Why can this protocol “only be used as a generic constraint”?

Question

I\'m attempting to do the following in Swift:

protocol ProtocolWithAlias {
    typealias T
}

protocol AnotherProtocol {
    func someFunc() -> ProtocolWithAl         


        

Answer 1

It's possible to implement this by inverting the control: instead of returning a value from someFunc we pass a consumer that can accept any type implementing ProtocolWithAlias and do something with it.

protocol ProtocolWithAlias {
    typealias T
}

protocol ProtocolConsumer {
    func consume(value: T)
}

protocol AnotherProtocol {
    func someFunc(consumer: ProtocolConsumer)
}

This trick is known as transforming the function to continuation passing style (CPS). Unfortunately I couldn't find any way to implement this without CPSing. The type system feature we are looking is existential types (and this thread has a nice explanation), but I think Swift doesn't support them (yet).

---

Why is the other answer not correct? What this signature tells:

func someFunc() -> T

is that this function can return a value of type T for any type implementing ProtocolWithAlias that the caller chooses, but we wanted it to be chosen by the callee.

It's not even possible to write a sensible implementation of this function. Lets pretend I have an implementation of someFunc: I could create a new class implementing ProtocolWithAlias and request someFunc to somehow create an instance of this class:

class Uninhabited: ProtocolWithAlias {
    typealias T = Int
    init(nope: Uninhabited) {}
}

...

let impossible: Uninhabited = someFunc()