Hi, I’m trying to solve folling case. I have a `List<IO<Either<E,A>>>` and I want...

# arrow

[dnowak]

Hi, I’m trying to solve folling case. I have a List<IO<Either<E,A>>> and I want to stop on first success. How to do this with Arrow?

[simon.vergauwen]

You can use traverse on List with EitherT.applicative(IO.applicative()).

And it should return you the shape EitherT<ForIO, E, List<A>> by calling value() you should be able to extract.

IO<Either<E, List,A>>

[simon.vergauwen]

Trying with ListK is probably easiest.
I can share a snippet later.

[dnowak]

Traverse seems to stop on first error… This is my simple test:

[dnowak]

import arrow.Kind
import arrow.core.Either
import arrow.core.ForListK
import arrow.core.extensions.list.traverse.traverse
import arrow.core.left
import arrow.core.right
import arrow.fx.ForIO
import <http://arrow.fx.IO|arrow.fx.IO>
import <http://arrow.fx.extensions.io|arrow.fx.extensions.io>.monad.monad
import arrow.fx.fix
import arrow.mtl.EitherT
import arrow.mtl.extensions.eithert.applicative.applicative
import arrow.mtl.value

data class Error(val message: String)
data class Result(val result: String)

fun evaluate(value: Int): IO<Either<Error, Result>> = IO {
    if ((value % 5) == 2) {
        Result("Success: $value").right()
    } else {
        Error("Error: $value").left()
    }
}

fun main() {
    val list: List<IO<Either<Error, Result>>> = List(10) { index -> evaluate(index) }
    val result: IO<Either<Error, Kind<ForListK, Result>>> = list.traverse(EitherT.applicative<Error, ForIO>(IO.monad())) {
        EitherT(it)
    }.value().fix()
    val runResult: Either<Error, Kind<ForListK, Result>> = result.unsafeRunSync()
    println(runResult)
}

[dnowak]

What I want to achive is to ignore all errors until I get a first success and the success should be the result.

[simon.vergauwen]

Oh you want to stop on the first success case. My bad.

[simon.vergauwen]

Okay, let me think about that. So you want to implement a traverse that uses handleErrorWith as composition instead, right? So the first success returns, and they execute in order.

[simon.vergauwen]

val ios: List<IO<Either<E, A>>> = TODO()

val eitherT: List<EitherT<ForIO, E, A>> = ios.map { EitherT }

val ioEA: IO<Either<E, A>> = eitherT.fold(EitherT.raiseError(RuntimeException("Initial error")) { acc, a ->
  acc.handleErrorwith { a }
}.value()

[simon.vergauwen]

The first success will short-circuit all remaining handleErrorWith calls.

So you can provide a list of IO<Either<E, A>> and it’ll return the first successful Either<E, A>.

[simon.vergauwen]

So it’s just folding over the original list, and composing all IOs together using handleErrorWith 🙂

[dnowak]

@simon.vergauwen I tried to implement your solution in Arrow 0.10.5. Unfortunately, I cant get it compile 😞.

[simon.vergauwen]

I wrote it from memory. Is there a snippet you can share? Otherwise I’ll whip up an example with imports later 😉

[dnowak]

import arrow.core.Either
import arrow.core.left
import arrow.core.right
import arrow.fx.ForIO
import <http://arrow.fx.IO|arrow.fx.IO>
import <http://arrow.fx.extensions.io|arrow.fx.extensions.io>.monad.monad
import arrow.mtl.EitherT
import arrow.mtl.extensions.eithert.applicativeError.raiseError

data class Error(val message: String)
data class Result(val result: String)

fun evaluate(value: Int): IO<Either<Error, Result>> = IO {
    if ((value % 5) == 2) {
        Result("Success: $value").right()
    } else {
        Error("Error: $value").left()
    }
}

val ios: List<IO<Either<Error, Result>>> = List(10) { index -> evaluate(index) }
val eitherT: List<EitherT<Error, ForIO, Result>> = ios.map { io -> EitherT(io) }
val ioEA: IO<Either<Error, Result>> = eitherT.fold(EitherT.raiseError(IO.monad(), Error("Initial"))) { acc, a ->
    acc.handleErrorwith { a }
}.value().fix()

fun main() {
    val result = ioEA.unsafeRunSync()
    println(result)
}

[simon.vergauwen]

What about this? It seems EitherT is still missing some useful operator like handleErrorWith. So I used the one from the underlying IO directly.

Untitled

[dnowak]

@simon.vergauwen Which version of Arrow have you used?

[simon.vergauwen]

All

[simon.vergauwen]

This is on master. I don’t have a 0.10.5 project checked out right now

[dnowak]

The only thing I changed to make it compile under 0.10.5 is that I extracted initial error to:

[dnowak]

val initial = EitherT
        .applicativeError<Error, ForIO>(IO.monad())
        .raiseError<Error>(Error("Initial error")) as arrow.mtl.EitherT<Error, ForIO, Result>

[dnowak]

And the result I get is: Left(Error(message=Initial))

[simon.vergauwen]

Yes, there that API functions seems to be missing. You could easily create your own overload, something like this.

You can also do fun <A, F, B> EitherT.Companion.raiseError(a: A): EitherT<A, ForIO, B> = EitherT(IO.just(a.left())

[simon.vergauwen]

You get initial?

[dnowak]

Yes

[dnowak]

https://gist.github.com/dnowak/b8120714c37310b70bf82029aad42176

[simon.vergauwen]

Hey @dnowak not sure if you solved it yet but you want an operator for EitherT that short-circuits on Throwable and E. For example handleBothWith.

fun <A, B> EitherT<A, IOPartialOf<Nothing>, B>.handleBothWith(
  f: (Either<Throwable, A>) -> EitherT<A, IOPartialOf<Nothing>, B>
): EitherT<A, IOPartialOf<Nothing>, B> =
  EitherT(value().fix().attempt().flatMap {
    when (it) {
      is Either.Left -> f(it)
      is Either.Right -> when (val bb = it.b) {
        is Either.Left -> f(Either.Right(bb.a))
        is Either.Right -> EitherT(IO.just(bb))
      }
    }.value()
  })

[simon.vergauwen]

This way you can recover from Throwable and Error while folding and the first success B will finish the chain.