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Monix 3.3.0
Better Stack Traces, Local improvements and new operators

The release is binary and source compatible with 3.x.x line. It is released for the following Scala and ScalaJS versions:

  • Scala 2.11: ScalaJS 0.6.x
  • Scala 2.12: ScalaJS 0.6.x and 1.3.x
  • Scala 2.13: ScalaJS 0.6.x and 1.3.x

Note that most likely, this is going to be the last release on ScalaJS 0.6.x. We can consider doing backports on-demand.


To use the new version, include this in your build.sbt:

libraryDependencies += "io.monix" %% "monix" % "3.3.0"

Or in case you just want any of the sub-projects:

// Local, Scheduler, Cancelable, CancelableFuture, Atomic, etc
libraryDependencies += "io.monix" %% "monix-execution" % "3.3.0"

// Utilities built for Cats-Effect, e.g. ConcurrentChannel, FutureLift
libraryDependencies += "io.monix" %% "monix-catnap" % "3.3.0"

// Task, Coeval
libraryDependencies += "io.monix" %% "monix-eval" % "3.3.0"

// Observable
libraryDependencies += "io.monix" %% "monix-reactive" % "3.3.0"

// Iterant
libraryDependencies += "io.monix" %% "monix-tail" % "3.3.0"


Better Stack Traces

This release includes a highly requested feature of better stack traces for Task and Coeval! Big thanks to @RaasAhsan and @djspiewak for providing the original implementation that we have ported.

They are enabled by default, but it is configurable. Refer to [Stack Traces section]({{ site.baseurl }}docs/3x/eval/ for more details.

We have measured about 10-30% performance hit in CACHED mode (the default) in microbenchmarks. If you have any performance tests, we would greatly appreciate any reports! If the hit is too big, you can disable the stack traces with -Dmonix.eval.stackTracingMode=none.

For the following code:


import monix.eval.Task
import monix.execution.Scheduler
import cats.implicits._
import scala.concurrent.duration._

object TestTracingApp extends App {
  implicit val s =

  def customMethod: Task[Unit] =

  val tracingTestApp: Task[Unit] = for {
    _ <- Task.shift
    _ <- Task.unit.attempt
    _ <- (Task(println("Started the program")), Task.unit).parTupled
    _ <- customMethod
    _ <- if (true) Task.raiseError(new Exception("boom")) else Task.unit
  } yield ()

  tracingTestApp.onErrorHandleWith(ex => Task(ex.printStackTrace())).runSyncUnsafe

The default (cached) stack trace is going to be:

java.lang.Exception: boom
        at guarantee @$.customMethod(TestTracingApp.scala:29)
        at flatMap @$.$anonfun$tracingTestApp$4(TestTracingApp.scala:35)
        at parTupled @$.$anonfun$tracingTestApp$2(TestTracingApp.scala:34)
        at parTupled @$.$anonfun$tracingTestApp$2(TestTracingApp.scala:34)
        at flatMap @$.$anonfun$tracingTestApp$2(TestTracingApp.scala:34)
        at flatMap @$.$anonfun$tracingTestApp$1(TestTracingApp.scala:33)
        at flatMap @$.delayedEndpoint$test$app$TestTracingApp$1(TestTracingApp.scala:32)

Before 3.3.0 and with stack traces disabled, stack traces are a mess:

java.lang.Exception: boom
        at monix.eval.internal.TaskRunLoop$.startFull(TaskRunLoop.scala:188)
        at monix.eval.internal.TaskRestartCallback.syncOnSuccess(TaskRestartCallback.scala:101)
        at monix.eval.internal.TaskRestartCallback$$anon$
        at monix.execution.internal.Trampoline.monix$execution$internal$Trampoline$$immediateLoop(Trampoline.scala:66)
        at monix.execution.internal.Trampoline.startLoop(Trampoline.scala:32)
        at monix.execution.schedulers.TrampolineExecutionContext$JVMNormalTrampoline.super$startLoop(TrampolineExecutionContext.scala:142)
        at monix.execution.schedulers.TrampolineExecutionContext$JVMNormalTrampoline.$anonfun$startLoop$1(TrampolineExecutionContext.scala:142)
        at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.scala:18)
        at scala.concurrent.BlockContext$.withBlockContext(BlockContext.scala:94)
        at monix.execution.schedulers.TrampolineExecutionContext$JVMNormalTrampoline.startLoop(TrampolineExecutionContext.scala:142)
        at monix.execution.internal.Trampoline.execute(Trampoline.scala:40)
        at monix.execution.schedulers.TrampolineExecutionContext.execute(TrampolineExecutionContext.scala:57)
        at monix.execution.schedulers.BatchingScheduler.execute(BatchingScheduler.scala:50)
        at monix.execution.schedulers.BatchingScheduler.execute$(BatchingScheduler.scala:47)
        at monix.execution.schedulers.AsyncScheduler.execute(AsyncScheduler.scala:31)
        at monix.eval.internal.TaskRestartCallback.onSuccess(TaskRestartCallback.scala:72)
        at monix.eval.internal.TaskRunLoop$.startFull(TaskRunLoop.scala:183)
        at monix.eval.internal.TaskRestartCallback.syncOnSuccess(TaskRestartCallback.scala:101)
        at monix.eval.internal.TaskRestartCallback.onSuccess(TaskRestartCallback.scala:74)
        at monix.eval.internal.TaskSleep$
        at java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(
        at java.util.concurrent.ForkJoinTask.doExec(
        at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(
        at java.util.concurrent.ForkJoinPool.runWorker(

Better Task => Future interop when using Local

Running Task isolates Local, which was not available in the Future, resulting in runToFuture. This release enables it and unblocks compelling use cases, such as reading proper request context in Akka HTTP Directive. We've created an AkkaHTTP Example that demonstrates it.

Latest behavior is:

implicit val s: Scheduler = Scheduler.Implicits.traced

val local = Local(0)

for {
  _  <- Task(local.update(1)).runToFuture
  value <- Future(local.get)
} yield println(s"Local value in Future $value")

println(s"Local value on the current thread = $value")

// => Local value on the current thread = 0
// => Local value in Future = 1

Task still isolates the Local, but the Future continuation keeps the same reference and can read it. Before the change, Local would be 0 in the Future.

More information about Local can be found in the new [Local documentation]({{ site.baseurl }}docs/3x/execution/local.html).

Relevant updates

  • #1205: Observable.mergePrioritizedList
  • #1209: Bring back Observable.transform and Transformer alias
  • #1198: Fix flatMapIterable calling recursively itself
  • #1213: Propagate Local isolation in runToFuture
  • #1217: Fix performance regression in bufferSliding
  • #1244: Add support for compression: Gzip and deflate
  • #1262: Fix bug in Task.runSyncUnsafe related to ContextSwitch
  • #1265: Implement Observable#bufferWhile and bufferWhileInclusive
  • #1267: Implement Asynchronous Stack Traces for Task
  • #1276: Add Task/Coeval convenience methods like .when
  • #1282: Add 'as' in Task and Coeval
  • #1284: Add left/right builders for Task and Coeval
  • #1286: Add none/some builders for Task and Coeval
  • #1291: tapEval, tapError methods at Task and Coeval
  • #1293: removing no-op onComplete() in Observable.takeByTimespan
  • #1299: Fix a bug in Local.value
  • #1307: Observable.fromIteratorBuffered

People who made this release possible

  • Adrian (@adrian-salajan)
  • Alexandru Nedelcu (@alexelcu)
  • ctoomey (@ctoomey)
  • Dmitro Pochapsky (@pchpsky)
  • Georgy Khotyan (@GKhotyan)
  • James Yoo (@jyoo980)
  • Kasper Kondzielski (@ghostbuster91)
  • Pau Alarcón (@paualarco)
  • Piotr Gawryś (@Avasil)
  • Sandeep Kota (@sandeepkota)
  • tafit3 (@tafit3)
  • Vladyslav (@VladPodilnyk)