Access Computed Property of a Protocol in a Protocol Extension

Question

Assume I have a protocol like

protocol TypedString : CustomStringConvertible, Equatable, Hashable { }

func == <S : TypedString, T : TypedString> (lhs : T, rhs : S) -> Bool {
    return lhs.description == rhs.description
}

I want to be able to provide a default implementation of Hashable:

extension TypedString {
    var hashValue = { return self.description.hashValue }
}

But the problem is that I get an error Use of unresolved identifier self.

How do I create a default implementation of Hashable using the string representation given by the description property defined in the CustomStringConvertible protocol?

---

The motivation is I want to create shallow wrappers around Strings, so I can semantically type-check my code. For example instead of writing func login (u : String, p : String) -> Bool I would instead write func login (u : UserName, p : Password) -> Bool where the (failable) initialisers for UserName and Password do validation. For example I might write

struct UserName : TypedString {
    let description : String

    init?(userName : String) {
        if userName.isEmpty { return nil; }

        self.description = userName
    }
}

Answer 1

What you want is totally possible, the error message you're receiving just isn't making it very clear what the problem is: your syntax for a computed property is incorrect. Computed properties need to be explicitly typed and don't use an equals sign:

extension TypedString {
    var hashValue: Int { return self.description.hashValue }
}

This works fine!