An issue has come to my attention that deserves wider discussion. Since at least Guile 1.8, (= 0.0 -0.0) has returned #t but (eqv? 0.0 -0.0) has returned #f, and this is still the case.
PLT Scheme agrees with us that (eqv? 0.0 -0.0) is #f, but MIT/GNU Scheme, SCM, Chicken, and Gauche all return #t in this case. Our current behavior violates the R5RS but is required by the R6RS. The R5RS requires that `eqv?' and `=' must agree for numbers of the same exactness. The R6RS requires that `eqv?' must return #f for arguments that yield different results (in the sense of eqv?) when passed as arguments to any other procedure that can be defined as a finite composition of Scheme’s standard arithmetic procedures. Since the R6RS also requires that (/ 0.0) yields +inf.0, and IEEE 754 requires not only this but also that (/ -0.0) yields -inf.0, that implies that (eqv? 0.0 -0.0) must be #f. IEEE 754 also requires that (= 0.0 -0.0) must return #t, and indeed this is the only sane option. Therefore I see no way to be compliant with both the R5RS and the R6RS at the same time. Personally, although I don't agree with the R6RS on everything, I think they got this part right. It's useful to have an equality predicate that can distinguish numbers that are distinguishable by other numerical operations. Given that everyone agrees that `eqv?' must distinguish 0 from 0.0, it is already not useful as a numerical `='. Any program that uses it this way is asking for trouble. Therefore, I don't have qualms about keeping our existing behavior, namely that (eqv? 0.0 -0.0) returns #f. What do you think? Mark