In the generics manifesto (https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md) conditional conformances via protocol extensions is under unlikely.
The manifesto notes: "This is an extremely powerful feature: is allows one to map the abstractions of one domain into another domain (e.g., every Matrix is a Graph). However, similar to private conformances, it puts a major burden on the dynamic-casting runtime to chase down arbitrarily long and potentially cyclic chains of conformances, which makes efficient implementation nearly impossible." Correct me if I’m completely wrong here but I think this is a super useful feature and is worth talking about. Firstly why its super useful if not immediately clear is cause you can do neat things like: extension Collection : Equatable where Element: Equatable And you can even make protocols in a module that you did not write conform to an arbitrary other protocol and give it a default implementation. I was wondering why this would put any more of a burden on the runtime than simple inheritance of protocols. The way this could be implemented is to augment the ConformanceTable for nominal types by looking up its protocol extension’s inheritance clauses. I can definitely see this impacting compile time but I don’t see why runtime performance will be any different than simple inheritance. Further, cyclic chains can be detected and broken (compiler error) during the second pass of semantic analysis. I had a (perhaps naïve) implementation here: https://github.com/apple/swift/pull/2233 and I think now would be a great time to talk about such a feature. Its possible I completely overlooked something but I would like a more a detailed explanation about why this isn’t possible. Regards, Manav Gabhawala _______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution
