> On Oct 20, 2017, at 7:36 AM, Xiaodi Wu <xiaodi...@gmail.com> wrote: > > > On Fri, Oct 20, 2017 at 07:21 David Zarzycki <d...@znu.io > <mailto:d...@znu.io>> wrote: > > >> On Oct 20, 2017, at 07:51, Xiaodi Wu via swift-dev <swift-dev@swift.org >> <mailto:swift-dev@swift.org>> wrote: >> >> On Fri, Oct 20, 2017 at 1:22 AM, Jonathan Hull <jh...@gbis.com >> <mailto:jh...@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@swift.org <mailto:swift-dev@swift.org>> wrote: >>> >>>> On Oct 19, 2017, at 4:29 PM, Xiaodi Wu via swift-dev <swift-dev@swift.org >>>> <mailto:swift-dev@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.
I don’t know about Brent, but I was only speaking about the semantics/behavior of == with respect to NaN. Trap on NaN for ‘==‘, but restore full IEEE behavior by using ‘&==‘. For ‘Float?’ we could treat NaN as nil for ‘==‘. ‘&==‘ would still have the IEEE behavior. This restores reflexivity and we can count on == being a true equivalence relation in our generic algorithms. Lack of proper handling of NaN is almost a stereotypical cause of bugs. Looking at code written for Floats, seeing &== will be a sign that the original programmer considered the possibility of NaN. Thanks, Jon
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