type t = NSString | NSMutableString | NSMutableArray

  module type Sig = sig
    val nstype : t
  end

  (* module type NSObject = sig *)
  (*   type t *)
  (*   val init : unit -> t *)
  (*   val description : t -> string *)
  (* end *)

  module Make (C : Sig) = struct
    type t
    let init () = create_object C.nstype
    let description p = call_method p "description" [||] |> string_of_nsstring
    include C
  end

  module NSString = Make(struct
      let nstype = NSString
    end)

  module NSMutableString = Make(struct
      let nstype = NSMutableString
      let set_string (p : t) (s : NSString.t) =
        call_method p "setString:" [|s|] |> ignore

    end)