[swift-evolution] Synthesizing Equatable, Hashable, and Comparable for tuple types

[Douglas Gregor]

> On Nov 20, 2017, at 6:07 PM, Tony Allevato via swift-evolution <swift-evolution at 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 at swift.org <mailto:swift-evolution at swift.org>> wrote:
> On Nov 20, 2017, at 5:48 PM, Kelvin Ma <kelvin13ma at gmail.com <mailto:kelvin13ma at 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 <clattner at nondot.org <mailto:clattner at nondot.org>> wrote:
>> 
>>> On Nov 20, 2017, at 5:39 PM, Kelvin Ma via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at 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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