[swift-evolution] [Proposal] Conditional Conformance on Protocols/Generic Types

[Austin Zheng]

Hello Dan,

You'll be pleased to learn that conforming generic types conditionally to protocols is on the roadmap (and is one of the highest priority items for the versions of Swift following 3.0): https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#conditional-conformances- <https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#conditional-conformances->

However, it's unlikely that protocols will gain conditional conformance: https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#conditional-conformances-via-protocol-extensions <https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#conditional-conformances-via-protocol-extensions>

That document originates from a mailing list post made some time ago, and is a decent overview as to what sorts of type system features the Swift core developers are interested in building.

Best,
Austin

> On Jun 4, 2016, at 2:12 PM, Dan Zimmerman via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Hey,
> 
> I was interested in adopting the ability for a type that's generic in some sense (either via generics or via associated types, in the case of protocols) to conform to other protocols conditionally based on its type parameter/associated type. For example:
> 
> ```
> extension CollectionType: Equatable where Generator.Element: Equatable {}
> 
> func ==<Collection: CollectionType where Collection.Generator.Element: Equatable>(left: Collection, right: Collection) -> Bool {
>  return zip(left, right).reduce(true) { accumulator, tuple in accumulator && (tuple.0 == tuple.1) }
> }
> ```
> 
> If this has already been proposed and knocked out/accepted please direct me to the right place.
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

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