public class Authentication {

    public typealias Credentials = (username: String, password: String)

    public enum Response {
        case authenticated(Token)
        case failed(String)
    }

    public let credentials: SafeObserver<Credentials>
    public let response: SafeSignal<Response>

    public init(client: Client) {

        let credentials = SafePublishSubject<Credentials>()

        self.response = credentials
            .flatMapLatest { username, password in
                return client.response(for: API.User.login(username: username, password: password))
            }
            .map { Response.authenticated($0) }
            .flatMapError { .just(Response.failed($0.reason)) }

        self.credentials = credentials.toObserver()
    }
}