open Batteries

module type N = sig
  type t
  val make: unit -> t
  val add_to_self: t -> t -> unit
  val as_float: t -> float
end

module Int: N = struct
  type t = int ref
  let make () = ref 1
  let add_to_self x y = x := !x + !y
  let as_float = (!) |- float_of_int
end

module Float: N = struct
  type t = float ref
  let make () = ref 0.5
  let add_to_self x y = x := !x +. !y
  let as_float = (!)
end

let which_module = function
  | "float" -> (module Float: N)
  | "int" -> (module Int: N)
  | _ -> invalid_arg "which_module"

let parse: string -> (module N with type t = 'a) * 'a = fun (type a) s ->
  let module C = (val (which_module s): N) in
  (module C: N), C.make ()

let do_repetitive_work: (module N with type t = 'a) -> 'a = fun (type a) m (x: a) ->
  let module C = (val m: N with type t = a) in
  let y = C.make ();
  C.add_to_self y x;
  C.add_to_self y (C.make ());
  y

let () = if !Sys.interactive then () else
  let m, x = parse Sys.argv.(1) in
  let module C = (val m: N) in
  do_repetitive_work (module C: N with type t = C.t) x
    |> C.as_float
    |> Printf.printf "%g\n"