[swift-dev] Rationalizing FloatingPoint conformance to Equatable

[David Zarzycki]

> On Oct 20, 2017, at 07:51, Xiaodi Wu via swift-dev <swift-dev at swift.org> wrote:
> 
> On Fri, Oct 20, 2017 at 1:22 AM, Jonathan Hull <jhull at gbis.com <mailto:jhull at gbis.com>> wrote:
> +1 for trapping unless using &==.  In the case of ‘Float?’ we could also map to nil.
> 
> This is probably a more appropriate discussion for evolution though...
> 
> 
>> On Oct 19, 2017, at 9:48 PM, Brent Royal-Gordon via swift-dev <swift-dev at swift.org <mailto:swift-dev at swift.org>> wrote:
>> 
>>> On Oct 19, 2017, at 4:29 PM, Xiaodi Wu via swift-dev <swift-dev at swift.org <mailto: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.
> 
> 
> Built-in floating point operators, as well as libc/libm math functions, are designed to propagate NaN correctly. This is not meant to be a thread about NaN, and we need to be cautious to define the scope of the problem to be solved from the outset. The tendency of having ever-expanding discussion where issues such as method names turn into discussions about the entire standard library go nowhere.
> 
> The question here is about `==` specifically and how to accommodate partial equivalence relations. For sanity, we start with the premise that NaN will forever be as it is.

I support Jonathan’s argument. If Swift wants to trap on NaN to improve self-consistency and simplicity, then the tradeoff might be worth it. The alternative, teaching the Equality protocol about NaNs, feels like “the tail wagging the dog".

In short: what IEEE requires of floating-point hardware is separable from IEEE’s opinions about language/library design.


Dave
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