on Wed Jun 08 2016, Thorsten Seitz <[email protected]> wrote:
> Ah, thanks, I forgot! I still consider this a bug, though (will have > to read up again what the reasons are for that behavior). Yes, but in the case of the issue we're discussing, the choices are: 1. Omit from the existential's API any protocol requirements that depend on Self or associated types, in which case it *can't* conform to itself because it doesn't fulfill the requirements. 2. Erase type relationships and trap at runtime when they don't line up. Matthew has been arguing against #2, but you can't “fix the bug” without it. > > -Thorsten > >> Am 08.06.2016 um 21:43 schrieb Austin Zheng <[email protected]>: >> >> It's not possible, even with Swift's current implementation of >> existentials. A protocol type P isn't considered to conform to >> itself, thus the following is rejected: >> >> let a : MyProtocol = // ... >> func myFunc<T : MyProtocol>(x: T) { >> // .... >> } >> myFunc(a) // "Cannot invoke 'myFunc' with an argument list of type >> MyProtocol" >> >> Changing how this works is probably worth a proposal by itself. >> >> Austin >> >> >> On Wed, Jun 8, 2016 at 12:34 PM, Thorsten Seitz via swift-evolution >> <[email protected] >> <mailto:[email protected]>> >> wrote: >> >> > Am 08.06.2016 um 20:33 schrieb Dave Abrahams via swift-evolution >> > <[email protected] >> > <mailto:[email protected]>>: >> > >> > >> > on Tue Jun 07 2016, Matthew Johnson <matthew-AT-anandabits.com> wrote: >> > >> >>> On Jun 7, 2016, at 9:15 PM, Dave Abrahams >> >>> <[email protected] >> >>> <mailto:[email protected]>> >> >>> wrote: >> >>> >> >>> >> >>> on Tue Jun 07 2016, Matthew Johnson <matthew-AT-anandabits.com >> >>> <http://matthew-at-anandabits.com/ >> >>> <http://matthew-at-anandabits.com/>>> wrote: >> >>> >> >> >> >>>>> On Jun 7, 2016, at 4:13 PM, Dave Abrahams via swift-evolution >> >>>>> <[email protected] >> >>>>> <mailto:[email protected]>> >> >>>>> wrote: >> >>>>> >> >>>>> >> >>>>> on Tue Jun 07 2016, Matthew Johnson >> >>>>> <[email protected] >> >>>>> <mailto:[email protected]>> >> >>>>> wrote: >> >>>>> >> >>>> >> >>>>>>> , but haven't realized >> >>>>>>> that if you step around the type relationships encoded in Self >> >>>>>>> requirements and associated types you end up with types that appear >> >>>>>>> to >> >>>>>>> interoperate but in fact trap at runtime unless used in exactly the >> >>>>>>> right way. >> >>>>>> >> >>>>>> Trap at runtime? How so? Generalized existentials should still be >> >>>>>> type-safe. >> >>>>> >> >>>>> There are two choices when you erase static type relationships: >> >>>>> >> >>>>> 1. Acheive type-safety by trapping at runtime >> >>>>> >> >>>>> FloatingPoint(3.0 as Float) + FloatingPoint(3.0 as Double) // trap >> >>>>> >> >>>>> 2. Don't expose protocol requirements that involve these relationships, >> >>>>> which would prevent the code above from compiling and prevent >> >>>>> FloatingPoint from conforming to itself. >> >>>>> >> >>>>>> Or are you talking about the hypothetical types / behaviors people >> >>>>>> think they want when they don’t fully understand what is happening... >> >>>>> >> >>>>> I don't know what you mean here. I think generalized existentials will >> >>>>> be nice to have, but I think most people will want them to do something >> >>>>> they can't possibly do. >> >>>> >> >>>> Exactly. What I meant is that people think they want that expression >> >>>> to compile because they don’t understand that the only thing it can do >> >>>> is trap. I said “hypothetical” because producing a compile time error >> >>>> rather than a runtime trap is the only sane thing to do. Your comment >> >>>> surprised me because I can’t imagine we would move forward in Swift >> >>>> with the approach of trapping. >> >>> >> >>> I would very much like to be able to create instances of “Collection >> >>> where Element == Int” so we can throw away the wrappers in the stdlib. >> >>> That will require some type mismatches to be caught at runtime via >> >>> trapping. >> >> >> >> For invalid index because the existential accepts a type erased index? >> > >> > Exactly. >> > >> >> How do you decide where to draw the line here? It feels like a very >> >> slippery slope for a language where safety is a stated priority to >> >> start adopting a strategy of runtime trapping for something as >> >> fundamental as how you expose members on an existential. >> > >> > If you don't do this, the alternative is that “Collection where Element >> > == Int” does not conform to Collection. That's weird and not very >> > useful. You could expose all the methods that were on protocol >> > extensions of Collection on this existential, unless they used >> > associated types other than the element type. But you couldn't pass the >> > existential to a generic function like >> > >> > func scrambled<C: Collection>(_ c: C) -> [C.Element] >> >> I don’t understand. Why couldn’t an existential be passed to that function? >> >> -Thorsten >> >> >> >> > >> >> IMO you should *have* to introduce unsafe behavior like that manually. >> > >> > Collection where Element == Int & Index == * >> > >> > ? >> > >> >> Collection indices are already something that isn’t fully statically >> >> safe so I understand why you might want to allow this. >> > >> > By the same measure, so are Ints :-) >> > >> > The fact that a type's methods have preconditions does *not* make it >> > “statically unsafe.” >> > >> >> But I don’t think having the language's existentials do this >> >> automatically is the right approach. Maybe there is another approach >> >> that could be used in targeted use cases where the less safe behavior >> >> makes sense and is carefully designed. >> > >> > Whether it makes sense or not really depends on the use-cases. There's >> > little point in generalizing existentials if the result isn't very useful. >> > The way to find out is to take a look at the examples we currently have >> > of protocols with associated types or Self requirements and consider >> > what you'd be able to do with their existentials if type relationships >> > couldn't be erased. >> > >> > We have known use-cases, currently emulated in the standard library, for >> > existentials with erased type relationships. *If* these represent the >> > predominant use cases for something like generalized existentials, it >> > seems to me that the language feature should support that. Note: I have >> > not seen anyone build an emulation of the other kind of generalized >> > existential. My theory: there's a good reason for that :-). >> > >> > -- >> > Dave >> > _______________________________________________ >> > swift-evolution mailing list >> > [email protected] >> > <mailto:[email protected]> >> > https://lists.swift.org/mailman/listinfo/swift-evolution >> > <https://lists.swift.org/mailman/listinfo/swift-evolution> >> >> _______________________________________________ >> swift-evolution mailing list >> [email protected] >> <mailto:[email protected]> >> https://lists.swift.org/mailman/listinfo/swift-evolution >> <https://lists.swift.org/mailman/listinfo/swift-evolution> >> > > _______________________________________________ > swift-evolution mailing list > [email protected] > https://lists.swift.org/mailman/listinfo/swift-evolution > -- Dave _______________________________________________ swift-evolution mailing list [email protected] https://lists.swift.org/mailman/listinfo/swift-evolution
