]> Jacques Garrigue wrote: > > # let f = (new c)#m ();; > > val f : '_a -> '_a = <fun> > > Unsound. RTFM (polymorphism restricted to values) > Try "let f x = (new c)#m () x" > Oups, sorry. I did'nt recognize this classical problem ... In fact this example was an attempt to simplify my next question, but I can see now that they are not related. > But I don't see why you need (). > You're right. It was there for historical reasons and I had not realized I did'nt need an argument anymore. > > 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. That's a shame because if one tries to use objects for manipulating lists : # exception Emptylist class type ['a] olist = object method hd : 'a method tl : 'a olist end class nil = object method ~hd: 'a . 'a = raise Emptylist method ~tl: 'a . 'a olist = raise Emptylist end let objnil = (new nil :> 'a olist);; Characters 14-21: let objnil = (new nil :> 'a olist);; ^^^^^^^ This expression cannot be coerced to type 'a olist = < hd : 'a; tl : 'a olist >; it has type nil = < hd : 'b. 'b; tl : 'c. 'c olist > but is here used with type < hd : 'a; tl : 'a #olist >. This simple coercion was not fully general. Consider using a double coercion. Too bad. Also this is the first time I get this error message. Does it mean that I have to try something like (a : b :> c) ? If not, what is a double coercion, and can you give a case where is it needed ? > > I believe it's correct, but you won't make me write the code without a > proof :-) This is why I like Ocaml :) ------------------- 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