Error combinaison when using parXXX in cats about interop-cats HOT 9 CLOSED

@larousso The errors are already accumulated in Cause[E] for ZIO. Could you please check if

(io1, io2).parTupled.mapErrorCause { 
  c => Cause.fail(NonEmptyList.fromListUnsafe(c.failures).reduce)
}

contains all the errors you're looking for?

from interop-cats.

Yes, this is working :

testM("zio") {
  import cats.implicits._
  import zio.interop.catz._
  assertM(
    (ZIO.fail(NonEmptyList.one("Error1")), ZIO.fail(NonEmptyList.one("Error2"))).parTupled.mapErrorCause { c =>
      Cause.fail(NonEmptyList.fromListUnsafe(c.failures).reduce)
    }.either,
    isLeft(
      equalTo(NonEmptyList.of("Error1", "Error2"))
    )
  )
}

from interop-cats.

@larousso In that case I think this issue can be closed? I feel like there wouldn't be much gain from a new Parallel instance with E: Semigroup constraint since Cause already does the accumulation here....

from interop-cats.

I really prefer the concise syntax with just

(io1, io2).parTupled

instead of

(io1, io2).parTupled.mapErrorCause { 
  c => Cause.fail(NonEmptyList.fromListUnsafe(c.failures).reduce)
}

but I understand that it's not fine for you to add a new instance with a E as a monoid so I can close the issue.

from interop-cats.

@larousso you can add it and there's a precedent with MonoidK/SemigroupK instances - https://github.com/zio/interop-cats/blob/master/interop-cats/shared/src/main/scala/zio/interop/cats.scala#L109 - but then I'm not sure how intuitive it would be to change the error behavior based on existence/nonexistence of instances for error (the same applies to the SemigroupK instance)

from interop-cats.

It seams to be the standard behavior for cats.
For exemple:

        testM("cats") {
          import cats._
          import cats.data._
          import cats.implicits._
          import zio.interop.catz._

          val et1 = EitherT.leftT[Task, String](NonEmptyList.one("Error1"))
          val et2 = EitherT.leftT[Task, String](NonEmptyList.one("Error2"))

          assertM(
            (et1, et2).parTupled.value,
            isLeft(
              equalTo(NonEmptyList.of("Error1", "Error2"))
            )
          )
        }

from interop-cats.

@larousso The difference is that you must specifically wrap your IO in EitherT to enable this behavior. There's no low-priority instance for Parallel[EitherT[...]] that doesn't require a Semigroup instance – therefore your errors will either be accumulated or your compilation will fail, there won't be a silent change of behavior towards non-accumulation from a missing instance.

from interop-cats.

For ZIO there's a silent change of behavior in SemigroupK currently. I'm not super fond of that as is, with a conditional Parallel instance there will be more silent changes of behavior...

/cc @zio/core for opinions...

from interop-cats.

ZIO cannot guarantee parallel error accumulation. This is because when one running effect fails, the others are canceled immediately... possibly before they begin.

You could do:

ZIO.uninterruptible {
  ...
}

If you wanted to "force" non-cancellation. That should allow you to get parallel error accumulation.

from interop-cats.