SolvedRxSwift Cannot flatMap a Single trait to a Completable trait

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Short description of the issue:

Cannot flatMap a Single trait to a Completable trait

Expected outcome:

I expect that the flatMap would let me convert to a Completable trait like in the example below.

What actually happens:

I get the following error:

 'flatMap' produces 'PrimitiveSequence<SingleTrait, R>', not the expected contextual result type 'PrimitiveSequence<SingleTrait, _>'

Self contained code example that reproduces the issue:

func convertExample() -> Completable {
    return Single<String>.create { single in
        return Disposables.create()
      .flatMap({ _ -> Completable in
        return Completable.empty()

RxSwift/RxCocoa/RxBlocking/RxTest version/commit



  • iOS
  • macOS
  • tvOS
  • watchOS
  • playgrounds

How easy is to reproduce? (chances of successful reproduce after running the self contained code)

  • easy, 100% repro
  • sometimes, 10%-100%
  • hard, 2% - 10%
  • extremely hard, %0 - 2%

Xcode version:


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Installation method:

  • CocoaPods
  • Carthage
  • Git submodules

I have multiple versions of Xcode installed:
(so we can know if this is a potential cause of your issue)

  • yes (which ones)
  • no

Level of RxSwift knowledge:
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and formulate the response in an appropriate manner)

  • just starting
  • I have a small code base
  • I have a significant code base
36 Answers

✔️Accepted Answer

I absolutely agree with @tarunon.
I encounter that situation almost every day.

Currently I implement that architecture by this way,

func createComplexCompletable() -> Completable {
    return singleA
        .flatMap { valueA -> Observable<Never> in
            otherCreateCompletableMethod(arg: valueA) // <- returns Completable

Looks terrible. 🙃

If I can write

func createComplexCompletable() -> Completable {
    return singleA
        .flatMap { valueA -> Completable in
            otherCreateCompletableMethod(arg: valueA)

Looks smart. 😎

Other Answers:

Single should be able to be converted into Maybe and Completable, and Completable only to Maybe (But Maybe cannot be converted to other traits).

I wrote a code something like below and it's working. It takes out the source observable once by asObservable and then convert it to other traits.

extension PrimitiveSequence where TraitType == SingleTrait {
    public func asMaybe() -> PrimitiveSequence<MaybeTrait, Element> {
        return self.asObservable().asMaybe()
    public func asCompletable() -> PrimitiveSequence<CompletableTrait, Never> {
        return self.asObservable().flatMap { _ in Observable<Never>.empty() }.asCompletable()

extension PrimitiveSequence where TraitType == CompletableTrait, ElementType == Swift.Never {
    public func asMaybe() -> PrimitiveSequence<MaybeTrait, Element> {
        return self.asObservable().asMaybe()

I've thought about this a bit, we can do something similar like RxJava did, it makes sense, but it might take a month or two to release it.

In my opinion, that make sense next flatMap overloading.

Source\Closure (T) -> Single<U> (T) -> Maybe<U> (T) -> Completable
Single<T> Single<U> Maybe<U> Completable
Maybe<T> Maybe<U> Maybe<U> Completable

Completable has no next(success), so there is no implementation of flatMap

We often make the function that has 1 argument and return Completable, but if we wish to use this function in PrimitiveSequence chaining, I think there is no way to use this function.
andThen can connect from Single to Completable, but we cannot receive next(success) value.

How do you think? @kzaher

Actually I believe that "flatmapCompletable" is totally necessary. In my case, I receive a stream from which I want to use data (Single), but the stream that goes next must not emit any value (Completable). Example:

private func addToken<T>(request : RequestBuilder<T>) -> Single<RequestBuilder<T>> {
        return Single.create {
            request.addHeaders([TokenRepository.headerKey : TokenRepository.sharedInstance.token!])
            return Disposables.create()

func secureRequest(request : RequestBuilder<Void>) -> Completable {
        return self.addToken(request: request)
            .flatMap {
                self.executeRequest(request: $0)

func executeRequest(request : RequestBuilder<Void>) -> Completable {
        return Completable.create(subscribe: { (observer) -> Disposable in
            request.execute({ (response, error) in
                    guard response != nil else {
                        let error = error ?? WebServiceError.unknown
            return Disposables.create()

I have been using RxJava2 and I think they manage better what is related to the changes between observable types than RxSwift... Single to Maybe, Single to Completable, flatmapCompletable, etc.

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