[swift-users] wishing I could cast (sort of) to protocol with associated type

Jordan Rose jordan_rose at apple.com
Wed Nov 2 15:58:16 CDT 2016

The only real way to do this today is to have two layers of protocol:

protocol SpecialControllerBase {
  var currentValueBase: SpecialValue? { get }
protocol SpecialController: SpecialControllerBase {
  associatedtype SpecialValueType : SpecialValue
  var currentValue: SpecialValueType? { get }
extension SpecialController {
  var currentValueBase: SpecialValue? { return self.currentValue }

Supporting this natively is the feature called generalized existentials, described in the “Generics Manifesto <https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#generalized-existentials>” of potential future Swift features. This has a lot of design and implementation considerations, so it’s not planned to happen right away, but it’s definitely a heavily-requested feature.


> On Nov 2, 2016, at 12:31, Robert Nikander via swift-users <swift-users at swift.org> wrote:
> Hi,
> In the following code, I want to test if x is a `SpecialController`. If it is, I want to get the `currentValue` as a `SpecialValue`. How do you do this? If not with a cast, then some other technique.
> I understand the error, and that SpecialController by itself is not a simple type to cast to. But it seems like what I’m saying is logically consistent and not that complicated. Is there really no way to *say* it in Swift?
>    protocol SpecialController {
>        associated type SpecialValueType : SpecialValue
>        var currentValue: SpecialValueType? { get }
>    }
>    ...
>    var x: AnyObject = ...
>    if let sc = x as? SpecialController {  // does not compile
> Rob
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users

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