[swift-dev] Rationalizing FloatingPoint conformance to Equatable

[Greg Titus]

> On Nov 1, 2017, at 9:16 AM, Ben Cohen via swift-dev <swift-dev at swift.org> wrote:
>> On Oct 31, 2017, at 10:11 PM, Chris Lattner via swift-dev <swift-dev at swift.org> wrote:
>> On Oct 31, 2017, at 9:07 AM, 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.
>> 
>> +100.  Swift isn’t the first language to face the problems of floating point, nor is it the first to try to shoehorn it into a framework like Equatable.  
> 
> Java and C# do not have this problem with their generic algorithms (albeit possibly because of limitations in their languages that Swift doesn’t have). Swift is setting itself up as a major language with confusing and unjustifiable behavior by comparison. That some other languages are also bad at this doesn’t seem relevant.

The common (and correct!) wisdom in _any_ programming language that uses IEEE floating point is that checking equality of two floating point values is almost always a terrible idea. Usually what you want in any real world code is to check for a difference less than some epsilon value, which depends upon context. There are just too many issues with values that aren’t exactly representable, rounding errors during computations, et cetera, for perfectly normal floats even if you completely left aside equality rules for NaN.

I completely understand the desire in this thread to make floating point really satisfy the axioms of Equatable, but the fact is, even if you did, using a generic algorithm that depends upon equatability with floating point types is almost always just a programming error waiting to happen. It’s implicit in the representation and use of floating point values themselves, no matter what particular implementation you decide on for == or &==.

If you really want to make the language better for developers, provide and emphasize fixed point or infinite precision or rational types for doing various things instead, and encourage them to shun floats as much as possible. If you really need to change anything about the standard library of Swift, my preferred solution would be to continue to provide ==(lhs : Float, rhs: Float) and != but NOT declare conformance to Equatable at all so that generic algorithms involving floats would fail to compile.

	- Greg