In Swift, how to cast to protocol with associated type?

Question

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

Answer 1

This doesn't work because SpecialController isn't a single type. You can think of associated types as a kind of generics. A SpecialController with its SpecialValueType being an Int is a completely different type from a SpecialController with its SpecialValueType being an String, just like how Optional is a completely different type from Optional.

Because of this, it doesn't make any sense to cast to SpecialValueType, because that would gloss over the associated type, and allow you to use (for example) a SpecialController with its SpecialValueType being an Int where a SpecialController with its SpecialValueType being a String is expected.

As compiler suggests, the only way SpecialController can be used is as a generic constraint. You can have a function that's generic over T, with the constraint that T must be a SpecialController. The domain of T now spans all the various concrete types of SpecialController, such as one with an Int associated type, and one with a String. For each possible associated type, there's a distinct SpecialController, and by extension, a distinct T.

To draw out the Optional analogy further. Imagine if what you're trying to do was possible. It would be much like this:

func funcThatExpectsIntOptional(_: Int?) {}

let x: Optional = "An optional string"
// Without its generic type parameter, this is an incomplete type. suppose this were valid
let y = x as! Optional
funcThatExpectsIntOptional(y) // boom.