> On Oct 26, 2017, at 9:34 AM, Xiaodi Wu <xiaodi...@gmail.com> wrote: > > On Thu, Oct 26, 2017 at 10:57 AM, Jonathan Hull <jh...@gbis.com > <mailto:jh...@gbis.com>> wrote: > >> On Oct 26, 2017, at 8:19 AM, Xiaodi Wu <xiaodi...@gmail.com >> <mailto:xiaodi...@gmail.com>> wrote: >> >> >> On Thu, Oct 26, 2017 at 07:52 Jonathan Hull <jh...@gbis.com >> <mailto:jh...@gbis.com>> wrote: >>> On Oct 25, 2017, at 11:22 PM, Xiaodi Wu <xiaodi...@gmail.com >>> <mailto:xiaodi...@gmail.com>> wrote: >>> >>> On Wed, Oct 25, 2017 at 11:46 PM, Jonathan Hull <jh...@gbis.com >>> <mailto:jh...@gbis.com>> wrote: >>> As someone mentioned earlier, we are trying to square a circle here. We >>> can’t have everything at once… we will have to prioritize. I feel like the >>> precedent in Swift is to prioritize safety/correctness with an option >>> ignore safety and regain speed. >>> >>> I think the 3 point solution I proposed is a good compromise that follows >>> that precedent. It does mean that there is, by default, a small >>> performance hit for floats in generic contexts, but in exchange for that, >>> we get increased correctness and safety. This is the exact same tradeoff >>> that Swift makes for optionals! Any speed lost can be regained by >>> providing a specific override for FloatingPoint that uses ‘&==‘. >>> >>> My point is not about performance. My point is that `Numeric.==` must >>> continue to have IEEE floating-point semantics for floating-point types and >>> integer semantics for integer types, or else existing uses of `Numeric.==` >>> will break without any way to fix them. The whole point of *having* >>> `Numeric` is to permit such generic algorithms to be written. But since >>> `Numeric.==` *is* `Equatable.==`, we have a large constraint on how the >>> semantics of `==` can be changed. >> >> It would also conform to the new protocol and have it’s Equatable >> conformance depreciated. Once we have conditional conformances, we can add >> Equatable back conditionally. Also, while we are waiting for that, Numeric >> can provide overrides of important methods when the conforming type is >> Equatable or FloatingPoint. >> >> >>> For example, if someone wants to write a generic function that works both >>> on Integer and FloatingPoint, then they would have to use the new protocol >>> which would force them to correctly handle cases involving NaN. >>> >>> What "new protocol" are you referring to, and what do you mean about >>> "correctly handling cases involving NaN"? The existing API of `Numeric` >>> makes it possible to write generic algorithms that accommodate both integer >>> and floating-point types--yes, even if the value is NaN. If you change the >>> definition of `==` or `<`, currently correct generic algorithms that use >>> `Numeric` will start to _incorrectly_ handle NaN. >> >> >> #1 from my previous email (shown again here): >>>> Currently, I think we should do 3 things: >>>> >>>> 1) Create a new protocol with a partial equivalence relation with >>>> signature of (T, T)->Bool? and automatically conform Equatable things to it >>>> 2) Depreciate Float, etc’s… Equatable conformance with a warning that it >>>> will eventually be removed (and conform Float, etc… to the partial >>>> equivalence protocol) >>>> 3) Provide an '&==‘ relation on Float, etc… (without a protocol) with the >>>> native Float IEEE comparison >> >> >> In this case, #2 would also apply to Numeric. You can think of the new >> protocol as a failable version of Equatable, so in any case where it can’t >> meet equatable’s rules, it returns nil. >> >> Again, Numeric makes possible the generic use of == with floating-point >> semantics for floating-point values and integer semantics for integer >> values; this design would not. > > Correct. I view this as a good thing, because another way of saying that is: > “it makes possible cases where == sometimes conforms to the rules of > Equatable and sometimes doesn’t." Under the solution I am advocating, > Numeric would instead allow generic use of '==?’. > > I suppose an argument could be made that we should extend ‘&==‘ to Numeric > from FloatingPoint, but then we would end up with the Rust situation you were > talking about earlier… > > This would break any `Numeric` algorithms that currently use `==` correctly. > There are useful guarantees that are common to integer `==` and IEEE > floating-point `==`; namely, they each model equivalence of their respective > types at roughly what IEEE calls "level 1" (as numbers, rather than as their > representation or encoding). Breaking that utterly eviscerates `Numeric`.
Nope. They would continue to work as they always have, but would have a depreciation warning on them. The authors of those algorithms would have a full depreciation cycle to update the algorithms. Fixits would be provided to make conversion easier.
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