Can I simulate traits/mixins in Swift?

Question

Does Swift have a way of mixing in traits, a la Scala? The section of the Swift book on using extensions to add protocols to existing classes comes tantalizingly close. However,

Answer 1

Here's my (not yet widely tested) way of doing what I think are Scala traits in Swift 2.1.1, Playgrounds-ready, two versions:

Less flexible:

protocol BigBadProtocol {
    func madFunc() -> String;
    // func otherFunc();
    // Maybe a couple more functions here.
}

protocol BlueMadFuncUser: BigBadProtocol {}

extension BlueMadFuncUser {
    func madFunc() -> String {
        return "Blue"
    }
}

protocol RedMadFuncUser: BigBadProtocol {}

extension RedMadFuncUser {
    func madFunc() -> String {
        return "Red"
    }
}

class ClearClass: BigBadProtocol {
    func madFunc() -> String {
        return "Clear"
    }
}

class BlueClass: BlueMadFuncUser {}

class RedClass: RedMadFuncUser {}

More flexible:

protocol BigBadProtocol {
    func madFunc() -> String;
    // func otherFunc();
    // Maybe a couple more functions here.
}

protocol BlueMadFuncUser {}

extension BigBadProtocol where Self: BlueMadFuncUser {
    func madFunc() -> String {
        return "Blue"
    }
}

protocol RedMadFuncUser {}

extension BigBadProtocol where Self: RedMadFuncUser {
    func madFunc() -> String {
        return "Red"
    }
}

class ClearClass: BigBadProtocol {
    func madFunc() -> String {
        return "Clear"
    }
}

class BlueClass: BigBadProtocol, BlueMadFuncUser {}

class RedClass: BigBadProtocol, RedMadFuncUser {}

Sanity check:

var classes: [BigBadProtocol] = [ClearClass(), BlueClass(), RedClass()]

// Prints "Clear, Blue, Red\n"
print((classes.map { $0.madFunc() }).joinWithSeparator(", "))

// Print another way for Playgrounds, which appears to bug out on the lines above
var s = ""
for klass in classes {
    s += klass.madFunc() + " "
}
print(s)

BlueMadFuncUser and RedMadFuncUser are two versions of a trait. My terminology might be off, but then you can independently create a second trait like that and mix and match in your classes as you please.

Would be much more challenging or boiler-plate-y to reuse logic like that with an inheritance-based approach.

I ended up wanting this pattern after finding it very useful in Hack for PHP, where from what I can tell traits are very similar to Scala's: https://docs.hhvm.com/hack/other-features/trait-and-interface-requirements)