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Scala example source code file (Future.scala)

This example Scala source code file (Future.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

async, atomicboolean, bindasync, bindsuspend, duration, future, long, now, seq, suspend, threading, threads, throwable, trampoline, unit

The Future.scala Scala example source code

package scalaz.concurrent

import java.util.concurrent.{Callable, ConcurrentLinkedQueue, ExecutorService, TimeoutException, ScheduledExecutorService, TimeUnit}
import java.util.concurrent.atomic.{AtomicInteger, AtomicBoolean, AtomicReference}

import collection.JavaConversions._
import scalaz.Tags.Parallel

import scalaz._
import scalaz.Free.Trampoline
import scalaz.syntax.monad._

import scala.concurrent.SyncVar
import scala.concurrent.duration._

/**
 * `Future` is a trampolined computation producing an `A` that may
 * include asynchronous steps. Like `Trampoline`, arbitrary
 * monadic expressions involving `map` and `flatMap` are guaranteed
 * to use constant stack space. But in addition, one may construct a
 * `Future` from an asynchronous computation, represented as a
 * function, `listen: (A => Unit) => Unit`, which registers a callback
 * that will be invoked when the result becomes available. This makes
 * `Future` useful as a concurrency primitive and as a control
 * structure for wrapping callback-based APIs with a more
 * straightforward, monadic API.
 *
 * Unlike the `Future` implementation in scala 2.10, `map` and
 * `flatMap` do NOT spawn new tasks and do not require an implicit
 * `ExecutionContext`. Instead, `map` and `flatMap` merely add to
 * the current (trampolined) continuation that will be run by the
 * 'current' thread, unless explicitly forked via `Future.fork` or
 * `Future.apply`. This means that `Future` achieves much better thread
 * reuse than the 2.10 implementation and avoids needless thread
 * pool submit cycles.
 *
 * `Future` also differs from the scala 2.10 `Future` type in that it
 * does not necessarily represent a _running_ computation. Instead, we
 * reintroduce nondeterminism _explicitly_ using the functions of the
 * `scalaz.Nondeterminism` interface. This simplifies our implementation
 * and makes code easier to reason about, since the order of effects
 * and the points of nondeterminism are made fully explicit and do not
 * depend on Scala's evaluation order.
 *
 * IMPORTANT NOTE: `Future` does not include any error handling and
 * should generally only be used as a building block by library
 * writers who want to build on `Future`'s capabilities but wish to
 * design their own error handling strategy. See
 * `scalaz.concurrent.Task` for a type that extends `Future` with
 * proper error handling -- it is merely a wrapper for
 * `Future[Throwable \/ A]` with a number of additional
 * convenience functions.
 */
sealed abstract class Future[+A] {
  import Future._

  def flatMap[B](f: A => Future[B]): Future[B] = this match {
    case Now(a) => Suspend(() => f(a))
    case Suspend(thunk) => BindSuspend(thunk, f)
    case Async(listen) => BindAsync(listen, f)
    case BindSuspend(thunk, g) =>
      Suspend(() => BindSuspend(thunk, g andThen (_ flatMap f)))
    case BindAsync(listen, g) =>
      Suspend(() => BindAsync(listen, g andThen (_ flatMap f)))
  }

  def map[B](f: A => B): Future[B] =
    flatMap(f andThen (b => Future.now(b)))

  /**
   * Run this computation to obtain an `A`, then invoke the given callback.
   * Also see `unsafePerformAsync`.
   */
  def unsafePerformListen(cb: A => Trampoline[Unit]): Unit =
    (this.step: @unchecked) match {
      case Now(a) => cb(a).run
      case Async(onFinish) => onFinish(cb)
      case BindAsync(onFinish, g) =>
        onFinish(x => Trampoline.delay(g(x)) map (_ unsafePerformListen cb))
    }


  @deprecated("use unsafePerformListen", "7.2")
  def listen(cb: A => Trampoline[Unit]): Unit =
    unsafePerformListen(cb)

  /**
   * 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, so this should provide a fairly
   * robust means of cancellation.
   */
  def unsafePerformListenInterruptibly(cb: A => Trampoline[Unit], cancel: AtomicBoolean): Unit =
    this.stepInterruptibly(cancel) match {
      case Now(a) if !cancel.get => cb(a).run
      case Async(onFinish) if !cancel.get =>
        onFinish(a =>
          if (!cancel.get) cb(a)
          else Trampoline.done(()))
      case BindAsync(onFinish, g) if !cancel.get =>
        onFinish(x =>
          if (!cancel.get) Trampoline.delay(g(x)) map (_ unsafePerformListenInterruptibly (cb, cancel))
          else Trampoline.done(()))
      case _ if cancel.get => ()
    }

  @deprecated("use unsafePerformListenInterruptibly", "7.2")
  def listenInterruptibly(cb: A => Trampoline[Unit], cancel: AtomicBoolean): Unit =
    unsafePerformListenInterruptibly(cb, cancel)

  /**
   * Evaluate this `Future` to a result, or another asynchronous computation.
   * This has the effect of stripping off any 'pure' trampolined computation at
   * the start of this `Future`.
   */
  @annotation.tailrec
  final def step: Future[A] = this match {
    case Suspend(thunk) => thunk().step
    case BindSuspend(thunk, f) => (thunk() flatMap f).step
    case _ => this
  }

  /** Like `step`, but may be interrupted by setting `cancel` to true. */
  @annotation.tailrec
  final def stepInterruptibly(cancel: AtomicBoolean): Future[A] =
    if (!cancel.get) this match {
      case Suspend(thunk) => thunk().stepInterruptibly(cancel)
      case BindSuspend(thunk, f) => (thunk() flatMap f).stepInterruptibly(cancel)
      case _ => this
    }
    else this

  /**
   * Begins running this `Future` and returns a new future that blocks
   * waiting for the result. Note that this will start executing side effects
   * immediately, and is thus morally equivalent to `unsafePerformIO`. The
   * resulting `Future` cannot be rerun to repeat the effects.
   *
   * Use with care.
   */
  def unsafeStart: Future[A] = {
    val latch = new java.util.concurrent.CountDownLatch(1)
    @volatile var result: Option[A] = None
    unsafePerformAsync { a => result = Some(a); latch.countDown }
    delay { latch.await; result.get }
  }

  @deprecated("use unsafeStart", "7.2")
  def start: Future[A] =
    unsafeStart

  /**
   * Run this `Future`, passing the result to the given callback once available.
   * Any pure, non-asynchronous computation at the head of this `Future` will
   * be forced in the calling thread. At the first `Async` encountered, control
   * switches to whatever thread backs the `Async` and this function returns.
   */
  def unsafePerformAsync(cb: A => Unit): Unit =
    unsafePerformListen(a => Trampoline.done(cb(a)))

  @deprecated("use unsafePerformAsync", "7.2")
  def runAsync(cb: A => Unit): Unit =
    unsafePerformAsync(cb)

  /**
   * 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(cb: A => Unit, cancel: AtomicBoolean): Unit =
    unsafePerformListenInterruptibly(a => Trampoline.done(cb(a)), cancel)

  @deprecated("use unsafePerformAsyncInterruptibly", "7.2")
  def runAsyncInterruptibly(cb: A => Unit, cancel: AtomicBoolean): Unit =
    unsafePerformAsyncInterruptibly(cb, cancel)

  /** Run this `Future` and block awaiting its result. */
  def unsafePerformSync: A = this match {
    case Now(a) => a
    case _ => {
      val latch = new java.util.concurrent.CountDownLatch(1)
      @volatile var result: Option[A] = None
      unsafePerformAsync { a => result = Some(a); latch.countDown }
      latch.await
      result.get
    }
  }

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

  /**
   * Run this `Future` and block until its result is available, or until
   * `timeoutInMillis` milliseconds have elapsed, at which point a `TimeoutException`
   * will be thrown and the `Future` will attempt to be canceled.
   */
  def unsafePerformSyncFor(timeoutInMillis: Long): A =
    unsafePerformSyncAttemptFor(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 `TimeoutException` as left value.
    * Will not report any other exceptions that may be raised during computation of `A`*/
  def unsafePerformSyncAttemptFor(timeoutInMillis: Long): Throwable \/ A = {
    val sync = new SyncVar[Throwable \/ A]
    val interrupt = new AtomicBoolean(false)
    unsafePerformAsyncInterruptibly(a => sync.put(\/-(a)), interrupt)
    sync.get(timeoutInMillis).getOrElse {
      interrupt.set(true)
      -\/(new TimeoutException(s"Timed out after $timeoutInMillis milliseconds"))
    }
  }

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

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

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


    /**
   * Returns a `Future` which returns a `TimeoutException` after `timeoutInMillis`,
   * and attempts to cancel the running computation.
   * This implementation will not block the future's execution thread
   */
  def timed(timeoutInMillis: Long)(implicit scheduler:ScheduledExecutorService): Future[Throwable \/ A] =
    //instead of run this though chooseAny, it is run through simple primitive,
    //as we are never interested in results of timeout callback, and this is more resource savvy
    async[Throwable \/ A] { cb =>
      val cancel = new AtomicBoolean(false)
      val done = new AtomicBoolean(false)
      scheduler.schedule(new Runnable {
        def run() {
          if (done.compareAndSet(false,true)) {
            cancel.set(true)
            cb(-\/(new TimeoutException(s"Timed out after $timeoutInMillis milliseconds")))
          }
        }
      }
      , timeoutInMillis, TimeUnit.MILLISECONDS)

      unsafePerformAsyncInterruptibly(a => if(done.compareAndSet(false,true)) cb(\/-(a)), cancel)
    }

  def timed(timeout: Duration)(implicit scheduler:ScheduledExecutorService = Strategy.DefaultTimeoutScheduler): Future[Throwable \/ A] =
    timed(timeout.toMillis)

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

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

   /**
   * Returns a `Future` that delays the execution of this `Future` by the duration `t`.
   */
  def after(t: Duration)(implicit scheduler:ScheduledExecutorService = Strategy.DefaultTimeoutScheduler): Future[A] =
    schedule((), t)(scheduler).flatMap(_ => this)

  def afterMillis(delay: Long)(implicit scheduler:ScheduledExecutorService = Strategy.DefaultTimeoutScheduler): Future[A] =
    after(FiniteDuration(delay, TimeUnit.MILLISECONDS))(scheduler)
}

object Future {
  case class Now[+A](a: A) extends Future[A]
  case class Async[+A](onFinish: (A => Trampoline[Unit]) => Unit) extends Future[A]
  case class Suspend[+A](thunk: () => Future[A]) extends Future[A]
  case class BindSuspend[A,B](thunk: () => Future[A], f: A => Future[B]) extends Future[B]
  case class BindAsync[A,B](onFinish: (A => Trampoline[Unit]) => Unit,
                            f: A => Future[B]) extends Future[B]

  // NB: considered implementing Traverse and Comonad, but these would have
  // to run the Future; leaving out for now

  implicit val futureInstance: Nondeterminism[Future] = new Nondeterminism[Future] {
    def bind[A,B](fa: Future[A])(f: A => Future[B]): Future[B] =
      fa flatMap f
    def point[A](a: => A): Future[A] = delay(a)

    def chooseAny[A](h: Future[A], t: Seq[Future[A]]): Future[(A, Seq[Future[A]])] = {
      Async { cb =>
        // The details of this implementation are a bit tricky, but the general
        // idea is to run all futures in parallel, returning whichever result
        // becomes available first.

        // To account for the fact that the losing computations are still
        // running, we construct special 'residual' Futures for the losers
        // that will first return from the already running computation,
        // then revert back to running the original Future.
        val won = new AtomicBoolean(false) // threads race to set this

        val fs = (h +: t).view.zipWithIndex.map { case (f, ind) =>
          val used = new AtomicBoolean(false)
          val ref = new AtomicReference[A]
          val listener = new AtomicReference[A => Trampoline[Unit]](null)
          val residual = Async { (cb: A => Trampoline[Unit]) =>
             if (used.compareAndSet(false, true)) { // get residual value from already running Future
               if (listener.compareAndSet(null, cb)) {} // we've successfully registered ourself with running task
               else cb(ref.get).run // the running task has completed, use its result
             }
             else // residual value used up, revert to original Future
               f.unsafePerformListen(cb)
          }
          (ind, f, residual, listener, ref)
        }.toIndexedSeq

        fs.foreach { case (ind, f, residual, listener, ref) =>
          f.unsafePerformListen { a =>
            ref.set(a)
            val notifyWinner =
              // If we're the first to finish, invoke `cb`, passing residuals
              if (won.compareAndSet(false, true))
                cb((a, fs.collect { case (i,_,rf,_,_) if i != ind => rf }))
              else {
                Trampoline.done(()) // noop; another thread will have already invoked `cb` w/ our residual
              }
            val notifyListener =
              if (listener.compareAndSet(null, finishedCallback))
                // noop; no listeners yet, any added after this will use result stored in `ref`
                Trampoline.done(())
              else // there is a registered listener, invoke it with the result
                listener.get.apply(a)
            notifyWinner *> notifyListener
          }
        }
      }
    }

    private[this] val finishedCallback: Any => Trampoline[Unit] =
      _ => sys.error("impossible, since there can only be one runner of chooseAny")

    // implementation runs all threads, dumping to a shared queue
    // last thread to finish invokes the callback with the results
    override def reduceUnordered[A, M](fs: Seq[Future[A]])(implicit R: Reducer[A, M]): Future[M] =
      fs match {
      case Seq() => Future.now(R.zero)
      case Seq(f) => f.map(R.unit)
      case other => Async { cb =>
        val results = new ConcurrentLinkedQueue[M]
        val c = new AtomicInteger(fs.size)

        fs.foreach { f =>
          f.unsafePerformListen { a =>
            // Try to reduce number of values in the queue
            val front = results.poll()
            if (front == null)
              results.add(R.unit(a))
            else
              results.add(R.cons(a, front))

            // only last completed f will hit the 0 here.
            if (c.decrementAndGet() == 0)
              cb(results.toList.foldLeft(R.zero)((a, b) => R.append(a, b)))
            else Trampoline.done(())
          }
        }
      }
    }
  }

  /** type for Futures which need to be executed in parallel when using an Applicative instance */
  type ParallelFuture[A] = Future[A] @@ Parallel

  /**
   * This Applicative instance runs Futures in parallel.
   *
   * It is different from the Applicative instance obtained from Monad[Future] which runs futures sequentially.
   */
  implicit val futureParallelApplicativeInstance: Applicative[ParallelFuture] =
    futureInstance.parallel

  /** Convert a strict value to a `Future`. */
  def now[A](a: A): Future[A] = Now(a)

  /**
   * Promote a non-strict value to a `Future`. Note that since `Future` 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): Future[A] = Suspend(() => Now(a))

  /**
   * Returns a `Future` 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 `Future`--nothing occurs until the `Future` is run.
   */
  def fork[A](a: => Future[A])(implicit pool: ExecutorService = Strategy.DefaultExecutorService): Future[A] =
    Future(a).join

  /**
   * 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](f: => Future[A]): Future[A] = Suspend(() => f)

  /**
   * Create a `Future` 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. See `Task.async` for a version that allows for asynchronous
   * exceptions.
   */
  def async[A](listen: (A => Unit) => Unit): Future[A] =
    Async((cb: A => Trampoline[Unit]) => listen { a => cb(a).run })

  /** Create a `Future` that will evaluate `a` using the given `ExecutorService`. */
  def apply[A](a: => A)(implicit pool: ExecutorService = Strategy.DefaultExecutorService): Future[A] = Async { cb =>
    pool.submit { new Callable[Unit] { def call = cb(a).run }}
  }

  /** Create a `Future` that will evaluate `a` after at least the given delay. */
  def schedule[A](a: => A, delay: Duration)(implicit pool: ScheduledExecutorService =
      Strategy.DefaultTimeoutScheduler): Future[A] =
    Async { cb =>
      pool.schedule(new Callable[Unit] {
        def call = cb(a).run
      }, delay.toMillis, TimeUnit.MILLISECONDS)
    }

  /** Calls `Nondeterminism[Future].gatherUnordered`.
   * @since 7.0.3
   */
  def gatherUnordered[A](fs: Seq[Future[A]]): Future[List[A]] =
    futureInstance.gatherUnordered(fs)

  def reduceUnordered[A, M](fs: Seq[Future[A]])(implicit R: Reducer[A, M]): Future[M] =
    futureInstance.reduceUnordered(fs)
}

Other Scala examples (source code examples)

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