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Date: Tue, 1 Jul 2008 17:14:37 +0200
From: Virgile Prevosto <virgile.prevosto@m4x.org>
To: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Open class types
Message-ID: <20080701171437.0cddf254@is005115>
In-Reply-To: <486A36DE.4040605@stork.ru>
References: <486A36DE.4040605@stork.ru>
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X-Spam: no; 0.00; subtype:01 subtype:01 ocaml:01 syntax:01 tutto:98 oggi:98 volta:98 polymorphic:01 polymorphic:01 caml-list:01 constraint:01 constraint:01 int:01 int:01 expression:02 

Hello,

Le mar 01 jui 2008 17:53:34 CEST,
SerP <serp@stork.ru> a =C3=A9crit :

> class type session_t =3D
>   object
>     method id:int;
>     method location: #location_t;
>   end;
>=20
...
>=20
> Error: This expression has type session =3D < id : int; location :
> location > but is here used with type
>          #session_t as 'a =3D < id : int; location : 'b.
> #location_t; .. >

My guess is that session_t is a bit too polymorphic: location is
a polymorphic method (indicated by the 'b. in the error message). Thus,
a class implementing session_t should have a location method able to
return any subtype of location_t, which might be a bit complicated to
say the least. A more sensible class type would require that the
location method returns a subtype of location_t, which may differ from
one implementation to the other, but is fixed for a given
implementation. By the way, this matches more or less your second
solution.
At the class/class type level, this can be done by adding a type
parameter to the class type and using a constraint as shown below
(warning, this is the regular ocaml syntax, not the revised one):

class type ['a]session_t =3D
  object
    method id:int
    method location: 'a
    constraint 'a =3D #location_t
  end

let check_session (session:'a #session_t) =3D begin
  print_int session#id;
  print_int session#location#a;
  print_string session#location#b;
end

--=20
E tutto per oggi, a la prossima volta.
Virgile