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[Caml-list] polymorphic methods
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Date: -- (:)
From: nadji@n...
Subject: Re: [Caml-list] polymorphic methods
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 :)

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