[swift-evolution] Introduce "associated_type" keyword

[John McCall]

> On Dec 5, 2015, at 4:35 PM, Loïc Lecrenier via swift-evolution <swift-evolution at swift.org> wrote:
> 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.

I think this is a great idea; re-using typealias for associated types was a mistake.

John.

> 
> 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
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution