wishing I could cast (sort of) to protocol with associated type


#1

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


(Jordan Rose) #2

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.

Jordan

···

On Nov 2, 2016, at 12:31, Robert Nikander via swift-users <swift-users@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
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