Improving the UI of generics

[Joe_Groff]

@anandabits

I think it'd be valuable to allow you to use generics syntax as sugar for associated types on protocols, so you could write:

protocol Collection<Element> {

associatedtype Index
}

How would this relate to multi-parameter protocols? If we use “generic protocol” syntax in this way is there some other syntax that we might use for multi-parameter protocols? What might that look like? I would hate to see multi-parameter protocols be precluded just because people have a hard time with associated types mostly due to current limitations in the type system that should be lifted.

I personally don't think that using the "generic protocols" for multi-parameter protocols is a good idea, since IMO the generic notation is the "good syntax", but multi-parameter protocols without a functional dependency are not the common case. Furthermore, the generics syntax implies an asymmetric relationship between Self and the arguments, which is appropriate for associated types, but completely artificial for multi-parameter protocols. I like the idea of a tuple-ish syntax like early versions of Rust had:

protocol (A, B): Protocol { }

func foo<A, B>() where (A, B): Protocol

or an application-like syntax similar to what's been proposed for Go 2:

protocol Protocol(A, B) { }

func foo<A, B>() where Protocol(A, B)

Since the set of protocols that need to be modeled this way is the minority, I think it's OK for it to have a somewhat more obscure syntax, if we ever support it at all.