[CE3] Kleisli Stack safety

[ronanM]

This code is Ok with CE2, but fail with StackOverflowError in CE3.

import cats.effect.{ExitCode, IO, IOApp}
import cats.syntax.all._
import cats.data.Kleisli

object MainKleisli extends IOApp {

  override def run(args: List[String]): IO[ExitCode] = {
    val N = 10000
    var i = 0

    List.fill(N)(0).traverse_(_ => Kleisli.liftF(IO(i += 1))).run("Go...") *>
      IO(println(s"i: ${i}")) *>
      IO(assert(i === N, s"i: $i != $N")).as(ExitCode.Success)
  }
}

[vasilmkd]

@ronanM Thanks for reporting and for the reproduction. I have a fix incoming.

[catostrophe]

@vasilmkd
this still fails:

  "should be stack safe in long parTraverse chains" in ticked { implicit ticker =>
      val N = 10000

      Ref[IO].of(0).flatMap { ref =>
        List
          .fill(N)(0)
          .parTraverse_(_ => Kleisli.liftF(ref.update(_ + 1)))
          .run("Go...") *> ref.get
      } must completeAs(N)
    }

[catostrophe]

Also typelevel/cats#3947

[vasilmkd]

Even with Cats Effect 3.2.0?

[catostrophe]

Yes. And it never worked on CE2

[vasilmkd]

Bummer. I'll take a look.

[vasilmkd]

I have a fix for parTraverse, I'll open a PR later.