> On Oct 31, 2017, at 09:07, Stephen Canon via swift-dev <swift-dev@swift.org>
> wrote:
>
> [Replying to the thread as a whole]
>
> There have been a bunch of suggestions for variants of `==` that either trap
> on NaN or return `Bool?`. I think that these suggestions result from people
> getting tunnel-vision on the idea of “make FloatingPoint equality satisfy
> desired axioms of Equatable / Comparable”. This is misguided. Our goal is
> (should be) to make a language usable by developers; satisfying axioms is
> only useful in as much as they serve that goal.
>
> Trapping or returning `Bool?` does not make it easier to write correct
> concrete code, and it does not enable writing generic algorithms that operate
> on Comparable or Equatable. Those are the problems to be solved.
>
> Why do they not help write correct concrete code? The overwhelming majority
> of cases in which IEEE 754 semantics lead to bugs are due to non-reflexivity
> of equality, so let’s focus on that. In the cases where this causes a bug,
> the user has code that looks like this:
>
> // Programmer fails to consider NaN behavior.
> if a == b {
> }
>
> but the correct implementation would be:
>
> // Programmer has thought about how to handle NaN here.
> if a == b || (a.isNaN && b.isNaN) {
> }
>
> W.r.t ease of writing correct *concrete* code, the task is to make *this*
> specific case cleaner and more intuitive. What does this look like under
> other proposed notions of equality? Suppose we make comparisons with NaN trap:
>
> // Programmer fails to consider NaN behavior. This now traps if a or b is
> NaN.
> // That’s somewhat safer, but almost surely not the desired behavior.
> if a == b {
> }
>
> // Programmer considers NaNs. They now cannot use `==` until they rule out
> // either a or b is NaN. This actually makes the code *more* complicated
> and
> // less readable. Alternatively, they use `&==` or whatever we call the
> unsafe
> // comparison and it’s just like what we had before, except now they have a
> // “weird operator”.
> if (!a.isNaN && !b.isNaN && a == b) || (a.isNaN && b.isNaN) {
> }
>
> Now what happens if we return Bool?
>
> // Programmer fails to consider NaN behavior. Maybe the error when they
> // wrote a == b clues them in that they should. Otherwise they just throw
> in
> // a `!` and move on. They have the same bug they had before.
> if (a == b)! {
> }
>
> // Programmer considers NaNs. Unchanged from what we have currently,
> // except that we replace || with ??.
> if a == b ?? (a.isNaN && b.isNaN) {
> }
>
> If we are going to do the work of introducing another notion of
> floating-point equality, it should directly solve non-reflexivity of equality
> *by making equality reflexive*. My preferred approach would be to simply
> identify all NaNs:
>
> // Programmer fails to consider NaN behavior. Now their code works!
> if a == b {
> }
>
> // Programmer thinks about NaNs, realizes they can simplify their existing
> code:
> if a == b {
> }
>
> What are the downsides of this?
>
> (a) it will confuse sometimes experts who expect IEEE 754 semantics.
> (b) any code that uses `a != a` as an idiom for detecting NaNs will be
> broken.
>
> (b) is by far the bigger risk. It *will* result in some bugs. Hopefully less
> than result from people failing to consider NaNs. The only real risk with (a)
> is that we get a biennial rant posted to hacker news about Swift equality
> being broken, and the response is basically “read the docs, use &== if you
> want that behavior”.
One more thought — and it’s crazy enough that I’m not even sure it’s worth
posting — does Swift’s `Equatable` semantics require that `(a == b) != (a !=
b)` always evaluate to `true`? Because it seems like the arguments for having
`.nan == .nan` return `false` would apply for `!=` as well. Without getting
into trapping, faulting or returning a `Bool?` / `Maybe` / `Tern`, I can’t
think of anything else that’d get a developer’s attention faster than the same
value being both not equal and not not equal to itself.
I mean, is that likely to cause any more bugs than having `.nan == .nan` return
`true`?
I *think* yes, but I tend to use `.isNaN`, so I’m not sure.
- Dave Sweeris
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