[swift-dev] Rationalizing FloatingPoint conformance to Equatable

[Xiaodi Wu]

On Tue, Oct 31, 2017 at 10:23 PM, David Sweeris <davesweeris at mac.com> wrote:

>
> On Oct 31, 2017, at 7:26 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>
> On Tue, Oct 31, 2017 at 5:56 PM, David Sweeris <davesweeris at mac.com>
> wrote:
>
>>
>> On Oct 31, 2017, at 09:07, Stephen Canon via swift-dev <
>> swift-dev at 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`?
>>
>
> Yes. `!=` is an extension method that cannot be overridden
>
>
> Wait, what? So if I have a `Password` type, and want to trigger extra
> logging if the `!=` function is called too many times within a second or
> something, that won't get called in generic code? That seems...
> unintuitive...
>

That's correct, as it is for all protocol extension methods (for example,
most of the collection APIs).
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