I’m having an issue/confusion with `fix()` . I tho...

# arrow

[kyleg]

I’m having an issue/confusion with fix() . I thought it converted Kind<F, A> into F<A>, where (for example) F is Option . But then I have this, and the compiler disagrees that my type is correct:

import arrow.core.Either
import arrow.core.Option
import arrow.core.extensions.option.traverse.traverse
import arrow.core.k
import <http://arrow.fx.IO|arrow.fx.IO>
import arrow.fx.extensions.fx
import <http://arrow.fx.extensions.io|arrow.fx.extensions.io>.applicative.applicative
import arrow.fx.fix
import <http://java.io|java.io>.File

interface MediaId
interface Entity
interface Foo { val source: Option<File> }
val pathArgs: Foo = TODO()
fun bar(x: String): IO<Sequence<File>> = TODO()
fun transform(it: File): Entity = TODO()
fun baz(x: Entity): IO<Either<Throwable, MediaId>> = TODO()


val result: IO<Option<Sequence<Either<Throwable, MediaId>>>> = pathArgs.source.traverse(IO.applicative()) {
    IO.fx {
        val files = bar(it.canonicalPath).bind()
        files.k().traverse(IO.applicative()) {
            val entity = transform(it)
            baz(entity)
        }.fix().bind()
    }
}.fix() // IO<Kind<ForOption, SequenceK<Either<Throwable, MediaId>>>> ???

The fix() at the end does turn it from Kind<IO, …> into IO<… but why doesn’t the fix() tagged onto the internal traverse not turn the Kind<ForOption, SequenceK into Option<SequenceK>?

[Jannis]

That is because traverse is defined to return Kind<F, Kind<G, A>> and for your types that is Kind<ForIO, Kind<ForOption, A>>. fix only ever deals with the outer layer, so you only get IO<Kind<ForOption, A>>. This sucks but there is no good way to have traverse return a value which has the inner Kind fixed already.
To deal with this you can either change your expected type to IO<OptionOf<A>> = IO<Kind<ForOption, A>> or use map { it.fix() } for the inner part.
Both of these "solutions" suck and I cannot wait for arrow-meta to once and for all end this with the higherkind compiler-plugin...