| From: Marius Vollmer <[EMAIL PROTECTED]> | Date: Sun, 16 Jul 2006 21:04:20 +0300 | | after enormous procrastination, I am now convinced that the change | in semantics of eqv? from 1.6 to 1.8 was wrong, and I think that we | should revert the eqv? behavior for the next 1.8.1 release. It's a | bug in the design and should be fixed. | | (Credit goes to Aubrey Jaffer for pointing out the bug in the first | place, and Kevin for making me understand.) | | R5RS is pretty clear about the relation between '=' and 'eqv?' and | we should follow that (even tho one could argue that R5RS has not | been written with signed zeros and NaNs in mind and R6RS will | likely be different in this area). | | Specifically, 'eqv?' would be changed to return '#t' when comparing | negative and positive zero: | | (eqv? 0.0 -0.0) => #t
Okay! | and should return #f for nans: | | (eqv? +nan.0 +nan.0) => #f IEEE-754 apparently chose to overload the floating-point equality operator to detect NaNs to avoid having to define new floating-point predicates. Guile already has a nan predicate. 754's low-level hack should not be exposed in Scheme. In the description of EQ?, R5RS states: `Eq?''s behavior on numbers and characters is implementation-dependent, but it will always return either true or false, and will return true only when `eqv?' would also return true. So if (eqv? +nan.0 +nan.0) returns false, then (eq? +nan.0 +nan.0) cannot return true and must return false. _Rationale:_ It will usually be possible to implement `eq?' much more efficiently than `eqv?', for example, as a simple pointer comparison instead of as some more complicated operation. If (eq? +nan.0 +nan.0) is required to return #f, then EQ? in Guile is prevented from being a simple pointer comparison. | If you need to test the sign of zero by turning it into the | corresponding infinity | | (> (/ 0.0) 0) => #t | (> (/ -0.0) 0) => #f | | To test for a NaN, use 'nan?'. | | Ok? _______________________________________________ Guile-user mailing list Guile-user@gnu.org http://lists.gnu.org/mailman/listinfo/guile-user
