Attempted to implement ValueForKey in Swift

valueForKey.swift

class SomeClass {
    
    var a: String?
    var b: [Int]?
    
    private func _u(named: String, value: AnyObject?, set: Bool) -> AnyObject? {
        switch named {
        case "a":
            if set {
                a = value as? String
            } else {
                return a
            }
        case "b":
            if set {
                b = value as? [Int]
            } else {
                return b
            }
        default:
            fatalError("Property named '\(named)' doesn't exist")
        }
        return nil
    }
    
    subscript(index: String) -> AnyObject? {
        get { return _u(index, value: nil, set: false) }
        set(newValue) { _u(index, value: newValue, set: true) }
    }
}

var t = SomeClass()
t["b"] = [1, 2, 3]
t["a"] = "monkey"
//t["x"] = 12 // crashes
println(t.a) // prints monkey