[swift-dev] Rationalizing FloatingPoint conformance to Equatable

[Xiaodi Wu]

On Fri, Oct 20, 2017 at 07:21 David Zarzycki <dave at znu.io> wrote:

>
>
> 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> 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> wrote:
>>
>> 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.
>>
>>
> 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.
>

IEEE 754 requires certain behaviors of conforming implementations and is
meant to allow for portable use of floating point. Swift’s floating point
facilities are conformant to that standard, and breaking IEEE 754
conformance has been repeatedly stated to be a non-starter.

The essential idea behind quiet NaN is that it can be used as input in
operations that take floating point values without raising errors. Since
breaking IEEE 754 conformance is a non-starter, trapping on NaN is outside
the realm of available solutions.

It has been pointed out, however, that IEEE does not require a specific
syntax for floating point equivalence, hence the question of whether it can
be spelled differently. However, trapping on NaN is emphatically not an
option.

But I really don’t want to discuss this at all here. The topic of this
thread is about the semantics of Equatable.
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