[swift-evolution] Introduce "associated_type" keyword

[Loïc Lecrenier]

Hi everyone :)

I propose introducing a new "associated_type" keyword that will replace "typealias" for declaring associated types in protocols.
I remember being confused by associated types when I started using Swift, and I think one reason why was the use of the typealias keyword to define them.
One reason was that I thought I knew what typealias did, and so I didn't stop to learn what it did inside a protocol. An other reason was the difficulty of finding help when searching for "typealias" instead of "associated types".
Then, when I thought I understood it, I started building an excessively protocol-oriented program as an exercise. And I still lost a lot of time fighting Swift by trying to use "real" typealias-es inside of protocols.

Conceptually, I had something like this:

protocol ProtA {
   typealias Container : SequenceType
}
protocol ProtB {
   typealias AnOtherAssocType : ProtA
   func foo(x: AnOtherAssocType.Container.Generator.Element, y: AnOtherAssocType.Container.Generator.Element) -> AnOtherAssocType.Container.Generator.Element
}

The function foo is very difficult to read, so I wanted to use a shortcut to Element by doing this:

protocol ProtB {
   typealias A : ProtA
   typealias Element = A.Container.Generator.Element
   func foo(x: Element, y: Element) -> Element
}

But by doing so, I didn't create a shortcut to Element, but an associated type with a default value of Element. (right?)
Then I tried to write extensions to ProtB where Element conforms to, say, Equatable, and couldn't make it work because A.Container.Generator.Element didn't conform to Equatable.

So, that was a rather long explanation of the reasons I think we should replace the typealias keyword by associated_type, and allow "real" typealias-es inside protocols.

Ideally, I would write

protocol ProtB {
   associated_type AnOtherAssocType : ProtA
   typealias Element = AnOtherAssocType.Container.Generator.Element
   func foo(x: Element, y: Element) -> Element
}

and it would be exactly the same as

protocol ProtB {
   associated_type AnOtherAssocType : ProtA
   func foo(x: A.Container.Generator.Element, y: A.Container.Generator.Element) -> A.Container.Generator.Element
}

There are probably some problems created by this proposal, but right now I can't see any :/

Thanks,

Loïc