On Nov 2, 2017, at 7:22 PM, Xiaodi Wu via swift-dev <swift-dev@swift.org> wrote:
> On Thu, Nov 2, 2017 at 5:22 PM, Matthew Johnson <matt...@anandabits.com
> <mailto:matt...@anandabits.com>> wrote:
>
>> On Nov 2, 2017, at 5:20 PM, Jonathan Hull via swift-dev <swift-dev@swift.org
>> <mailto:swift-dev@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).
This is a real concern, which I don’t think can be dismissed easily. It’s
really the only major concern that I have, however.
> 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.
What algorithms do you have in mind? You want to detect and handle NaN for a
generic algorithm written against Numeric context via x != x, I assume?
If we went down the road that I sketched out, I think we would (completely
arbitrarily) order NaN after +Inf under Comparable. Anything else would be
fairly inconsistent.
Broadly I agree that if we can find some other way to make things work, we
should do it, but this also isn’t a *terrible* option.
– Steve
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