[swift-evolution] [swift-evolution-announce] [Review] SE-0089: Replace protocol<P1, P2> syntax with Any<P1, P2>

[Matthew Johnson]

> On Jun 16, 2016, at 10:36 AM, Paul Cantrell via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> On Jun 16, 2016, at 8:29 AM, Thorsten Seitz via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at 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
> 
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