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>?

Answer 1

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.

Answer 2

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.