Publisher mapError with the Combine Framework

Question

I have a setup like this with an error shown below

enum DataError: Error { case failed(error: Error) } enum UIError: Error { case failed(error: Error) } struct SomeData { } func foo() -> AnyPublisher<Result<[SomeData]?, DataError>, Never> { ... } func foo2() -> AnyPublisher<Result<[SomeData]?, UIError>, Never> { return foo() .mapError { error -> UIError switch error { .... } return UIError.failed(error: $0) } .eraseToAnyPublisher() ===> // Error Cannot convert return expression of type 'AnyPublisher<Result<[SomeData]?, DataError>, UIError>' to return type 'AnyPublisher<Result<[T], UIError>, Never>' }

The error message seem pretty straight forward but cant quite resolve it.

Without seeing your implementation of foo() this is largely pointless, as your code can never be tested in the compiler. — matt
– matt, Commented Mar 4, 2020 at 21:54

Apostolos · Accepted Answer · 2020-03-04 19:59:11Z

You will probably need something like this:

The Publisher returns no Error. The Result has the error so you need to mapError the Result.

I modified the Error enums a bit to make it easier to understand.

 enum DataError: Error { case failed } enum UIError: Error { case failed } func foo2() -> AnyPublisher<Result<[SomeData]?, UIError>, Never> { return foo().map { result in result.mapError { _ -> UIError in UIError.failed } }.eraseToAnyPublisher() }

David Pasztor · Accepted Answer · 2020-03-02 17:00:51Z

There's no point in using Result in Combine, since your Publishers will always either emit a value or complete with failure. They should never emit a value, which contains an error.

Once you get rid of your usage of Result, the errors go away nicely.

func foo() -> AnyPublisher<[SomeData], DataError> { ... } func foo2() -> AnyPublisher<[SomeData], UIError> { return foo() .mapError { error -> UIError in switch error { ... } return UIError.failed(error: error) } .eraseToAnyPublisher() }

Either with Result or in a Publisher, you usually don't need to use Optionals, since in case of error, there simply won't be a value, there's no need to return nil value, unlike with completion handlers.

Thanks @David for the suggestion; point taken, however it doesn't quite answer the questions. Do you have any response to resolving the current implementations error?
@user3355301 why do you want to fix a solution that has a fundamental flaw in it rather than fix that fundamental flaw?
There is a Result publisher so Result is useful — it is a publisher.

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