Example.swift

protocol LoginProvider {
  func login()
}

class EmailLoginProvider: LoginProvider {
  var email: String
  var password: String
  init(email: String, password: String) {
    self.email = email
    self.password = password
  }
  
  func login() {
    print("loggin in using email \(self.email)")
  }
}

class FacebookLoginProvider: LoginProvider {
  func login() {
    print("logging in with facebook")
  }
}

protocol LoginProviderConvertible {
  func toLoginProvider() -> LogginProviderConvertible
}

enum AppLoginProviders {
  case Email(email: String, password: String)
  case Facebook
}


extension AppLoginProviders: LoginProviderConvertible {
  func toLoginProvider() -> LogginProviderConvertible {
    switch(self) {
      case .Email(let email, let password): return EmailLoginProvider(email: email, password: password)
      case .Facebook: return FacebookLoginProvider()
    }
  }
}


func performLogin(loginProvider: LoginProvider) {
  loginProvider.login()
}

func performLogin(loginProviderConvertible: loginProviderConvertible) {
  performLogin(loginProviderConvertible.toLoginProvider())
}

performLogin(FacebookLoginProvider())
performLogin(.Email(email: "email", password: "password"))