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

[Joe Groff]

> On Nov 28, 2017, at 10:52 AM, Vladimir.S <svabox at gmail.com> wrote:
> 
> On 27.11.2017 20:28, Joe Groff via swift-evolution wrote:
>>> On Nov 20, 2017, at 5:43 PM, Chris Lattner via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.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.
>> It would be a tractable intermediate problem to introduce built-in conformances for tuples (and perhaps metatypes) to Equatable/Hashable/Comparable without breaching the more general topic of allowing these types to have general protocol conformances. I think that would cover the highest-value use cases.
> 
> So, shouldn't we do this first step ASAP and then design a good common solution to allow tuples/metatypes/funcs to confirm to custom protocols in some next version of Swift?
> I really believe this is the good practical decision and will be supported by community if such proposal will be on the table.
> Is there any drawback in such step?

The expected behavior of tuple Equatable/Hashable/Comparable seems obvious to me (though I could well be missing something), and any behavior we hardcode should be naturally replaceable by a generalized conformance mechanism, so it's primarily a "small matter of implementation". There would be some implementation cost to managing the special case in the compiler and runtime; the tradeoff seems worth it to me in this case, but others might reasonably disagree. Not speaking for the entire core team, I would personally support considering a proposal and implementation for builtin tuple Equatable/Hashable/Comparable conformance.

-Joe