From: Zefram <zefram@fysh.org>
To: Mark H Weaver <mhw@netris.org>
Cc: 16365@debbugs.gnu.org
Subject: bug#16365: (* 0 +inf.0) rationale is flawed
Date: Tue, 21 Jun 2016 15:21:30 +0100 [thread overview]
Message-ID: <20160621142130.GA1170@fysh.org> (raw)
In-Reply-To: <87vb12skz7.fsf@netris.org>
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
prev parent reply other threads:[~2016-06-21 14:21 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-01-06 0:17 bug#16365: (* 0 +inf.0) rationale is flawed Zefram
2016-06-21 12:41 ` Andy Wingo
2016-06-21 13:57 ` Mark H Weaver
2016-06-21 14:21 ` Zefram [this message]
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