> On Jun 16, 2016, at 10:36 AM, Paul Cantrell via swift-evolution > <swift-evolution@swift.org> wrote: > >> On Jun 16, 2016, at 8:29 AM, Thorsten Seitz via swift-evolution >> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: >> >> Protocols are a mechanism for deriving types from each other whereas >> generics are a way to parameterize types. My point was that Swift's other >> way to parameterize types, namely by associated types, is very similar to >> generics with wildcards when looking a the existentials of such protocols. > > This has been a point of confusion for me as well. I keep hearing that > associated types are different from generic protocols, but this seems like a > distinction without a difference. > > Suppose Swift allowed generic protocols. How would a hypothetical > Collection<Foo> be different in practice from the proposed existential > Any<Collection where .Element == Foo>?
If Collection were a Java-like generic protocol you would have 5 generic parameters, all of which must be explicitly provided with arguments when forming an existential, although the arguments could be wildcards. This is a bit unwieldy. You would probably end up using a typealias in either case. > > Yes, in the realm of type theory and compiler internals they might > represented differently, sure. But in practice, in terms of what code can > actually do? I know of only two differences: > > 1. A type can only conform to any given protocol with one set of type > parameters. (Nothing can be both Collection<Foo> and Collection<Bar>.) This is a pretty huge difference. Multiple conformances are on the unlikely list for Swift and generic protocol syntax implies multiple conformances are possible (as is the case in at least some languages). > > 2. When a type conforms to Collection, it declares “associatedtype Foo” > instead of “: Collection<Foo>”, and Foo can be inferred by the compiler in > some circumstances. That’s handy, but it’s a syntactic difference. > > Is there a deeper difference I’m missing? > > Cheers, P > > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution
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