Hello. Is there a way to transform an `Either` suc...

# arrow

[cavega]

Hello. Is there a way to transform an Either such that if Left meets some specific condition (i.e. filter out some error code that should not throw an error) the downstream Either via the fold operation gets routed to ifRight?

I did something like this (where Either<List<MyErrors>, <Data>>):

return someService()
            .map { it.toEither().mapLeft {
                it.filter { it.code != "some-failure-code" }
            } }

But all that does in return an empty list of MyErrors to the downstream fold’s isLeft.

[Scott Christopher]

If you want to convert a Left potentially to a Right depending on its value, you can make use of handleErrorWith - https://arrow-kt.io/docs/arrow/typeclasses/applicativeerror/#kindf-ahandleerrorwith

[Scott Christopher]

in your example, something like:

someService()
  .map { it.toEither().handleErrorWith {
    when (val errs = it.filter { it.code != "some-failure-code" }) {
      errs.isEmpty() -> Right(Unit) // or some other value to recover to
      else -> Left(errs)
    }
  } }

[simon.vergauwen]

Yes, at the very end of your code you could turn a Left into Right by inspecting Left and returning any arbitrary Either using handleErrorWith.

Or use handleError if you only want to return a Right.

val either: Either<String, Int> = ...

either.handleErrorWith { str ->
  Either.catch { str.toInt() }
}

either.handleError { str -> 0 }

[cavega]

Awesome. Thank you! 🙇