Simultaneously mapping over publisher values and errors

31 Dec 2020 ⇐ Notes archive

(This is an entry in my technical notebook. There will likely be typos, mistakes, or wider logical leaps — the intent here is to “let others look over my shoulder while I figure things out.”)

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Dalton nerd sniped me earlier today when he asked:

Combine question: I’d love a “map”-like operator that is failable — like tryMap — but where I end up with a strongly-typed Error. I’m picturing something akin to,

func mapResult<NewOutput, NewFailure>(_ transform: @escaping (Result<Output, Failure>) -> Result<NewOutput, NewFailure>) -> AnyPublisher<NewOutput, NewFailure>.

Does this exist in the framework?

It doesn’t, yet Result and Result.publisher make this wicked fun to pull off.

The transform argument is also a specialized form of Either.bimap hiding in plain sight.