Calling protocol default implementation from regular method, when protocol has associated type

Question

I have a protocol that has a static method with a default parameter. I want to change the default value in a class that implements the protocol. Essentially doing what is easily done with classes and super.
I only have a solution when the Protocol has no associated type.

The following code works, but as soon as you uncomment the associated type declaration, it doesn't compile.

protocol Protocol {
//    associatedtype AssociatedType
}

extension Protocol {
    func sayHello(name: String = "World") {
        print("Hello, \(name)!")
    }
}

class Class<T>: Protocol {
    typealias AssociatedType = T

    func sayHello(name: String = "Stack Overflow") {
        // Uncommenting the Protocol.AssociatedType causes:
        // Protocol can only be used as a generic constraint because it has associated type requirements
        (self as Protocol).sayHello(name)
    }
}

Class<()>().sayHello()

I do understand why it doesn't compile: Protocol has no concrete type for AssociatedType.
So maybe the question should read "Can I explicitly specialize a protocol?", to which I believe the answer is no.

I have a partial workaround. But even when it works, it sucks.
Especially when you consider that I'm writing a library where sayHello is public, so the following workaround forces me to have a second protocol, which has to be public, but is useless.
Here's the workaround:

protocol Parent {}

protocol Protocol: Parent {
    associatedtype AssociatedType
}

extension Parent {
    func sayHello(name: String = "World") {
        print("Hello, \(name)!")
    }
}

class Class<T>: Protocol {
    typealias AssociatedType = T

    func sayHello(name: String = "Stack Overflow") {
        (self as Parent).sayHello(name)
    }
}

Class<()>().sayHello()

But this doesn't work for me, because my sayHello uses the associated type. So it can't be extracted to another protocol.

Just to be sure I'm clear, here's what I'd like, only substituting the class for a protocol:

class Protocol<T> {
    func sayHello(name: String = "World") {
        print("Hello, \(name)!")
    }
}

class Class<T>: Protocol<T> {
    override func sayHello(name: String = "Stack Overflow") {
        super.sayHello(name)
    }
}

Class<()>().sayHello()

Answer 1

Inspired by Rob Napier's answer, here's what I went with; good old overloading for defaults:

protocol Protocol {
    associatedtype AssociatedType
}

extension Protocol {
    func sayHello(name: String = "World") {
        print("Hello, \(name)!")
    }
}

class Class<T>: Protocol {
    typealias AssociatedType = T

    func sayHello() {
        self.sayHello("Stack Overflow")
    }
}

Class<()>().sayHello()      // Hello, Stack Overflow!
Class<()>().sayHello("you") // Hello, you!

This does fit my needs, but doesn't answer the question. So I'm not 100% satisfied.
I believe Rust gets this one right by allowing traits/protocols to be generic both using X<T> and associated types.

Answer 2

You're trying to reinvent inheritance in protocols, and there is no such thing. But it is trivial to get what you're talking about; just say what you mean. You don't mean "I want to do the thing I inherited." You mean "I want to do some common behavior." Just provide a name for that common behavior. This removes all ambiguity about which one you mean.

protocol Protocol {
        associatedtype AssociatedType
}

extension Protocol {
    // Put the default behavior on the protocol, not on the instance
    // Of course you could also put it on the instance if that were convenient.
    static func defaultSayHello(_ name: String = "World") {
        print("Hello, \(name)!")
    }

    // If you want a default on the instance, too, provide one that we an override
    func sayHello(_ name: String = "World") {
        Self.defaultSayHello(name)
    }
}

class Class<T>: Protocol {
    typealias AssociatedType = T

    func sayHello(name: String = "Stack Overflow") {
        // Now the default behavior lives on my type
        Class.defaultSayHello(name)
    }
}

// But other types can get default behavior
class OtherClass<T>: Protocol {
    typealias AssociatedType = T
}

Class<()>().sayHello() // Hello, Stack Overflow!
OtherClass<()>().sayHello() // Hello, World!

The one frustrating part about this is that Swift provides no way to limit defaultSayHello to implementers of Protocol. So technically anyone can call it. It can sometimes be worth prefixing it with an _ to indicate that outsiders shouldn't. This is a basic access control problem in protocols, having nothing to do with this specific question; it comes up all the time when you want "things my implementers can use on themselves, but shouldn't be called randomly." Swift doesn't have a solution for that today.