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 MAA18955 for caml-redistribution; Mon, 18 Jan 1999 12:10:26 +0100 (MET)
Received: from nez-perce.inria.fr (nez-perce.inria.fr [192.93.2.78]) by pauillac.inria.fr (8.7.6/8.7.3) with ESMTP id LAA31469 for <caml-list@pauillac.inria.fr>; Mon, 18 Jan 1999 11:05:52 +0100 (MET)
Received: from post.tepkom.ru (relay.tepkom.ru [195.9.240.162])
	by nez-perce.inria.fr (8.8.7/8.8.7) with ESMTP id LAA01488
	for <caml-list@inria.fr>; Mon, 18 Jan 1999 11:05:48 +0100 (MET)
Received: from localhost (msk@localhost)
	by post.tepkom.ru (8.8.7/8.8.7) with ESMTP id NAA25552;
	Mon, 18 Jan 1999 13:05:42 +0300
Date: Mon, 18 Jan 1999 13:05:42 +0300 (MSK)
From: Anton Moscal <msk@post.tepkom.ru>
To: Jerry Jackson <jrj@channelpoint.com>
cc: "'caml-list@inria.fr'" <caml-list@inria.fr>
Subject: Re: null values and sentinels
In-Reply-To: <8A24EC12044FD21195E200600895E0B34BFBC2@goat.channelpoint.com>
Message-ID: <Pine.LNX.4.03.9901181301500.25253-100000@post.tepkom.ru>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: weis



On Tue, 12 Jan 1999, Jerry Jackson wrote:

> Hello,
> 
> I'd like to know what the recommended solution to the following 
> situation is...
> 
> I'd like to create a type like:
> 
> # type foo = {xxx: int; mutable yyy: foo};;
> 
> so that I can add nodes by mutating the "yyy" slots.  Unfortunately,
> it seems that I must create a sentinel value of type "foo" to stand
> for a null pointer.  This works but I'd like to be able to use pattern
> matching on the value of yyy so as to get compile-time warnings if I
> forget to check for null.

The only known to me solution is the following:

declare

    let rec foo_nil = {xxx=1; yyy = foo_nil};

and then use foo_nil as Nil constant. The main problem is the
following: foo_nil can't be used in the matching.
 
Anton E.Moscal