Ignoring synthesized conformances for a second, think about how you would manually implement a conformance of a tuple type to a protocol. You would need some way to statically “iterate” over all the component types of the tuple — in fact this is the same as having variadic generics.
If we had variadic generics, we could implement tuples conforming to protocols, either by refactoring the compiler to allow conforming types to be non-nominal, or by reworking things so that a tuple is a nominal type with a single variadic generic parameter. Slava > On Nov 20, 2017, at 9:06 PM, Tony Allevato via swift-evolution > <swift-evolution@swift.org> wrote: > > This is something I've wanted to look at for a while. A few weeks ago I > pushed out https://github.com/apple/swift/pull/12598 > <https://github.com/apple/swift/pull/12598> to extend the existing synthesis > to handle structs/enums when a field/payload has a tuple of things that are > Equatable/Hashable, and in that PR it was (rightly) observed, as Chris just > did, that making tuples conform to protocols would be a more general solution > that solves the same problem you want to solve here. > > I'd love to dig into this more, but last time I experimented with it I got > stuck on places where the protocol conformance machinery expects > NominalTypeDecls, and I wasn't sure where the right place to hoist that logic > up to was (since tuples don't have a corresponding Decl from what I can > tell). Any pointers? > > On Mon, Nov 20, 2017 at 5:51 PM Chris Lattner via swift-evolution > <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: > On Nov 20, 2017, at 5:48 PM, Kelvin Ma <kelvin1...@gmail.com > <mailto:kelvin1...@gmail.com>> wrote: >> the end goal here is to use tuples as a compatible currency type, to that >> end it makes sense for these three protocols to be handled as “compiler >> magic” and to disallow users from manually defining tuple conformances >> themselves. i’m not a fan of compiler magic, but Equatable, Hashable, and >> Comparable are special because they’re the basis for a lot of standard >> library functionality so i think the benefits of making this a special >> supported case outweigh the additional language opacity. > > I understand your goal, but that compiler magic can’t exist until there is > something to hook it into. Tuples can’t conform to protocols right now, so > there is nothing that can be synthesized. > > -Chris > > >> >> On Mon, Nov 20, 2017 at 8:42 PM, Chris Lattner <clatt...@nondot.org >> <mailto:clatt...@nondot.org>> wrote: >> >>> On Nov 20, 2017, at 5:39 PM, Kelvin Ma via swift-evolution >>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: >>> >>> when SE-185 >>> <https://github.com/apple/swift-evolution/blob/master/proposals/0185-synthesize-equatable-hashable.md> >>> went through swift evolution, it was agreed that the next logical step >>> <https://www.mail-archive.com/swift-evolution@swift.org/msg26162.html> is >>> synthesizing these conformances for tuple types, though it was left out of >>> the original proposal to avoid mission creep. I think now is the time to >>> start thinking about this. i’m also tacking on Comparable to the other two >>> protocols because there is precedent in the language from SE-15 >>> <https://github.com/apple/swift-evolution/blob/master/proposals/0015-tuple-comparison-operators.md> >>> that tuple comparison is something that makes sense to write. >>> >>> EHC conformance is even more important for tuples than it is for structs >>> because tuples effectively have no workaround whereas in structs, you could >>> just manually implement the conformance. >> >> In my opinion, you’re approaching this from the wrong direction. The >> fundamental problem here is that tuples can’t conform to a protocol. If >> they could, synthesizing these conformances would be straight-forward. >> >> If you’re interested in pushing this forward, the discussion is “how do >> non-nominal types like tuples and functions conform to protocols”? >> >> -Chris >> >> >> >> >> > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org <mailto:swift-evolution@swift.org> > https://lists.swift.org/mailman/listinfo/swift-evolution > <https://lists.swift.org/mailman/listinfo/swift-evolution> > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution
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