> On Nov 20, 2017, at 6:07 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?
Tuples won’t have a corresponding Decl, because they’re structural. The
protocol conformance machinery (e.g., the ConformanceLookupTable) would need to
be generalized to cover the various kinds of types that can be made to conform
to a protocol: nominal types, function types, tuple types, metatypes, and
protocol composition types. Similarly, the assumption that one can only write
an extension on a nominal type is fairly widespread in the compiler.
All of these are fine refactors along the way, and could be used to improve the
compiler long before we’re ready to turn it into a user-facing language feature.
- Doug
>
> 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
>>
>>
>>
>>
>>
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