[swift-evolution] [pitch] Comparison Reform

Stephen Canon scanon at apple.com
Tue Apr 25 09:34:38 CDT 2017

> On Apr 24, 2017, at 10:06 PM, Jonathan Hull via swift-evolution <swift-evolution at swift.org> wrote:
> As I am thinking about it more, this means that for == and <
> NaN == NaN
> -0 == +0
> +Inf < NaN
> Since this would break from IEEE, I think we should also consider taking the opportunity to make == and < work with a default tolerance.  That is, 'a == b' would check that (a - b) < epsilon for some reasonable choice of epsilon that works for common usage.  I know this would make the hash function harder to write, but I think it is worthwhile.

It’s possible to tinker with IEEE comparisons and maybe improve them or at least work to make them compatible in some way with the expectations of Comparable. Making them have a tolerance by default is out of the question, however: as soon as you add a tolerance, you give up transitivity of equality, which is a much, much worse violation of the axioms than anything inflicted by the presence of NaN in the IEEE 754 model.

– Steve

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