Received: (at 16365) by debbugs.gnu.org; 21 Jun 2016 14:21:36 +0000 From debbugs-submit-bounces <at> debbugs.gnu.org Tue Jun 21 10:21:36 2016 Received: from localhost ([127.0.0.1]:49851 helo=debbugs.gnu.org) by debbugs.gnu.org with esmtp (Exim 4.84_2) (envelope-from <debbugs-submit-bounces <at> debbugs.gnu.org>) id 1bFMYa-00056M-6R for submit <at> debbugs.gnu.org; Tue, 21 Jun 2016 10:21:36 -0400 Received: from river.fysh.org ([87.98.248.19]:43731 ident=Debian-exim) by debbugs.gnu.org with esmtp (Exim 4.84_2) (envelope-from <zefram@HIDDEN>) id 1bFMYX-00056B-QC for 16365 <at> debbugs.gnu.org; Tue, 21 Jun 2016 10:21:34 -0400 Received: from zefram by river.fysh.org with local (Exim 4.84_2 #1 (Debian)) id 1bFMYU-00034A-24; Tue, 21 Jun 2016 15:21:30 +0100 Date: Tue, 21 Jun 2016 15:21:30 +0100 From: Zefram <zefram@HIDDEN> To: Mark H Weaver <mhw@HIDDEN> Subject: Re: bug#16365: (* 0 +inf.0) rationale is flawed Message-ID: <20160621142130.GA1170@HIDDEN> References: <20140106001719.GI21945@HIDDEN> <87r3bqzpbd.fsf@HIDDEN> <87vb12skz7.fsf@HIDDEN> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87vb12skz7.fsf@HIDDEN> X-Spam-Score: -1.4 (-) X-Debbugs-Envelope-To: 16365 Cc: Andy Wingo <wingo@HIDDEN>, 16365 <at> debbugs.gnu.org X-BeenThere: debbugs-submit <at> debbugs.gnu.org X-Mailman-Version: 2.1.18 Precedence: list List-Id: <debbugs-submit.debbugs.gnu.org> List-Unsubscribe: <https://debbugs.gnu.org/cgi-bin/mailman/options/debbugs-submit>, <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=unsubscribe> List-Archive: <https://debbugs.gnu.org/cgi-bin/mailman/private/debbugs-submit/> List-Post: <mailto:debbugs-submit <at> debbugs.gnu.org> List-Help: <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=help> List-Subscribe: <https://debbugs.gnu.org/cgi-bin/mailman/listinfo/debbugs-submit>, <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=subscribe> Errors-To: debbugs-submit-bounces <at> debbugs.gnu.org Sender: "Debbugs-submit" <debbugs-submit-bounces <at> debbugs.gnu.org> X-Spam-Score: -1.4 (-) Mark H Weaver wrote: > I also suspect that (/ 0 <anything_but_exact_0>) should be 0, >although that conflicts with R6RS. We should probably investigate the >rationale behind R6RS's decision to specify that (/ 0 0.0) returns a NaN >before changing that, though. I think R6RS makes sense for (/ 0 0.0). A flonum zero really represents a range of values including both small non-zero numbers and actual zero. The mathematical result of the division could therefore be either zero or undefined. To return zero for it would be picking a particular result, on the assumption that the flonum zero actually represented a non-zero value, and that's not justified. So to use the flonum behaviour seems the best thing available. (/ 0 3.5) is a different case. Here the mathematical result is an exact zero, and I'm surprised that R6RS specifies that this should be an inexact zero. This seems inconsistent with (* 1.0 0), for which it specifies that the result may be either 0 or 0.0. I'd also question R6RS in the related case of (/ 0.0 0). Mathematically this division is definitely an error, regardless of whether the dividend represents zero or a non-zero number. So it would make sense for this to raise an exception in the same manner as (/ 3 0) or (/ 0 0), rather than get flonum treatment as R6RS specifies. But deviating from R6RS, even with a good rationale for other behaviour, would be a bad idea. The questionable R6RS requirements are not crazy, just suboptimal. The case I originally raised, (* 0 +inf.0), is one for which R6RS offers the choice. -zefram
bug-guile@HIDDEN:bug#16365; Package guile.
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From: Mark H Weaver <mhw@HIDDEN>
To: Andy Wingo <wingo@HIDDEN>
Subject: Re: bug#16365: (* 0 +inf.0) rationale is flawed
References: <20140106001719.GI21945@HIDDEN> <87r3bqzpbd.fsf@HIDDEN>
Date: Tue, 21 Jun 2016 09:57:32 -0400
In-Reply-To: <87r3bqzpbd.fsf@HIDDEN> (Andy Wingo's message of "Tue, 21 Jun
2016 14:41:58 +0200")
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Andy Wingo <wingo@HIDDEN> writes:
> Thoughts, Mark?
Sorry for the long delay on this, but briefly, I agree that my rationale
was flawed, and that we should make (* 0 <anything>) == 0 in all cases
in 2.2. I also suspect that (/ 0 <anything_but_exact_0>) should be 0,
although that conflicts with R6RS. We should probably investigate the
rationale behind R6RS's decision to specify that (/ 0 0.0) returns a NaN
before changing that, though.
I hope to work more on this soon.
Thanks,
Mark
> On Mon 06 Jan 2014 01:17, Zefram <zefram@HIDDEN> writes:
>
>> Commit 5e7918077a4015768a352ab19e4a8e94531bc8aa says
>>
>> A note on the rationale for (* 0 +inf.0) being a NaN and not exact 0:
>> The R6RS requires that (/ 0 0.0) return a NaN value, and that (/ 0.0)
>> return +inf.0. We would like (/ x y) to be the same as (* x (/ y)),
>>
>> This identity doesn't actually hold. For example, on guile 2.0.9 with
>> IEEE double flonums:
>>
>> scheme@(guile-user)> (/ (expt 2.0 -20) (expt 2.0 -1026))
>> $36 = 6.857655085992111e302
>> scheme@(guile-user)> (* (expt 2.0 -20) (/ (expt 2.0 -1026)))
>> $37 = +inf.0
>>
>> This case arises because the dynamic range of this flonum format is
>> slightly asymmetric: 2^-1026 is representable, but 2^1026 overflows.
>>
>> So the rationale for (* 0 +inf.0) yielding +nan.0 is flawed. As the
>> supposed invariant and the rationale are not in the actual documentation
>> (only mentioned in the commit log) this is not necessarily a bug.
>> But worth thinking again to determine whether the case for adopting
>> the flonum behaviour here is still stronger than the obvious case for
>> the exact zero to predominate. (Mathematically, multiplying zero by an
>> infinite number does yield zero. Let alone multiplying it by a merely
>> large finite number, which is what the flonum indefinite `infinity'
>> really represents.)
>>
>> -zefram
bug-guile@HIDDEN:bug#16365; Package guile.
Full text available.Received: (at 16365) by debbugs.gnu.org; 21 Jun 2016 12:42:10 +0000 From debbugs-submit-bounces <at> debbugs.gnu.org Tue Jun 21 08:42:10 2016 Received: from localhost ([127.0.0.1]:48720 helo=debbugs.gnu.org) by debbugs.gnu.org with esmtp (Exim 4.84_2) (envelope-from <debbugs-submit-bounces <at> debbugs.gnu.org>) id 1bFL0M-0002Lw-1L for submit <at> debbugs.gnu.org; Tue, 21 Jun 2016 08:42:10 -0400 Received: from pb-sasl1.pobox.com ([64.147.108.66]:52556 helo=sasl.smtp.pobox.com) by debbugs.gnu.org with esmtp (Exim 4.84_2) (envelope-from <wingo@HIDDEN>) id 1bFL0K-0002Lp-Ht for 16365 <at> debbugs.gnu.org; Tue, 21 Jun 2016 08:42:08 -0400 Received: from sasl.smtp.pobox.com (unknown [127.0.0.1]) by pb-sasl1.pobox.com (Postfix) with ESMTP id B622E22AEF; Tue, 21 Jun 2016 08:42:06 -0400 (EDT) DKIM-Signature: v=1; a=rsa-sha1; c=relaxed; d=pobox.com; h=from:to:cc :subject:references:date:in-reply-to:message-id:mime-version :content-type; s=sasl; bh=yIfwRR9zfSwwK+DZbRAxssILQIU=; b=n6dpbu w7k8Lu1zlFTuWAR0kvCxCVdEf9CJ2v1chb+oz2cqeUJ5Uxl2p0pGsEapq6ngVvdh AL1vZ2IRmD0VC3u1go4+/vAgezrAmN2eRLBLgLgMOERnIYXiB6nFYuCcoNSJyCzO MYcnT4dVYXSpOgwKkTEO4wkjKMySrlISDy8Ck= DomainKey-Signature: a=rsa-sha1; c=nofws; d=pobox.com; h=from:to:cc :subject:references:date:in-reply-to:message-id:mime-version :content-type; q=dns; s=sasl; b=u0Mkq+0RatoPt8Ekg2uBBrFISv06tsz9 WULj+fTxppwfF5bOjGu/dEy4QknJ9acMqPZ1F1H7JQNKffwnrSBQrQ7Yon7EhM/A 4brgnYcrpnaKQGEP25rflAENsxFbtsDL5WJVr6T0ZsA/BfecopfUP4S0UbuwTyK0 VoZ1tXm1qUs= Received: from pb-sasl1.nyi.icgroup.com (unknown [127.0.0.1]) by pb-sasl1.pobox.com (Postfix) with ESMTP id AE77322AED; Tue, 21 Jun 2016 08:42:06 -0400 (EDT) Received: from clucks (unknown [88.160.190.192]) (using TLSv1 with cipher ECDHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by pb-sasl1.pobox.com (Postfix) with ESMTPSA id C26BB22AEC; Tue, 21 Jun 2016 08:42:05 -0400 (EDT) From: Andy Wingo <wingo@HIDDEN> To: mhw@HIDDEN Subject: Re: bug#16365: (* 0 +inf.0) rationale is flawed References: <20140106001719.GI21945@HIDDEN> Date: Tue, 21 Jun 2016 14:41:58 +0200 In-Reply-To: <20140106001719.GI21945@HIDDEN> (zefram@HIDDEN's message of "Mon, 6 Jan 2014 00:17:19 +0000") Message-ID: <87r3bqzpbd.fsf@HIDDEN> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Pobox-Relay-ID: 8FB0BE02-37AD-11E6-AE4A-C1836462E9F6-02397024!pb-sasl1.pobox.com X-Spam-Score: -1.4 (-) X-Debbugs-Envelope-To: 16365 Cc: Zefram <zefram@HIDDEN>, 16365 <at> debbugs.gnu.org X-BeenThere: debbugs-submit <at> debbugs.gnu.org X-Mailman-Version: 2.1.18 Precedence: list List-Id: <debbugs-submit.debbugs.gnu.org> List-Unsubscribe: <https://debbugs.gnu.org/cgi-bin/mailman/options/debbugs-submit>, <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=unsubscribe> List-Archive: <https://debbugs.gnu.org/cgi-bin/mailman/private/debbugs-submit/> List-Post: <mailto:debbugs-submit <at> debbugs.gnu.org> List-Help: <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=help> List-Subscribe: <https://debbugs.gnu.org/cgi-bin/mailman/listinfo/debbugs-submit>, <mailto:debbugs-submit-request <at> debbugs.gnu.org?subject=subscribe> Errors-To: debbugs-submit-bounces <at> debbugs.gnu.org Sender: "Debbugs-submit" <debbugs-submit-bounces <at> debbugs.gnu.org> X-Spam-Score: -1.4 (-) Thoughts, Mark? On Mon 06 Jan 2014 01:17, Zefram <zefram@HIDDEN> writes: > Commit 5e7918077a4015768a352ab19e4a8e94531bc8aa says > > A note on the rationale for (* 0 +inf.0) being a NaN and not exact 0: > The R6RS requires that (/ 0 0.0) return a NaN value, and that (/ 0.0) > return +inf.0. We would like (/ x y) to be the same as (* x (/ y)), > > This identity doesn't actually hold. For example, on guile 2.0.9 with > IEEE double flonums: > > scheme@(guile-user)> (/ (expt 2.0 -20) (expt 2.0 -1026)) > $36 = 6.857655085992111e302 > scheme@(guile-user)> (* (expt 2.0 -20) (/ (expt 2.0 -1026))) > $37 = +inf.0 > > This case arises because the dynamic range of this flonum format is > slightly asymmetric: 2^-1026 is representable, but 2^1026 overflows. > > So the rationale for (* 0 +inf.0) yielding +nan.0 is flawed. As the > supposed invariant and the rationale are not in the actual documentation > (only mentioned in the commit log) this is not necessarily a bug. > But worth thinking again to determine whether the case for adopting > the flonum behaviour here is still stronger than the obvious case for > the exact zero to predominate. (Mathematically, multiplying zero by an > infinite number does yield zero. Let alone multiplying it by a merely > large finite number, which is what the flonum indefinite `infinity' > really represents.) > > -zefram
bug-guile@HIDDEN:bug#16365; Package guile.
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From: Zefram <zefram@HIDDEN>
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Subject: (* 0 +inf.0) rationale is flawed
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Commit 5e7918077a4015768a352ab19e4a8e94531bc8aa says
A note on the rationale for (* 0 +inf.0) being a NaN and not exact 0:
The R6RS requires that (/ 0 0.0) return a NaN value, and that (/ 0.0)
return +inf.0. We would like (/ x y) to be the same as (* x (/ y)),
This identity doesn't actually hold. For example, on guile 2.0.9 with
IEEE double flonums:
scheme@(guile-user)> (/ (expt 2.0 -20) (expt 2.0 -1026))
$36 = 6.857655085992111e302
scheme@(guile-user)> (* (expt 2.0 -20) (/ (expt 2.0 -1026)))
$37 = +inf.0
This case arises because the dynamic range of this flonum format is
slightly asymmetric: 2^-1026 is representable, but 2^1026 overflows.
So the rationale for (* 0 +inf.0) yielding +nan.0 is flawed. As the
supposed invariant and the rationale are not in the actual documentation
(only mentioned in the commit log) this is not necessarily a bug.
But worth thinking again to determine whether the case for adopting
the flonum behaviour here is still stronger than the obvious case for
the exact zero to predominate. (Mathematically, multiplying zero by an
infinite number does yield zero. Let alone multiplying it by a merely
large finite number, which is what the flonum indefinite `infinity'
really represents.)
-zefram
Zefram <zefram@HIDDEN>:bug-guile@HIDDEN.
Full text available.bug-guile@HIDDEN:bug#16365; Package guile.
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