[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
## bug#16365: (* 0 +inf.0) rationale is flawed

**From**: |
Zefram |

**Subject**: |
bug#16365: (* 0 +inf.0) rationale is flawed |

**Date**: |
Tue, 21 Jun 2016 15:21:30 +0100 |

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