user_with_sns.swift

class User {}

// Twitter
protocol Twitter {
//    var username: String { get }
}
extension Twitter {
    var username: String {
        get { return "tryswiftconf" }
    }
}
func showTwitter(user: Twitter) {
    print(user.username)
}
extension User: Twitter {}

// GitHub
protocol GitHub {
//    var username: String { get }
}
extension GitHub {
    var username: String {
        get { return "lattner" }
    }
}
func showGitHub(user: GitHub) {
    print(user.username)
}
extension User: GitHub {}


let user = User()
showTwitter(user) // tryswiftconf
showGitHub(user) // lattner