Andy Wingo <wi...@pobox.com> 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 <zef...@fysh.org> 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