A TaskLocal is like a ThreadLocal that is pure and with a flexible scope, being processed in the context of the Task data type.

This data type wraps monix.execution.misc.Local.

Just like a ThreadLocal, usage of a TaskLocal is safe, the state of all current locals being transported over async boundaries (aka when threads get forked) by the Task run-loop implementation, but only when the Task reference gets executed with Task.Options.localContextPropagation set to true, or it uses a monix.execution.schedulers.TracingScheduler.

One way to achieve this is with Task.executeWithOptions, a single call is sufficient just before runToFuture:

{
  import monix.execution.Scheduler.Implicits.global

  val t = Task(42)
  t.executeWithOptions(_.enableLocalContextPropagation)
    // triggers the actual execution
    .runToFuture
}

Another possibility is to use Task.runToFutureOpt instead of runToFuture and specify the set of options implicitly:

{
  import monix.execution.Scheduler.Implicits.global
  implicit val options: Task.Options = Task.defaultOptions.enableLocalContextPropagation

  val t = Task(42)
  // Options passed implicitly
  val f = t.runToFutureOpt
}

Full example:

import monix.eval.{Task, TaskLocal}

val task: Task[Unit] =
  for {
    local <- TaskLocal(0)
    value1 <- local.read // value1 == 0
    _ <- local.write(100)
    value2 <- local.read // value2 == 100
    value3 <- local.bind(200)(local.read.map(_ * 2)) // value3 == 200 * 2
    value4 <- local.read // value4 == 100
    _ <- local.clear
    value5 <- local.read // value5 == 0
  } yield {
    // Should print 0, 100, 400, 100, 0
    println("value1: " + value1)
    println("value2: " + value2)
    println("value3: " + value3)
    println("value4: " + value4)
    println("value5: " + value5)
  }

// For transporting locals over async boundaries defined by
// Task, any Scheduler will do, however for transporting locals
// over async boundaries managed by Future and others, you need
// a `TracingScheduler` here:
import monix.execution.Scheduler.Implicits.traced

// Triggering actual execution,
// runToFutureOpt is not needed if `TracingScheduler` is used
val result = task.runToFuture