Set a given Publishers Failure type to Never in Combine

Question

Is there a way to transform a given AnyPublisher<AnyType, SomeError> to AnyPublisher<AnyType, Never>?

Gerous

Answer 1

A publisher with Never as error type mean that it can't throw error at all. It will always deliver a value.

To obtain a publisher that can never throw errors you have 2 solutions:

1/ Catch all possible errors:

let publisher: AnyPublisher<AnyType, SomeError> = //...

publisher.catch { error in
  // handle the error here. The `catch` operator requires to
  // return a "fallback value" as a publisher
  return Just(/* ... */) // as an example
}

2/ If you are sure that there no errors can be thrown by the publisher, you can use .assertNoFailure(), that will convert your publisher. Note that is an error pass through the .assertNoFailure(), your app will crash immediately.

Bisset

Ah,I didn't know about assertNoFailure. Thank you! – Gerous

@Bisset It doesn't work, because catch will ( and just too ) completes publisher immediately. There is no generic solution for the author's problem. – Obbard

I didn't used catch for a while now, but I seems to recall that catch does complete the publisher (in case it catches an error), but replace the publisher with a new one (here Just). So after the catch you do have a publisher with Never as error type (and you will have the value provided by Just in a sink or assign). – Bisset

Answer 2

Use the replaceError operator. This requires that you emit an AnyType value that will be sent down the pipeline from this point if an error arrives from upstream.

For example:

URLSession.shared.dataTaskPublisher(for: url)
    .map {$0.data} // *
    .replaceError(with: Data()) // *
    // ...

From this point on down the pipeline, we are guaranteed that either the Data from the data task completion or (if there is a networking error) an empty Data will be received. The Failure type from this point down the pipeline is Never.

Hormonal

I think this is the best answer because it is the most direct and elegant way to achieve what the question is asking for. The accepted answer uses catch which needs to to wrapped in the flatmap if we don't want to end the communication and most of all, this is coming from Matt Neuburg 😃 – Antigen

By the way, just realised replaceError always ends the stream. So catch has more flexibility - you could wrap it in a flatmap to continue the stream – Antigen