From: Jacques Garrigue <garrigue@kurims.kyoto-u.ac.jp>
To: ben@socialtools.net
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] does class polymorphism need to be so complicated?
Date: Fri, 22 Aug 2003 12:59:53 +0900 [thread overview]
Message-ID: <20030822125953S.garrigue@kurims.kyoto-u.ac.jp> (raw)
In-Reply-To: <3F455730.2030407@socialtools.net>
From: Benjamin Geer <ben@socialtools.net>
> Benjamin Geer wrote:
> > I'm still curious to know why the example I gave (returning a
> > mysql_connection from a method that was typed to return a #connection)
> > didn't compile, though ("This method has type string -> mysql_connection
> > which is less general than 'a. string -> (#connection as 'a)").
Because #connection is _not_ an interface. It is just a polymorphic
type including all types having more methods than connection.
So returning a #connection means that the returned object already has
all the methods in the world, with all types imaginable. Nonsense.
#connection only makes sense on the left-hand side of an arrow.
By the way, the typing of #connection really uses rows (as by Remy and
later Pottier), but in a simplified way. So the unification Jacques
Carrette is talking about is already there from the beginning of ocaml.
> Curiously, it works if the class is parameterised instead of the method:
>
> class type ['a] driver =
> object
> constraint 'a = #connection
> method get_connection : db_name:string -> 'a
> end
>
> class mysql_driver =
> object (self : 's)
> constraint 's = #connection #driver
> method get_connection ~(db_name:string) =
> new mysql_connection db_name
> end
The meaning is completely different. Your class type says that
get_connection must at least have all methods of connection.
But in mysql_driver, you end up with a get_connection returning a
(monomorphic) mysql_connection. No progress whatsoever.
Jacques Garrigue
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next prev parent reply other threads:[~2003-08-22 4:00 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-08-20 15:42 Benjamin Geer
2003-08-20 16:05 ` Brian Hurt
2003-08-20 16:19 ` Richard Jones
2003-08-20 16:25 ` Benjamin Geer
2003-08-20 17:09 ` brogoff
2003-08-20 17:25 ` Jacques Carette
2003-08-20 23:34 ` Jacques Garrigue
2003-08-21 13:27 ` Jacques Carette
2003-08-20 18:19 ` Benjamin Geer
2003-08-20 20:39 ` brogoff
2003-08-20 21:04 ` Benjamin Geer
2003-08-21 0:28 ` Jacques Garrigue
2003-08-21 8:17 ` Benjamin Geer
2003-08-21 8:58 ` Jacques Garrigue
2003-08-21 9:38 ` Benjamin Geer
2003-08-21 11:44 ` Remi Vanicat
2003-08-21 13:11 ` Richard Jones
2003-08-21 16:41 ` Remi Vanicat
2003-08-21 18:04 ` brogoff
2003-08-21 20:20 ` Benjamin Geer
2003-08-21 23:35 ` Benjamin Geer
2003-08-22 3:59 ` Jacques Garrigue [this message]
2003-08-22 7:12 ` Benjamin Geer
2003-08-21 13:38 ` Benjamin Geer
2003-08-21 0:58 ` brogoff
2003-08-20 23:40 ` Benjamin Geer
2003-08-21 1:29 ` Jacques Garrigue
2003-08-21 9:19 ` Benjamin Geer
2003-08-21 18:44 ` Chris Clearwater
2003-08-20 20:43 ` Issac Trotts
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