[swift-evolution] Tuples as RawRepresentable

[Karl]

> On 16 Oct 2016, at 00:33, Haravikk via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 
>> On 15 Oct 2016, at 18:21, Nevin Brackett-Rozinsky <nevin.brackettrozinsky at gmail.com <mailto:nevin.brackettrozinsky at gmail.com>> wrote:
>> 
>> Tuples cannot conform to protocols, so despite the existence of an “==” operator for certain tuples, no tuple conforms to Equatable.
>> 
>> This is problematic, because it means that a function which takes a generic Equatable parameter cannot be called with a tuple argument, even though an applicable “==” operator exists.
>> 
>> (Ditto for “Comparable”, mutatis mutandis.)
>> 
>> Nevin
> 
> Yeah, since the operators are implemented though perhaps some kind of magic can be used? It seems strange that the following is valid:
> 
> 	struct Foo : Equatable { let value:(Int, Int) }
> 	func == (lhs:Foo, rhs:Foo) -> Bool { return lhs.value == rhs.value }
> 
> Yet tuples can't just be Equatable etc. But I'm wondering whether that falls into separate issue territory, such that it should be done first as its own proposal?
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Tuples are non-nominal (named) types, and that’s why they can’t conform to protocols. There is something in the generics manifesto about possibly allowing them to do that in the future, though: 

https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#extensions-of-structural-types <https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#extensions-of-structural-types>
> Extensions of structural types
> 
> Currently, only nominal types (classes, structs, enums, protocols) can be extended. One could imagine extending structural types—particularly tuple types—to allow them to, e.g., conform to protocols. For example, pulling together variadic generics, parameterized extensions, and conditional conformances, one could express "a tuple type is Equatable if all of its element types are Equatable":
> 
> extension<...Elements : Equatable> (Elements...) : Equatable {   // extending the tuple type "(Elements...)" to be Equatable
> }


- Karl

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