]> From: nadji@noos.fr > 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 ?) Unsound. RTFM (polymorphism restricted to values) Try "let f x = (new c)#m () x" But I don't see why you need (). > 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 ? Sorry, but there is no handling of instanciation via subtyping. Currently subtyping and instanciation are orthogonal concepts: you cannot subtype in an .mli, and you cannot instanciate an explicitly polymorphic type when subtyping. I believe it's correct, but you won't make me write the code without a proof :-) Jacques Garrigue ------------------- To unsubscribe, mail caml-list-request@inria.fr Archives: http://caml.inria.fr Bug reports: http://caml.inria.fr/bin/caml-bugs FAQ: http://caml.inria.fr/FAQ/ Beginner's list: http://groups.yahoo.com/group/ocaml_beginners