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