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From: Didier Cassirame <didier.cassirame@gmail.com>
To: "caml-list@inria.fr" <caml-list@inria.fr>
Content-Type: text/plain; charset=UTF-8
Subject: [Caml-list] parameterized classes, modules & polymorphic variants

Dear caml-list,

I have been trying recently to combine classes, modules and variants
in the following fashion:

module A1 = struct

  class ['a] t = object
       constraint 'a = [>`a]
       method m : 'a -> string = function `a -> "a" | `a1 -> "a1" | _ -> "_"
   end

end;;

module A2 = struct

  class ['a] t = object
       constraint 'a = [>`a]
       method m : 'a -> string = function `a -> "a" | `a2 -> "a2" | _ -> "_"
   end

end;;

module A3 = struct

  class ['a] t = object
       constraint 'a = [>`a]
       method m : 'a -> string = function `a -> "a" | `a3 -> "a3" | _ -> "_"
   end

end;;

module type A = sig

  class ['a] t : object
       constraint 'a = [>`a]
       method m : 'a -> string
   end

end;;

type m = (module A);;

let l: m list = [ (module A1); (module A2); (module A3)];;

--------------------------------

Unfortunately the list typecheck fails. However, making a list of
class instances from A1.t, A2.t, A3.t succeed, with the type:

 [> `a | `a1 | `a2 | `a3 ] ct list

ct being defined as equal to A.t.

I thought that perhaps I should parameterize the type m from the type
parameter 'a of A.t to solve my problem, but I am not sure of the
syntax, or if it's the problem. Does anyone have an idea?

didier