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Date: Mon, 15 Jul 2002 01:16:53 +0200
From: nadji@noos.fr
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To: caml-list@inria.fr
Subject: [Caml-list] polymorphic methods
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Hello,

While playing with the current cvs version of ocaml (3.04+15),
several questions came to me concerning the semantics of
polymorphic methods. For example :

# class c = object
  method ~m: 'a . unit -> 'a -> 'a = fun _ x -> x
end;;
    class c : object method m : unit -> 'a -> 'a end
# let f = (new c)#m ();;
val f : '_a -> '_a = <fun>

Shouldn't the type of f be : 'a -> 'a  ?
(Would it be sound/implementable/easy to add ?)

Second question:
# class ['a] d = object
  method m: 'a -> 'a = fun x -> x
end;;
    class ['a] d : object method m : 'a -> 'a end
# class subd = object
  method ~m: 'a. 'a -> 'a = fun x -> x
end;;
    class subd : object method m : 'a -> 'a end
# let v = (new subd :> 'a d);;
Characters 9-17:
  let v = (new subd :> 'a d);;
           ^^^^^^^^
This expression cannot be coerced to type 'a d = < m : 'a -> 'a >;
it has type subd = < m : 'b. 'b -> 'b > but is here used with type 'a d

Or even the easier :
# class d' = object
  method m x = x + 0
end;;
    class d' : object method m : int -> int end
# let v' = (new subd :> d');;
Characters 10-18:
  let v' = (new subd :> d');;
            ^^^^^^^^
This expression cannot be coerced to type d' = < m : int -> int >;
it has type subd = < m : 'a. 'a -> 'a > but is here used with type d'

But the type ('b. 'b -> 'b) is more general than ('a -> 'a)
or (int -> int), no ?
Can someone give me some hints why I can't coerce subd ?

Btw, thanks for the upcoming release, it seems _really_ excellent !
Nadji

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