impl.switch
      (fun () -> "")
      (fun x xs -> string_of_item x ^ " " ^ inner xs)
      ys
  in
  inner l