From mboxrd@z Thu Jan  1 00:00:00 1970
Received: (from weis@localhost) by pauillac.inria.fr (8.7.6/8.7.3) id NAA26160 for caml-redistribution; Thu, 18 Feb 1999 13:21:49 +0100 (MET)
Received: from concorde.inria.fr (concorde.inria.fr [192.93.2.39]) by pauillac.inria.fr (8.7.6/8.7.3) with ESMTP id UAA06299 for <caml-list@pauillac.inria.fr>; Wed, 17 Feb 1999 20:11:27 +0100 (MET)
Received: from morgon.inria.fr (morgon.inria.fr [128.93.8.33])
	by concorde.inria.fr (8.8.7/8.8.7) with ESMTP id UAA27273;
	Wed, 17 Feb 1999 20:11:24 +0100 (MET)
Received: (from remy@localhost)
	by morgon.inria.fr (8.8.7/8.8.7) id UAA07513;
	Wed, 17 Feb 1999 20:14:04 +0100
Message-ID: <19990217201403.48891@morgon.inria.fr>
Date: Wed, 17 Feb 1999 20:14:03 +0100
From: Didier Remy <remy@morgon.inria.fr>
To: Christopher Jeris <cjeris@math.mit.edu>
Cc: Xavier Leroy <Xavier.Leroy@inria.fr>, caml-list@inria.fr
Subject: Re: anonymous record types in variants
Reply-To: Didier.Remy@inria.fr
References: <19990217103259.47130@pauillac.inria.fr> <Pine.SUN.4.03.9902171247250.28377-100000@severi.mit.edu>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
X-Mailer: Mutt 0.85e
In-Reply-To: <Pine.SUN.4.03.9902171247250.28377-100000@severi.mit.edu>; from Christopher Jeris on Wed, Feb 17, 1999 at 01:09:06PM -0500
Organization: INRIA, BP 105, F-78153 Le Chesnay Cedex
Phone: (33) 1 3963 5317 -- Sec: (33) 1 3963 5570 -- Fax: (33) 1 3963 5684
Web: http://cristal.inria.fr/~remy
Sender: weis

> But an analogous construction already exists in the object system:

I was expecting this question...  

Yes, it is possible to have anonymous records and still keep principal
types, using `polymorphic' records (a restriction of `extensible records')
--this is the way objects are typed.

However, this will cost at runtime, unless there are very severe,
complicated compile-time/link-time optimizations. (In the case of objects,
this cost is expected because you are programming by message invocation.)

Since there are already objects in Ocaml, the need for
polymorphic/extensible records is not so strong. This would be convenient
but would not add much expressivity (you can always use objects).  Hence the
gain may be insufficient to justify a complication of the language and of
the implementation.

> Another thing that kind of confuses me is open object types.  Can the open
> object type < one : int; .. > be defined directly in any way ?
> 
>   type foo = < one : int; .. >

Yes, but since this is a polymorphic type you have to write:

        type 'a foo  = 'a constraint 'a = < one : int; ..>;;

This captures the polymorphic row variable `..' (that you are not allowed to
name directly) within the constrained type parameter 'a.

    Didier.