[swift-dev] Rationalizing FloatingPoint conformance to Equatable

Xiaodi Wu xiaodi.wu at gmail.com
Thu Nov 2 18:21:53 CDT 2017


On Thu, Nov 2, 2017 at 5:22 PM, Matthew Johnson <matthew at anandabits.com>
wrote:

>
> On Nov 2, 2017, at 5:20 PM, Jonathan Hull via swift-dev <
> swift-dev at swift.org> wrote:
>
> It looks like we have a good solution.  Per Steve and David’s suggestions:
>
> 1) Make FloatingPoint == reflexive
>
> 2) Add &== to FloatingPoint for those who specifically want IEEE behavior
>
> 3) Add a warning + fixit to ‘a != a’
>
> We should take this to evolution…
>
>
> Looks like a winner to me.
>

Again, there remain several problems with this design. In the concrete
context, the syntax `&==` suggests that it is a compatibility, legacy, or
specialized function not to be preferred over `==`. This makes Swift
deviate from every other programming language, creating a new footgun for
experienced developers, and encourages a performance hit where one is not
demonstrably necessary (most operations that ask about UI coordinates, say,
will never have NaN as an input). Again also, this design eliminates the
possibility of writing a class of useful algorithms that use Numeric. It
also doesn't address the problem of sorting (as NaN would still compare
unordered to all other values). These are serious questions that require
careful consideration.

In initiating this conversation, my goal has been to start a conversation
about possible designs for `Equatable` and `Comparable` that permis useful
generic algorithms to work correctly with floating point without hampering
performance or changing syntax for concrete numerics. Unless this is
clearly impossible, the leap of changing the behavior of `==` for a numeric
type should absolutely not be the proposed solution, in my view.


> On Nov 1, 2017, at 10:02 AM, Stephen Canon via swift-dev <
> swift-dev at swift.org> wrote:
>
>
>
> On Nov 1, 2017, at 12:51 PM, Greg Titus via swift-dev <swift-dev at swift.org>
> wrote:
>
> 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.
>
>
> The common wisdom is fundamentally bogus. One should *often* use a
> tolerance for floating-point comparison, but there’s a whole host of
> situations in which exact equality is perfectly appropriate. Of particular
> note for us, those situations include most generic contexts based on
> Equatable or Comparable.
>
> s.contains(2)
>
> should not return `true` if `s` contains something two-ish. It should
> return `true` if and only if `s` contains the actual value `2`.
>
> One might want to have an additional method on sets of
> FloatingPoint-conforming types that uses a tolerance, but contains should
> do precisely what it says on the tin.
>
> 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.
>
>
> Fixed point representations have their place[1], but rationals are
> strictly worse than floating-point representations in every way except for
> their ability to exactly represent 1/3 in toy problems and efficiency of
> division until you saturate the size of the denominator. I am not
> exaggerating. They waste space, they have a highly non-uniform distribution
> of representable values, they have many redundant representations, and
> denominators grow exponentially in almost every non-toy problem.
>
> Floating-point is the worst approximation to the real numbers except for
> all the others.
>
> – Steve
>
> [1] Unfortunately they are generally unusable for libraries due to
> complete lack of scale invariance. This can be worked around, but doing so
> requires more hand-holding than floating-point.
> _______________________________________________
> swift-dev mailing list
> swift-dev at swift.org
> https://lists.swift.org/mailman/listinfo/swift-dev
>
>
> _______________________________________________
> swift-dev mailing list
> swift-dev at swift.org
> https://lists.swift.org/mailman/listinfo/swift-dev
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-dev/attachments/20171102/c00f1073/attachment.html>


More information about the swift-dev mailing list