Hi, I'm new to arrow and already passionate, and try to use IO with Either to have a typed error but...

# arrow

[rcfgnu]

Hi, I'm new to arrow and already passionate, and try to use IO with Either to have a typed error but I'm stuck with the composition of dependent function like:

fun x() : IO<Either<Failure,String>>
    fun y(xResult: String) : IO<Either<Failure,Int>>
    fun z(yResult: Int) : IO<Either<Failure,String>>

It's my design totally wrong?

[isto]

you need EitherT if you want to compute in a context where Either is inside another monad (in your case IO), https://arrow-kt.io/docs/0.10/arrow/mtl/eithert/

[Jorge Castillo]

That's a good option also

[rcfgnu]

thank a lot you for the suggestions, I try to understand and follow this approach

[isto]

the syntax can be quite hard to figure out first, but in the example case it would go something like this with 0.10.5 syntax ...

[isto]

data class Failure(val reason: String)

fun x() : IO<Either<Failure,String>> = IO { "x".right() }
fun y(xResult: String) : IO<Either<Failure,Int>> { return IO { Failure(xResult).left() } }
fun z(yResult: Int) : IO<Either<Failure,String>> { return IO { "$yResult".right() } }

fun compute(): IO<Either<Failure, String>> = EitherT.monad<Failure, ForIO>(IO.monad()).fx.monad {
  val xResult = ! EitherT(x())
  val yResult = ! EitherT(y(xResult))
  ! EitherT(z(yResult))
}.value().fix()

[rcfgnu]

thank you so much very helpful