Hello :wave: Is this the idiomatic way of `bind`ing `Either` and `EitherNel` in the same `either { …...

# arrow

[James Baker]

Hello 👋 Is this the idiomatic way of `bind`ing Either and EitherNel in the same either { … } block?

fun foo(): Either<E, A>
fun A.bar(): EitherNel<E, B>

fun baz(): EitherNel<E, B> = either {
    val a = foo().toEitherNel().bind()
    a.bar().bind()
}

Thanks in advance 😄

[simon.vergauwen]

Hey James,

Sadly Kotlin cannot have specific support for this, but once we have context receivers we can define:

context(Raise<NonEmptyList<E>>)
fun Either<E, A>.bind(): A = when(this) {
  is Right -> value
  is Left -> raise(nonEmptyListOf(value))
}

[Youssef Shoaib [MOD]]

Once withError makes it into a release (it's already merged, just not released yet) you can also do:

fun baz(): EitherNel<E, B> = either {
  withError({ e: E -> e.nel() }) {
    val a = foo().bind()
    a.bar().bind()
  }
}

[James Baker]

I've just started using withError in 1.2.0 Cheers!

[Youssef Shoaib [MOD]]

Also you can define a method that does the withError({ e: E -> e.nel() }) {} wrapping for you. Call it withNel or something