How associated(typealias) type and Self in protocols work?

Question

Sorry, I am a newbie in programming, I try to express what i want to ask. Please forgive me. I have seen something like this in protocol.

protocol Pro1 {
    typealias Element
    // ...
}
protocol Pro2: Pro1 {
    typealias Element = Self
    // ...
}

Element in protocols, does this Element relate to each other?

And I don't understand what the following expression means:

typealias Element = Self

Thanks so much.

Answer 1

Pro1

Writing this

protocol Pro1 {
    typealias Element
}

you are just telling that there will be a type named Element.

Pro2

Adding this

protocol Pro2: Pro1 {
    typealias Element = Self
}

you are telling to the compiler that Element will be the same type of the type that is implementing Pro2.

So yes, there is a relation between the Element in Pro1 and Pro2.

Adding methods to Pro1

Let's declare 2 methods that will use Element in Pro1

protocol Pro1 {
    typealias Element
    func a() -> Element
    func b(elm: Element)
}

Conforming to Pro1

Now a class conform to Pro1 will be like this.

class Foo: Pro1 {
    func a() -> String {
        return ""
    }
    func b(elm: String) {
    }
}

As you can see we are forced by the compiler to set the return type of a and the param of b to be of the same type.

Conforming to Pro2

Now let's try to conform another class to Pro2. Again Pro1 will force us to declare methods a and b where the return type of a is equals to the param of b. Furthermore Pro2 will force us to set this type equals to the type of the current type Boo.

So the previous class will to conform to Pro2 because String is different from Foo

class Foo: Pro2 {
    func a() -> String { // <- compiler error
        return ""
    }
    func b(elm: String) { // <- compiler error

    }
}

But if we declare a new class and replace Element with Boo it will work because the constraints of both protocols are satisfied.

class Boo: Pro2 {
    func a() -> Boo {
        return Boo()
    }
    func b(elm: Boo) {

    }
}