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 IAA06454 for caml-red; Thu, 1 Feb 2001 08:41:04 +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 UAA04788 for <caml-list@pauillac.inria.fr>; Wed, 31 Jan 2001 20:05:10 +0100 (MET)
Received: from lakeland.eecs.harvard.edu (lakeland.eecs.harvard.edu [140.247.62.173])
	by nez-perce.inria.fr (8.11.1/8.10.0) with ESMTP id f0VJ59T08206
	for <caml-list@inria.fr>; Wed, 31 Jan 2001 20:05:09 +0100 (MET)
Received: by lakeland.eecs.harvard.edu (Postfix, from userid 32148)
	id 6ACDB1FD1E; Wed, 31 Jan 2001 14:05:03 -0500 (EST)
Date: Wed, 31 Jan 2001 14:05:03 -0500
From: Christian Lindig <lindig@eecs.harvard.edu>
To: Caml Mailing List <caml-list@inria.fr>
Cc: George Russell <ger@informatik.uni-bremen.de>,
        Archisman Rudra <archi@mosaic.mrl.nyu.edu>
Subject: NaN Test in OCaml
Message-ID: <20010131140503.D2418@lakeland.eecs.harvard.edu>
Mail-Followup-To: Christian Lindig <lindig>,
	Caml Mailing List <caml-list@inria.fr>,
	George Russell <ger@informatik.uni-bremen.de>,
	Archisman Rudra <archi@mosaic.mrl.nyu.edu>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
User-Agent: Mutt/1.2.5i
Sender: weis@pauillac.inria.fr


George Russell <ger@informatik.uni-bremen.de> has suggested on
comp.lang.ml the following test to find out whether a float is NaN:

        x is not a NaN <=> (x = x)

Doing this leads to interesting results with OCaml 3.0:

    # let nan x = not (x = x);;
    val nan : 'a -> bool = <fun>
    # nan (1.0 /. 0.0);;
    - : bool = false            (* correct *)
    # nan (0.0 /. 0.0);;
    - : bool = false            (* should be true *)

The following definition of nan uses a type annotation and has a
different result:

    # let nan (x:float) = not (x = x);;
    val nan : float -> bool = <fun>
    # nan (0.0 /. 0.0);;
    - : bool = true             (* correct *)
    # nan (1.0 /. 0.0);;
    - : bool = false            (* correct *)

Is this a bug or a feature? Anyway, I guess this again shows the subtleties
of equality.

-- Christian 
        
-- 
Christian Lindig          Harvard University - DEAS
lindig@eecs.harvard.edu   33 Oxford St, MD 242, Cambridge MA 02138
phone: +1 (617) 496-7157  http://www.eecs.harvard.edu/~lindig/