typelevel / typelevel/fs2

Counter-intuitive behavior with `interruptScope` and `IOLocal`s

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  • #2843 by @armanbilge — closed without merging
bug
Dominant language
Scala
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Description

Context: https://github.com/armanbilge/bayou/issues/1#issuecomment-1062827082

//> using scala "3.1.1"
//> using lib "co.fs2::fs2-core:3.2.5"

import cats.effect._
import fs2._

object Bayou extends IOApp.Simple:
  // def run = s.compile.lastOrError.flatMap(IO.println) // List(2, 1)
  // def run = s.scope.compile.lastOrError.flatMap(IO.println) // List(2, 1)
  def run = s.interruptScope.compile.lastOrError.flatMap(IO.println) // List()

  def s = Stream.eval(IOLocal(List.empty[Int])).flatMap { local =>
    Stream.eval(local.update(1 :: _)).flatMap { _ =>
      Stream.eval(local.update(2 :: _)).flatMap { _ =>
        Stream.eval(local.get)
      }
    }
  }

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by running the Scala 3.1.1 reproduction with fs2-core 3.2.5 and compare the plain, scoped, and interrupt-scoped stream results. Read the behavior around interruptScope and IOLocal, then establish the expected result and verify it with a regression test.

Written by the indexing model from the issue text.

Assessment

Tech stack
scala
Domain
stream-processing
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
35/100

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