rx+lift.swift

func lift<A1, A2, R>(_ f: @escaping (A1, A2) -> R) -> ((Observable<A1>, Observable<A2>) -> Observable<R>) {
    return { (a1: Observable<A1>, a2: Observable<A2>) -> Observable<R> in
        return Observable.combineLatest(a1, a2).map(f)
    }
}

func lift<A, R>(_ f: @escaping (A) -> Observable<R>) -> ((Observable<A>) -> Observable<R>) {
    return { (a: Observable<A>) -> Observable<R> in
        a.flatMap(f)
    }
}

func lift <A1, A2, R>(_ f: @escaping (A1, A2) -> Observable<R>) -> ((Observable<A1>, Observable<A2>) -> Observable<R>) {
    return { (a1: Observable<A1>, a2:Observable<A2>) -> Observable<R> in
        Observable.combineLatest(a1, a2).flatMap(f)
    }
}

struct Credentials {
    let username: String
    let password: String
}

let loginSignal = PublishSubject<Void>()
let usernameSubject = BehaviorSubject<String>(value: "")
let passwordSubject = BehaviorSubject<String>(value: "")


enum LoginResult {
    case success, failure
}

func login(credentials: Credentials) -> Observable<LoginResult> {
    if credentials.username == "pasi" && credentials.password == "password1" {
        return Observable.just(.success)
    } else {
        return Observable.just(.failure)
    }
}

let createCredentials = lift(Credentials.init)
let login_ = lift(login)

login_(loginSignal.withLatestFrom(createCredentials(usernameSubject, passwordSubject))).debug("login").subscribe()

usernameSubject.onNext("pasi")
passwordSubject.onNext("pasword")
loginSignal.onNext(())
// login -> Event next(failure)

passwordSubject.onNext("password1")
loginSignal.onNext(())
// login -> Event next(success)