Hi, is there a support in Arrow for transforming: - `() -> Either<Failure, Result>` and ...

# arrow

[dnowak]

Hi, is there a support in Arrow for transforming:
• () -> Either<Failure, Result> and
• (Failure) -> Error
to () -> Either<Error, Result.

All I want to do is to “decorate” a function returning Either with error mapping.

[simon.vergauwen]

Hey @dnowak,

[simon.vergauwen]

That is certainly possible, but you’ll probably want to adapt/lift those functions to fit a more general signature.

Either#redeemWith takes:
• (Failure) -> Either<Error, B>
• (Result) -> Either<Error, B>
So with that you can apply the desired transformations.

val initial: () -> Either<Failure, Result> = TODO()
val error: (Failure) -> Error = TODO()
val success: () -> Either<Error, Result> = TODO()

val r: Either<Error, Result> =
  initial()
    .redeemWith({ fail: Failure ->
       error(fail).left()
    }, { _: Result ->
      success()
    })

[dnowak]

Hi @simon.vergauwen, sorry for not being accurate. I just want to map the error in Either but have a as a result a function.

val initlial: () -> Either<Failure, Result> = TODO()
val mapError: (Failure) -> Error = TODO()
val mapped: () -> Either<Error, Result> = {
    initial().mapLeft(::mapError)
}

[dnowak]

And actually what I’m looking for would be an extension function to to apply error mapping to a function that returns Either

val mapped = initial.mapLeft(::mapError)

[simon.vergauwen]

Ah, such functionality does not exist out of the box but you could achieve it by composing two different things inside Arrow Core.

intial.andThen { it.mapLeft(::mapError) }

[simon.vergauwen]

Where andThen is a function for composing functions with each other.

[dnowak]

Thanks 🙂