[swift-evolution] [swift-evolution-announce] [Review] Replace `typealias` keyword with `associatedtype` for associated type declarations

Kevin Ballard kevin at sb.org
Sun Jan 3 20:09:54 CST 2016

On Sun, Jan 3, 2016, at 03:41 AM, Loïc Lecrenier via swift-evolution wrote:
> Hi Drew,
> Thanks for the review, just a quick remark:
> “Real” type aliases are already forbidden inside protocols, so this proposal wouldn’t change that.
> (According to the grammar, a protocol body can only contain: property, method, initializer, subscript, or associated type member declarations)
> In your example, secondstype and usecstype were associated types with initial values. To convince yourself, try to create this function
> func bar(_: Foo) { }
> and you should see the "can only be used as a generic constraint because it has Self or associated type requirements” error.
> I initially wanted to allow type aliases inside protocols, and I was told type aliases weren’t requirements, so they shouldn’t be defined inside protocols, which makes sense to me.
> We might want to reconsider this, but I think it is outside the scope of this proposal.

I agree that protocols shouldn't have typealiases, but I would be in favor of protocol extensions having them:

extension SequenceType {
    typealias Element = Generator.Element

But it is definitely outside the scope of this proposal.

-Kevin Ballard

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