How can I constrain a Swift protocol to a concrete type?

Question

Take a look at the following hypothetical code:

class Stream<S: SequenceType where S.Generator.Element: Character> {
    init(_ sequence: S) {}
}

Doesn't compile. I get "S.Generator.Element constrained to non-protocol type Character". This is a bummer, man. I've thought of two possibilities:

class Stream<S: SequenceType where S.Generator.Element: ExtendedGraphemeClusterLiteralType> {
}

This constraint works because Character is the only thing I know of to implement that protocol. The problem is that now I have an ExtendedGraphemeClusterLiteralType instead of a Character so I'm forced to cast, which I can live with.

The other possibility is just to define my own protocol, e.g., CharacterType, and have Character implement that through an extension. (This is probably also safer.) This is likely the approach I will actually take, but I wondered if anyone knew a way around this limitation other than this one?

Answer 1

Try:

class Stream<S: SequenceType where S.Generator.Element == Character> {
//                                                     ^^
    init(_ sequence: S) {}
}