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Scala example source code file (Task.scala)

This example Scala source code file (Task.scala) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Scala by Example" TM.

Learn more about this Scala project at its project page.

Java - Scala tags/keywords

atomicboolean, boolean, duration, executorservice, list, long, monaderror, nondeterminism, seq, some, task, threading, threads, throwable, unit

The Task.scala Scala example source code

package scalaz.concurrent

import java.util.concurrent.{ScheduledExecutorService, ConcurrentLinkedQueue, ExecutorService}
import java.util.concurrent.atomic.{AtomicBoolean, AtomicInteger}

import scalaz._
import scalaz.Tags.Parallel
import scalaz.syntax.monad._
import scalaz.std.list._
import scalaz.Free.Trampoline
import scalaz.\/._

import collection.JavaConversions._
import scala.concurrent.duration._

/**
 * `Task[A]` wraps a `scalaz.concurrent.Future[Throwable \/ A]`,
 * with some convenience functions for handling exceptions. Its
 * `Monad` and `Nondeterminism` instances are derived from [[Future]].
 *
 * `Task` (and `Future`) differ in several key ways from the `Future`
 * implementation in Scala 2.10 , and have a number of advantages. See the
 * documentation for [[scalaz.concurrent.Future]] for more information.
 *
 * `Task` is exception-safe when constructed using the primitives
 * in the companion object, but when calling the constructor, you
 * are responsible for ensuring the exception safety of the provided
 * `Future`.
 */
class Task[+A](val get: Future[Throwable \/ A]) {

  def flatMap[B](f: A => Task[B]): Task[B] =
    new Task(get flatMap {
      case -\/(e) => Future.now(-\/(e))
      case \/-(a) => Task.Try(f(a)) match {
        case e @ -\/(_) => Future.now(e)
        case \/-(task) => task.get
      }
    })

  def map[B](f: A => B): Task[B] =
    new Task(get map { _ flatMap {a => Task.Try(f(a))} })

  /** 'Catches' exceptions in the given task and returns them as values. */
  def attempt: Task[Throwable \/ A] =
    new Task(get map {
      case -\/(e) => \/-(-\/(e))
      case \/-(a) => \/-(\/-(a))
    })

  /**
   * Returns a new `Task` in which `f` is scheduled to be run on completion.
   * This would typically be used to release any resources acquired by this
   * `Task`.
   */
  def onFinish(f: Option[Throwable] => Task[Unit]): Task[A] =
    new Task(get flatMap {
      case -\/(e) =>
        Task.Try(f(Some(e))).fold(e2 => Future.now(-\/(e2)), _.get *> Future.now(-\/(e)))
      case r =>
        Task.Try(f(None)).fold(e => Future.now(-\/(e)), _.get *> Future.now(r))
    })

  /**
   * Calls `attempt` and handles some exceptions using the given partial
   * function, calling Task.now on the result. Any nonmatching exceptions
   * are reraised.
   */
  def handle[B>:A](f: PartialFunction[Throwable,B]): Task[B] =
    handleWith(f andThen Task.now)

  /**
   * Calls `attempt` and handles some exceptions using the given partial
   * function. Any nonmatching exceptions are reraised.
   */
  def handleWith[B>:A](f: PartialFunction[Throwable,Task[B]]): Task[B] =
    attempt flatMap {
      case -\/(e) => f.lift(e) getOrElse Task.fail(e)
      case \/-(a) => Task.now(a)
    }

  /**
   * Runs this `Task`, and if it fails with an exception, runs `t2`.
   * This is rather coarse-grained. Use `attempt`, `handle`, and
   * `flatMap` for more fine grained control of exception handling.
   */
  def or[B>:A](t2: Task[B]): Task[B] =
    new Task(this.get flatMap {
      case -\/(e) => t2.get
      case a => Future.now(a)
    })

  /**
   * Run this `Task` and block until its result is available. This will
   * throw any exceptions generated by the `Task`. To return exceptions
   * in an `\/`, use `unsafePerformSyncAttempt`.
   */
  def unsafePerformSync: A =
    get.unsafePerformSync match {
      case -\/(e) => throw e
      case \/-(a) => a
    }

  @deprecated("use unsafePerformSync", "7.2")
  def run: A =
    unsafePerformSync

  /** Like `unsafePerformSync`, but returns exceptions as values. */
  def unsafePerformSyncAttempt: Throwable \/ A =
    try get.unsafePerformSync catch { case t: Throwable => -\/(t) }

  @deprecated("use unsafePerformSyncAttempt", "7.2")
  def attemptRun: Throwable \/ A =
    unsafePerformSyncAttempt

  /**
   * Run this computation to obtain an `A`, so long as `cancel` remains false.
   * Because of trampolining, we get frequent opportunities to cancel
   * while stepping through the trampoline, this should provide a fairly
   * robust means of cancellation.
   */
  def unsafePerformAsyncInterruptibly(f: (Throwable \/ A) => Unit, cancel: AtomicBoolean): Unit =
    get.unsafePerformAsyncInterruptibly(f, cancel)

  @deprecated("use unsafePerformAsyncInterruptibly", "7.2")
  def runAsyncInterruptibly(f: (Throwable \/ A) => Unit, cancel: AtomicBoolean): Unit =
    unsafePerformAsyncInterruptibly(f, cancel)

  /**
   * Similar to `unsafePerformAsyncInterruptibly(f,cancel)` except instead of interrupting by setting cancel to true,
   * It returns the function, that, when applied will interrupt the task.
   *
   * This allows "deterministic" completion of task computation
   * even if it was interrupted.
   * That means task will complete even when interrupted,
   * but with `TaskInterrupted` exception.
   *
   * Note 1: When Interrupted, the `f` callback will run in thread that called the `Interrupting` function () => Unit
   * Note 2: If task has handler like attempt, it won't get consulted for handling TaskInterrupted excpetion
   * @param f
   * @return
   */
  def unsafePerformAsyncInterruptibly(f: (Throwable \/ A) => Unit) : () => Unit = {
    val completed : AtomicBoolean = new AtomicBoolean(false)
    val a = Actor[Option[Throwable \/ A]] ({
      case Some(r) if ! completed.get =>
        completed.set(true)
        f(r)
      case None if ! completed.get  =>
        completed.set(true)
        f(left(Task.TaskInterrupted))
      case _ => () //already completed
    })(Strategy.Sequential)

    get.unsafePerformAsyncInterruptibly(r => a ! Some(r), completed)
    () => { a ! None }
  }

  @deprecated("use unsafePerformAsyncInterruptibly", "7.2")
  def runAsyncInterruptibly(f: (Throwable \/ A) => Unit) : () => Unit =
    unsafePerformAsyncInterruptibly(f)

  /**
   * Run this computation to obtain either a result or an exception, then
   * invoke the given callback. Any pure, non-asynchronous computation at the
   * head of this `Task` will be forced in the calling thread. At the first
   * `Async` encountered, control to whatever thread backs the `Async` and
   * this function returns immediately.
   */
  def unsafePerformAsync(f: (Throwable \/ A) => Unit): Unit =
    get.unsafePerformAsync(f)

  @deprecated("use unsafePerformAsync", "7.2")
  def runAsync(f: (Throwable \/ A) => Unit): Unit =
    unsafePerformAsync(f)

  /**
   * Run this `Task` and block until its result is available, or until
   * `timeoutInMillis` milliseconds have elapsed, at which point a `TimeoutException`
   * will be thrown and the `Task` will attempt to be canceled.
   */
  def unsafePerformSyncFor(timeoutInMillis: Long): A =
    get.unsafePerformSyncFor(timeoutInMillis) match {
      case -\/(e) => throw e
      case \/-(a) => a
    }

  def unsafePerformSyncFor(timeout: Duration): A =
    unsafePerformSyncFor(timeout.toMillis)

  @deprecated("use unsafePerformSyncFor", "7.2")
  def runFor(timeoutInMillis: Long): A =
    unsafePerformSyncFor(timeoutInMillis)

  @deprecated("use unsafePerformSyncFor", "7.2")
  def runFor(timeout: Duration): A =
    unsafePerformSyncFor(timeout)

  /**
   * Like `unsafePerformSyncFor`, but returns exceptions as values. Both `TimeoutException`
   * and other exceptions will be folded into the same `Throwable`.
   */
  def unsafePerformSyncAttemptFor(timeoutInMillis: Long): Throwable \/ A =
    get.unsafePerformSyncAttemptFor(timeoutInMillis).join

  @deprecated("use unsafePerformSyncAttemptFor", "7.2")
  def attemptRunFor(timeoutInMillis: Long): Throwable \/ A =
    unsafePerformSyncAttemptFor(timeoutInMillis)

  def unsafePerformSyncAttemptFor(timeout: Duration): Throwable \/ A =
    unsafePerformSyncAttemptFor(timeout.toMillis)

  @deprecated("use unsafePerformSyncAttemptFor", "7.2")
  def attemptRunFor(timeout: Duration): Throwable \/ A =
    unsafePerformSyncAttemptFor(timeout)

  /**
   * A `Task` which returns a `TimeoutException` after `timeoutInMillis`,
   * and attempts to cancel the running computation.
   */
  def timed(timeoutInMillis: Long)(implicit scheduler:ScheduledExecutorService): Task[A] =
    new Task(get.timed(timeoutInMillis).map(_.join))

  def timed(timeout: Duration)(implicit scheduler:ScheduledExecutorService = Strategy.DefaultTimeoutScheduler): Task[A] =
    timed(timeout.toMillis)

  @deprecated("use timed", "7.2")
  def unsafePerformTimed(timeout: Duration)(implicit scheduler:ScheduledExecutorService = Strategy.DefaultTimeoutScheduler): Task[A] =
    timed(timeout)

  @deprecated("use timed", "7.2")
  def unsafePerformTimed(timeoutInMillis: Long)(implicit scheduler:ScheduledExecutorService): Task[A] =
    timed(timeoutInMillis)

  /**
   * Retries this task if it fails, once for each element in `delays`,
   * each retry delayed by the corresponding duration, accumulating
   * errors into a list.
   * A retriable failure is one for which the predicate `p` returns `true`.
   */
  def retryAccumulating(delays: Seq[Duration], p: (Throwable => Boolean) = _.isInstanceOf[Exception]): Task[(A, List[Throwable])] =
    retryInternal(delays, p, true)

  @deprecated("use retryAccumulating", "7.2")
  def unsafePerformRetryAccumulating(delays: Seq[Duration], p: (Throwable => Boolean) = _.isInstanceOf[Exception]): Task[(A, List[Throwable])] =
    retryAccumulating(delays, p)

  /**
   * Retries this task if it fails, once for each element in `delays`,
   * each retry delayed by the corresponding duration.
   * A retriable failure is one for which the predicate `p` returns `true`.
   */
  def retry(delays: Seq[Duration], p: (Throwable => Boolean) = _.isInstanceOf[Exception]): Task[A] =
    retryInternal(delays, p, false).map(_._1)

  @deprecated("use retry", "7.2")
  def unsafePerformRetry(delays: Seq[Duration], p: (Throwable => Boolean) = _.isInstanceOf[Exception]): Task[A] =
    retry(delays, p)

  private def retryInternal(delays: Seq[Duration],
                            p: (Throwable => Boolean),
                            accumulateErrors: Boolean): Task[(A, List[Throwable])] = {
      def help(ds: Seq[Duration], es: => Stream[Throwable]): Future[Throwable \/ (A, List[Throwable])] = {
        def acc = if (accumulateErrors) es.toList else Nil
          ds match {
            case Seq() => get map (_. map(_ -> acc))
            case Seq(t, ts @_*) => get flatMap {
              case -\/(e) if p(e) =>
                help(ts, e #:: es) after t
              case x => Future.now(x.map(_ -> acc))
            }
        }
      }
      Task.async { help(delays, Stream()).unsafePerformAsync }
    }

  /** Ensures that the result of this Task satisfies the given predicate, or fails with the given value. */
  def ensure(failure: => Throwable)(f: A => Boolean): Task[A] =
    flatMap(a => if(f(a)) Task.now(a) else Task.fail(failure))

  /**
   * Delays the execution of this `Task` by the duration `t`.
   */
  def after(t: Duration): Task[A] =
    new Task(get after t)
}

object Task {

  implicit val taskInstance: Nondeterminism[Task] with BindRec[Task] with Catchable[Task] with MonadError[Task,Throwable] =
    new Nondeterminism[Task] with BindRec[Task] with Catchable[Task] with MonadError[Task, Throwable] {
      val F = Nondeterminism[Future]
      def point[A](a: => A) = Task.point(a)
      def bind[A,B](a: Task[A])(f: A => Task[B]): Task[B] =
        a flatMap f
      def chooseAny[A](h: Task[A], t: Seq[Task[A]]): Task[(A, Seq[Task[A]])] =
        new Task ( F.map(F.chooseAny(h.get, t map (_ get))) { case (a, residuals) =>
          a.map((_, residuals.map(new Task(_))))
        })
      override def gatherUnordered[A](fs: Seq[Task[A]]): Task[List[A]] = {
        new Task (F.map(F.gatherUnordered(fs.map(_ get)))(eithers =>
          Traverse[List].sequenceU(eithers)
        ))
      }
      def fail[A](e: Throwable): Task[A] = new Task(Future.now(-\/(e)))
      def attempt[A](a: Task[A]): Task[Throwable \/ A] = a.attempt
      def tailrecM[A, B](f: A => Task[A \/ B])(a: A): Task[B] = Task.tailrecM(f)(a)
      def raiseError[A](e: Throwable): Task[A] = fail(e)
      def handleError[A](fa: Task[A])(f: Throwable => Task[A]): Task[A] =
        fa.handleWith { case t => f(t) }
    }

  /** signals task was interrupted **/
  case object TaskInterrupted extends InterruptedException {
    override def fillInStackTrace = this
  }

  def point[A](a: => A) = new Task(Future.delay(Try(a)))

  /** A `Task` which fails with the given `Throwable`. */
  def fail(e: Throwable): Task[Nothing] = new Task(Future.now(-\/(e)))

  /** Convert a strict value to a `Task`. Also see `delay`. */
  def now[A](a: A): Task[A] = new Task(Future.now(\/-(a)))

  /**
   * Promote a non-strict value to a `Task`, catching exceptions in
   * the process. Note that since `Task` is unmemoized, this will
   * recompute `a` each time it is sequenced into a larger computation.
   * Memoize `a` with a lazy value before calling this function if
   * memoization is desired.
   */
  def delay[A](a: => A): Task[A] = suspend(now(a))

  /**
   * Produce `f` in the main trampolining loop, `Future.step`, using a fresh
   * call stack. The standard trampolining primitive, useful for avoiding
   * stack overflows.
   */
  def suspend[A](a: => Task[A]): Task[A] = new Task(Future.suspend(
    Try(a.get) match {
      case -\/(e) => Future.now(-\/(e))
      case \/-(f) => f
  }))

  /** Create a `Task` that will evaluate `a` using the given `ExecutorService`. */
  def apply[A](a: => A)(implicit pool: ExecutorService = Strategy.DefaultExecutorService): Task[A] =
    new Task(Future(Try(a))(pool))

  /**
   * Create a `Task` that starts evaluating `a` using the given `ExecutorService` right away.
   * This will start executing side effects immediately, and is thus morally equivalent to
   * `unsafePerformIO`. The resulting `Task` cannot be rerun to repeat the effects.
   * Use with care.
   */
  def unsafeStart[A](a: => A)(implicit pool: ExecutorService = Strategy.DefaultExecutorService): Task[A] =
    new Task(Future(Task.Try(a))(pool).unsafeStart)

  /**
   * Returns a `Task` that produces the same result as the given `Future`,
   * but forks its evaluation off into a separate (logical) thread, using
   * the given `ExecutorService`. Note that this forking is only described
   * by the returned `Task`--nothing occurs until the `Task` is run.
   */
  def fork[A](a: => Task[A])(implicit pool: ExecutorService = Strategy.DefaultExecutorService): Task[A] =
    apply(a).join


  /**
   * Create a `Task` from an asynchronous computation, which takes the form
   * of a function with which we can register a callback. This can be used
   * to translate from a callback-based API to a straightforward monadic
   * version.
   */
  def async[A](register: ((Throwable \/ A) => Unit) => Unit): Task[A] =
    new Task(Future.async(register))

  def schedule[A](a: => A, delay: Duration)(implicit pool: ScheduledExecutorService =
    Strategy.DefaultTimeoutScheduler): Task[A] = new Task(Future.schedule(Try(a), delay))

  /**
   * Like `Nondeterminism[Task].gatherUnordered`, but if `exceptionCancels` is true,
   * exceptions in any task try to immediately cancel all other running tasks. If
   * `exceptionCancels` is false, in the event of an error, all tasks are run to completion
   * before the error is returned.
   * @since 7.0.3
   */
  def gatherUnordered[A](tasks: Seq[Task[A]], exceptionCancels: Boolean = false): Task[List[A]] =
    reduceUnordered[A, List[A]](tasks, exceptionCancels)

  def reduceUnordered[A, M](tasks: Seq[Task[A]], exceptionCancels: Boolean = false)(implicit R: Reducer[A, M]): Task[M] =
    if (!exceptionCancels) taskInstance.reduceUnordered(tasks)
    else tasks match {
      // Unfortunately we cannot reuse the future's combinator
      // due to early terminating requirement on task
      // when task fails.  This also makes implementation a bit trickier
      case Seq() => Task.now(R.zero)
      case Seq(t) => t.map(R.unit)
      case _ => new Task(Future.Async { cb =>
        val interrupt = new AtomicBoolean(false)
        val results = new ConcurrentLinkedQueue[M]
        val togo = new AtomicInteger(tasks.size)

        tasks.foreach { t =>
          val handle: (Throwable \/ A) => Trampoline[Unit] = {
            case \/-(success) =>
              // Try to reduce number of values in the queue
              val front = results.poll()
              if (front == null)
                results.add(R.unit(success))
              else
                results.add(R.cons(success, front))

              // only last completed f will hit the 0 here.
              if (togo.decrementAndGet() == 0)
                cb(\/-(results.toList.foldLeft(R.zero)((a, b) => R.append(a, b))))
              else
                Trampoline.done(())
            case e@(-\/(failure)) =>
              // Only allow the first failure to invoke the callback, so we
              // race to set `togo` to 0 here.
              // If we win, invoke the callback with our error, otherwise, noop
              @annotation.tailrec
              def firstFailure: Boolean = {
                val current = togo.get
                if (current > 0) {
                  if (togo.compareAndSet(current,0)) true
                  else firstFailure
                }
                else false
              }

              if (firstFailure) // invoke `cb`, then cancel any computation not running yet
                // food for thought - might be safe to set the interrupt first
                // but, this may also kill `cb(e)`
                // could have separate AtomicBooleans for each task
                cb(e) *> Trampoline.delay { interrupt.set(true); () }
              else
                Trampoline.done(())
          }
          t.get.unsafePerformListenInterruptibly(handle, interrupt)
        }
      })
    }

  /** Utility function - evaluate `a` and catch and return any exceptions. */
  def Try[A](a: => A): Throwable \/ A =
    try \/-(a) catch { case e: Throwable => -\/(e) }

  def fromMaybe[A](ma: Maybe[A])(t: => Throwable): Task[A] =
    ma.cata(Task.now, Task.fail(t))

  def fromDisjunction[A <: Throwable, B](x: A \/ B): Task[B] =
    x.fold(Task.fail, Task.now)

  def tailrecM[A, B](f: A => Task[A \/ B])(a: A): Task[B] =
    f(a).flatMap {
      case -\/(a0) => tailrecM(f)(a0)
      case \/-(b) => point(b)
    }

  /** type for Tasks which need to be executed in parallel when using an Applicative instance */
  type ParallelTask[A] = Task[A] @@ Parallel

  /** This Applicative instance runs Tasks in parallel.
   *
   * It is different from the Applicative instance obtained from Monad[Task] which runs tasks sequentially.
   */
  implicit val taskParallelApplicativeInstance: Applicative[ParallelTask] =
    taskInstance.parallel
}

Other Scala examples (source code examples)

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