protocol Serializable {
    func serialize() -> String
}

extension Serializable {
    func serialize() -> String {
        return "\(self)"
    }
}

extension String: Serializable {}
extension Int: Serializable {}

extension Dictionary: Serializable where Key == String {
    func serialize() -> String {
        for (key, value) in self {
            result.append("\(key):")
            if let serializableElement = value as? Serializable {
                result.append((serializableElement.serialize()))
            } else {
                let mirror = Mirror(reflecting: value)
                print("self: \(self.customMirror.subjectType), element: \(mirror.subjectType)")
            }
        }
        return result
    }
}

let x = ["key11" : 5, "key2" : ["inner" : "hhhhh" ]].serialize()

print(x)