[swift-dev] Rationalizing FloatingPoint conformance to Equatable

Brent Royal-Gordon brent at architechies.com
Thu Oct 19 23:48:33 CDT 2017

> On Oct 19, 2017, at 4:29 PM, Xiaodi Wu via swift-dev <swift-dev at swift.org> wrote:
> D) Must floating-point IEEE-compliant equivalence be spelled `==`?
> In my view, this is something open for debate. I see no reason why it cannot be migrated to `&==` if it were felt that `==` *must* be a full equivalence relation. I believe this is controversial, however.

I actually got partway through writing up a pitch on this yesterday, but my opinion is that NaNs are so exceptional, and so prone to misuse, that we ought to treat them like integer arithmetic overflows: trap when they're detected, unless you use an `&` variant operator which indicates you know what you're doing.

I strongly suspect that, in practice, most float-manipulating code is not prepared to handle NaN and will not do anything sensible in its presence. For example, Apple platforms use floating-point types for geometry, color components, GPS locations, etc. Very little of this code will do anything sensible in the presence of a NaN. Arguably, it'd be better to exclude them through the type system, but I don't think that's a realistic possibility—we would need to have done that in a more source-break-friendly era. But that doesn't have to mean we're completely stuck.

Brent Royal-Gordon

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