path: root/src/library/scala/concurrent/Future.scala

/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2003-2011, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

package scala.concurrent



import java.util.concurrent.{ ConcurrentLinkedQueue, TimeUnit, Callable }
import java.util.concurrent.TimeUnit.{ NANOSECONDS => NANOS, MILLISECONDS ⇒ MILLIS }
import java.lang.{ Iterable => JIterable }
import java.util.{ LinkedList => JLinkedList }
import java.{ lang => jl }
import java.util.concurrent.atomic.{ AtomicReferenceFieldUpdater, AtomicInteger, AtomicBoolean }

import scala.util.{ Try, Success, Failure }
import scala.concurrent.util.Duration
import scala.Option

import scala.annotation.tailrec
import scala.collection.mutable.Stack
import scala.collection.mutable.Builder
import scala.collection.generic.CanBuildFrom



/** The trait that represents futures.
 *
 *  Asynchronous computations that yield futures are created with the `future` call:
 *
 *  {{{
 *  val s = "Hello"
 *  val f: Future[String] = future {
 *    s + " future!"
 *  }
 *  f onSuccess {
 *    case msg => println(msg)
 *  }
 *  }}}
 *
 *  @author  Philipp Haller, Heather Miller, Aleksandar Prokopec, Viktor Klang
 *
 *  @define multipleCallbacks
 *  Multiple callbacks may be registered; there is no guarantee that they will be
 *  executed in a particular order.
 *
 *  @define caughtThrowables
 *  The future may contain a throwable object and this means that the future failed.
 *  Futures obtained through combinators have the same exception as the future they were obtained from.
 *  The following throwable objects are not contained in the future:
 *  - `Error` - errors are not contained within futures
 *  - `InterruptedException` - not contained within futures
 *  - all `scala.util.control.ControlThrowable` except `NonLocalReturnControl` - not contained within futures
 *
 *  Instead, the future is completed with a ExecutionException with one of the exceptions above
 *  as the cause.
 *  If a future is failed with a `scala.runtime.NonLocalReturnControl`,
 *  it is completed with a value instead from that throwable instead instead.
 *
 *  @define nonDeterministic
 *  Note: using this method yields nondeterministic dataflow programs.
 *
 *  @define forComprehensionExamples
 *  Example:
 *
 *  {{{
 *  val f = future { 5 }
 *  val g = future { 3 }
 *  val h = for {
 *    x: Int <- f // returns Future(5)
 *    y: Int <- g // returns Future(5)
 *  } yield x + y
 *  }}}
 *
 *  is translated to:
 *
 *  {{{
 *  f flatMap { (x: Int) => g map { (y: Int) => x + y } }
 *  }}}
 */
trait Future[+T] extends Awaitable[T] {
self =>

  /* Callbacks */

  /** When this future is completed successfully (i.e. with a value),
   *  apply the provided partial function to the value if the partial function
   *  is defined at that value.
   *
   *  If the future has already been completed with a value,
   *  this will either be applied immediately or be scheduled asynchronously.
   *
   *  $multipleCallbacks
   */
  def onSuccess[U](pf: PartialFunction[T, U]): this.type = onComplete {
    case Failure(t) => // do nothing
    case Success(v) => if (pf isDefinedAt v) pf(v) else { /*do nothing*/ }
  }

  /** When this future is completed with a failure (i.e. with a throwable),
   *  apply the provided callback to the throwable.
   *
   *  $caughtThrowables
   *
   *  If the future has already been completed with a failure,
   *  this will either be applied immediately or be scheduled asynchronously.
   *
   *  Will not be called in case that the future is completed with a value.
   *
   *  $multipleCallbacks
   */
  def onFailure[U](callback: PartialFunction[Throwable, U]): this.type = onComplete {
    case Failure(t) => if (isFutureThrowable(t) && callback.isDefinedAt(t)) callback(t) else { /*do nothing*/ }
    case Success(v) => // do nothing
  }

  /** When this future is completed, either through an exception, a timeout, or a value,
   *  apply the provided function.
   *
   *  If the future has already been completed,
   *  this will either be applied immediately or be scheduled asynchronously.
   *
   *  $multipleCallbacks
   */
  def onComplete[U](func: Try[T] => U): this.type


  /* Miscellaneous */

  /** Creates a new promise.
   */
  def newPromise[S]: Promise[S]


  /* Projections */

  /** Returns a failed projection of this future.
   *
   *  The failed projection is a future holding a value of type `Throwable`.
   *
   *  It is completed with a value which is the throwable of the original future
   *  in case the original future is failed.
   *
   *  It is failed with a `NoSuchElementException` if the original future is completed successfully.
   *
   *  Blocking on this future returns a value if the original future is completed with an exception
   *  and throws a corresponding exception if the original future fails.
   */
  def failed: Future[Throwable] = {
    def noSuchElem(v: T) =
      new NoSuchElementException("Future.failed not completed with a throwable. Instead completed with: " + v)

    val p = newPromise[Throwable]

    onComplete {
      case Failure(t) => p success t
      case Success(v) => p failure noSuchElem(v)
    }

    p.future
  }


  /* Monadic operations */

  /** Asynchronously processes the value in the future once the value becomes available.
   *
   *  Will not be called if the future fails.
   */
  def foreach[U](f: T => U): Unit = onComplete {
    case Success(r) => f(r)
    case Failure(_)  => // do nothing
  }

  /** Creates a new future by applying a function to the successful result of
   *  this future. If this future is completed with an exception then the new
   *  future will also contain this exception.
   *
   *  $forComprehensionExample
   */
  def map[S](f: T => S): Future[S] = {
    val p = newPromise[S]

    onComplete {
      case Failure(t) => p failure t
      case Success(v) =>
        try p success f(v)
        catch {
          case t => p complete resolver(t)
        }
    }

    p.future
  }

  /** Creates a new future by applying a function to the successful result of
   *  this future, and returns the result of the function as the new future.
   *  If this future is completed with an exception then the new future will
   *  also contain this exception.
   *
   *  $forComprehensionExample
   */
  def flatMap[S](f: T => Future[S]): Future[S] = {
    val p = newPromise[S]

    onComplete {
      case Failure(t) => p failure t
      case Success(v) =>
        try {
          f(v) onComplete {
            case Failure(t) => p failure t
            case Success(v) => p success v
          }
        } catch {
          case t: Throwable => p complete resolver(t)
        }
    }

    p.future
  }

  /** Creates a new future by filtering the value of the current future with a predicate.
   *
   *  If the current future contains a value which satisfies the predicate, the new future will also hold that value.
   *  Otherwise, the resulting future will fail with a `NoSuchElementException`.
   *
   *  If the current future fails or times out, the resulting future also fails or times out, respectively.
   *
   *  Example:
   *  {{{
   *  val f = future { 5 }
   *  val g = f filter { _ % 2 == 1 }
   *  val h = f filter { _ % 2 == 0 }
   *  await(g, 0) // evaluates to 5
   *  await(h, 0) // throw a NoSuchElementException
   *  }}}
   */
  def filter(pred: T => Boolean): Future[T] = {
    val p = newPromise[T]

    onComplete {
      case Failure(t) => p failure t
      case Success(v) =>
        try {
          if (pred(v)) p success v
          else p failure new NoSuchElementException("Future.filter predicate is not satisfied by: " + v)
        } catch {
          case t: Throwable => p complete resolver(t)
        }
    }

    p.future
  }

  /** Creates a new future by mapping the value of the current future if the given partial function is defined at that value.
   *
   *  If the current future contains a value for which the partial function is defined, the new future will also hold that value.
   *  Otherwise, the resulting future will fail with a `NoSuchElementException`.
   *
   *  If the current future fails or times out, the resulting future also fails or times out, respectively.
   *
   *  Example:
   *  {{{
   *  val f = future { -5 }
   *  val g = f collect {
   *    case x if x < 0 => -x
   *  }
   *  val h = f collect {
   *    case x if x > 0 => x * 2
   *  }
   *  await(g, 0) // evaluates to 5
   *  await(h, 0) // throw a NoSuchElementException
   *  }}}
   */
  def collect[S](pf: PartialFunction[T, S]): Future[S] = {
    val p = newPromise[S]

    onComplete {
      case Failure(t) => p failure t
      case Success(v) =>
        try {
          if (pf.isDefinedAt(v)) p success pf(v)
          else p failure new NoSuchElementException("Future.collect partial function is not defined at: " + v)
        } catch {
          case t: Throwable => p complete resolver(t)
        }
    }

    p.future
  }

  /** Creates a new future that will handle any matching throwable that this
   *  future might contain. If there is no match, or if this future contains
   *  a valid result then the new future will contain the same.
   *
   *  Example:
   *
   *  {{{
   *  future (6 / 0) recover { case e: ArithmeticException ⇒ 0 } // result: 0
   *  future (6 / 0) recover { case e: NotFoundException   ⇒ 0 } // result: exception
   *  future (6 / 2) recover { case e: ArithmeticException ⇒ 0 } // result: 3
   *  }}}
   */
  def recover[U >: T](pf: PartialFunction[Throwable, U]): Future[U] = {
    val p = newPromise[U]

    onComplete {
      case Failure(t) if pf isDefinedAt t =>
        try { p success pf(t) }
        catch { case t: Throwable => p complete resolver(t) }
      case otherwise => p complete otherwise
    }

    p.future
  }

  /** Creates a new future that will handle any matching throwable that this
   *  future might contain by assigning it a value of another future.
   *
   *  If there is no match, or if this future contains
   *  a valid result then the new future will contain the same result.
   *
   *  Example:
   *
   *  {{{
   *  val f = future { Int.MaxValue }
   *  future (6 / 0) recoverWith { case e: ArithmeticException => f } // result: Int.MaxValue
   *  }}}
   */
  def recoverWith[U >: T](pf: PartialFunction[Throwable, Future[U]]): Future[U] = {
    val p = newPromise[U]

    onComplete {
      case Failure(t) if pf isDefinedAt t =>
        try {
          p completeWith pf(t)
        } catch {
          case t: Throwable => p complete resolver(t)
        }
      case otherwise => p complete otherwise
    }

    p.future
  }

  /** Zips the values of `this` and `that` future, and creates
   *  a new future holding the tuple of their results.
   *
   *  If `this` future fails, the resulting future is failed
   *  with the throwable stored in `this`.
   *  Otherwise, if `that` future fails, the resulting future is failed
   *  with the throwable stored in `that`.
   */
  def zip[U](that: Future[U]): Future[(T, U)] = {
    val p = newPromise[(T, U)]

    this onComplete {
      case Failure(t)  => p failure t
      case Success(r) => that onSuccess {
        case r2 => p success ((r, r2))
      }
    }

    that onFailure {
      case f => p failure f
    }

    p.future
  }

  /** Creates a new future which holds the result of this future if it was completed successfully, or, if not,
   *  the result of the `that` future if `that` is completed successfully.
   *  If both futures are failed, the resulting future holds the throwable object of the first future.
   *
   *  Using this method will not cause concurrent programs to become nondeterministic.
   *
   *  Example:
   *  {{{
   *  val f = future { sys.error("failed") }
   *  val g = future { 5 }
   *  val h = f orElse g
   *  await(h, 0) // evaluates to 5
   *  }}}
   */
  def fallbackTo[U >: T](that: Future[U]): Future[U] = {
    val p = newPromise[U]

    onComplete {
      case Failure(t) => that onComplete {
        case Failure(_) => p failure t
        case Success(v) => p success v
      }
      case Success(v) => p success v
    }

    p.future
  }

  /** Applies the side-effecting function to the result of this future, and returns
   *  a new future with the result of this future.
   *
   *  This method allows one to enforce that the callbacks are executed in a
   *  specified order.
   *
   *  Note that if one of the chained `andThen` callbacks throws
   *  an exception, that exception is not propagated to the subsequent `andThen`
   *  callbacks. Instead, the subsequent `andThen` callbacks are given the original
   *  value of this future.
   *
   *  The following example prints out `5`:
   *
   *  {{{
   *  val f = future { 5 }
   *  f andThen {
   *    case r => sys.error("runtime exception")
   *  } andThen {
   *    case Failure(t) => println(t)
   *    case Success(v) => println(v)
   *  }
   *  }}}
   */
  def andThen[U](pf: PartialFunction[Try[T], U]): Future[T] = {
    val p = newPromise[T]

    onComplete {
      case r =>
        try if (pf isDefinedAt r) pf(r)
        finally p complete r
    }

    p.future
  }

  /** Creates a new future which holds the result of either this future or `that` future, depending on
   *  which future was completed first.
   *
   *  $nonDeterministic
   *
   *  Example:
   *  {{{
   *  val f = future { sys.error("failed") }
   *  val g = future { 5 }
   *  val h = f either g
   *  await(h, 0) // evaluates to either 5 or throws a runtime exception
   *  }}}
   */
  def either[U >: T](that: Future[U]): Future[U] = {
    val p = self.newPromise[U]

    val completePromise: PartialFunction[Try[U], _] = {
      case Failure(t) => p tryFailure t
      case Success(v) => p trySuccess v
    }

    self onComplete completePromise
    that onComplete completePromise

    p.future
  }

}



/** Future companion object.
 *
 *  @define nonDeterministic
 *  Note: using this method yields nondeterministic dataflow programs.
 */
object Future {
  
  def apply[T](body: =>T)(implicit executor: ExecutionContext): Future[T] = impl.Future(body)
  
}