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  1.  
  2. open Batteries
  4. module type N = sig
  5. type t
  6. val make: unit -> t
  7. val add_to_self: t -> t -> unit
  8. val as_float: t -> float
  9. end
  11. module Int: N = struct
  12. type t = int ref
  13. let make () = ref 1
  14. let add_to_self x y = x := !x + !y
  15. let as_float = (!) |- float_of_int
  16. end
  18. module Float: N = struct
  19. type t = float ref
  20. let make () = ref 0.5
  21. let add_to_self x y = x := !x +. !y
  22. let as_float = (!)
  23. end
  25. let which_module = function
  26. | "float" -> (module Float: N)
  27. | "int" -> (module Int: N)
  28. | _ -> invalid_arg "which_module"
  30. let parse: string -> (module N with type t = 'a) * 'a = fun (type a) s ->
  31. let module C = (val (which_module s): N) in
  32. (module C: N), C.make ()
  34. let do_repetitive_work: (module N with type t = 'a) -> 'a -> 'a = fun (type a) m (x: a) ->
  35. let module C = (val m: N with type t = a) in
  36. let y = C.make ();
  37. C.add_to_self y x;
  38. C.add_to_self y (C.make ());
  39. y
  41. let () = if !Sys.interactive then () else
  42. let m, x = parse Sys.argv.(1) in
  43. let module C = (val m: N) in
  44. do_repetitive_work (module C: N with type t = C.t) x
  45. |> C.as_float
  46. |> Printf.printf "%g\n"
  47.  
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