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Date: Wed, 7 Jan 2015 16:30:09 +0100
From: Goswin von Brederlow <goswin-v-b@web.de>
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Subject: Re: [Caml-list] Problem with universal functions in a module

Somehow the example code got lost:

module M = struct
  type _ t = Int : int -> int t | Float : float -> float t
  let print : type a . a t -> unit = function
    | Int i -> Printf.printf "%d\n" i
    | Float f -> Printf.printf "%f\n" f
  (* should be abstract *)
  type box = Box : 'a t -> box
  let box t = Box t

  (*
  let call_unboxed fn (Box t) = fn t
  Error: This expression has type a#0 t but an expression was expected of type
         a#0 t
       The type constructor a#0 would escape its scope
  *)

  (* should be abstract *)
  type 'a helper = { fn : 'b . 'b t -> 'a }

  (*
  let helper fn = { fn }
  Error: This field value has type 'c t -> 'd which is less general than
         'b. 'b t -> 'a
  *)

  let call helper (Box t) = helper.fn t
end  

open M
let t = [box (Int 1); box (Float 2.0)]  

(* Works but is not possible if box is abstract *)
let () = List.iter (function Box x -> print x) t

(* Use conversion helper to call unboxed values with a function *)
let h = { fn = print }
let () = List.iter (call h) t

MfG
	Goswin