Hello everyone! I use v0.11.0-SNAPSHOT and I encou...

# arrow

[Skotar]

Hello everyone! I use v0.11.0-SNAPSHOT and I encountered an issue (at least in my understanding):

class OneError
class TwoError

private fun oneEither(): Either<OneError, String> = OneError().left()
private fun twoEither(value: String): Either<TwoError, Int> = value.toInt().right()

fun either() {
   oneEither()
     .flatMap { twoEither(it) }
     .fold(
       { throw Exception(it::class.simpleName) },
       { ... }
     )
}

private fun oneIO(): IO<OneError, String> = IO.raiseError(OneError())
private fun twoIO(value: String): IO<TwoError, Int> = IO.just(value.toInt())

fun io() {
   oneIO()
     .flatMap { twoIO(it) }
     .unsafeRunSyncEither()
     .fold(
       { throw Exception(it::class.simpleName) },
       { ... }
     )
}

The function either throws java.lang.Exception: OneError as I expect. But the function io throws OneError cannot be cast to class TwoError. When I look at the signature of flatMap (fun <E, A, B, E2 : E> IOOf<E, A>.flatMap(f: (A) -> IOOf<E2, B>): IO<E2, B>), I think that it shouldn't even compile.
Can you tell me if my understanding is correct? I expect that IO with two types should behave like Either.

[pakoito]

does it throw or does it fail compilation?

[pakoito]

@simon.vergauwen @aballano flatMap should not map the error side AFAIK

[Skotar]

The function io compiles and throws OneError cannot be cast to class TwoErro. I think that the left side shouldn't be even touched like in Either case.

[raulraja]

Please create an issue in arrow fx and tag @simon.vergauwen and myself, thanks.

[Skotar]

Thanks for the response! Done. https://github.com/arrow-kt/arrow-fx/issues/149