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From: Michael Wohlwend <micha-1@fantasymail.de>
To: peng.zang@gmail.com, caml-list@yquem.inria.fr
Subject: Re: [Caml-list] oo type question
Date: Thu, 6 Mar 2008 16:45:30 +0100
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X-Spam: no; 0.00; donnerstag:01 subtype:01 casts:01 someclass:01 peng:98 polymorphic:01 polymorphic:01 caml-list:01 functions:01 int:01 int:01 coerce:02 objects:02 ids:03 ids:03 

Am Donnerstag, 6. M=E4rz 2008 14:18:29 schrieb Peng Zang:
> let s =3D new store ;;
> s#add (o1 :> <id:int>);;
> s#add (o2 :> <id:int>);;

> Note how I coerce o1 and o2 both to a subtype before adding.  Now the
> problem goes away because to the s#add method o1 and o2 have the same typ=
e.

yes, that's the easy way, but then I have to do casts all over the place, n=
ot=20
very nice

> BUT, what I think you really want is a polymorphic method, not a
> polymorphic class.  The manual has a decent explanation under polymohrphic
> methods in the Objects section.  So I think you want this:
>   method add : 'a. <id:int; ..> as 'a -> unit =3D
>     fun o -> ids <- o#id :: ids

I know that version and of course it works. I only  didn't understand (and=
=20
still do not) why it's not possible to just declare
method add (a: #someClass) ... which seems natural for me. And it works for=
=20
normal functions that way.

If you have many classes (actually it's for adding widgets to group widgets=
)=20
it seems just a bit too complicated and not a natural way to declare such a=
=20
simple thing.


thanks,
 Michael