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Standard library for the Scala Programming Language
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/*                     __                                               *\
**     ________ ___   / /  ___     Scala API                            **
**    / __/ __// _ | / /  / _ |    (c) 2003-2013, LAMP/EPFL             **
**  __\ \/ /__/ __ |/ /__/ __ |    http://scala-lang.org/               **
** /____/\___/_/ |_/____/_/ | |                                         **
**                          |/                                          **
\*                                                                      */

package scala.concurrent

import scala.util.{ Try, Success, Failure }

/** Promise is an object which can be completed with a value or failed
 *  with an exception.
 *
 *  @define promiseCompletion
 *  If the promise has already been fulfilled, failed or has timed out,
 *  calling this method will throw an IllegalStateException.
 *
 *  @define allowedThrowables
 *  If the throwable used to fail this promise is an error, a control exception
 *  or an interrupted exception, it will be wrapped as a cause within an
 *  `ExecutionException` which will fail the promise.
 *
 *  @define nonDeterministic
 *  Note: Using this method may result in non-deterministic concurrent programs.
 */
trait Promise[T] {

  // used for internal callbacks defined in
  // the lexical scope of this trait;
  // _never_ for application callbacks.
  private implicit def internalExecutor: ExecutionContext = Future.InternalCallbackExecutor

  /** Future containing the value of this promise.
   */
  def future: Future[T]

  /** Returns whether the promise has already been completed with
   *  a value or an exception.
   *
   *  $nonDeterministic
   *
   *  @return    `true` if the promise is already completed, `false` otherwise
   */
  def isCompleted: Boolean

  /** Completes the promise with either an exception or a value.
   *
   *  @param result     Either the value or the exception to complete the promise with.
   *
   *  $promiseCompletion
   */
  def complete(result: Try[T]): this.type =
    if (tryComplete(result)) this else throw new IllegalStateException("Promise already completed.")

  /** Tries to complete the promise with either a value or the exception.
   *
   *  $nonDeterministic
   *
   *  @return    If the promise has already been completed returns `false`, or `true` otherwise.
   */
  def tryComplete(result: Try[T]): Boolean

  /** Completes this promise with the specified future, once that future is completed.
   *
   *  @return   This promise
   */
  final def completeWith(other: Future[T]): this.type = {
    other onComplete { this complete _ }
    this
  }
  
  /** Attempts to complete this promise with the specified future, once that future is completed.
   *
   *  @return   This promise
   */
  final def tryCompleteWith(other: Future[T]): this.type = {
    other onComplete { this tryComplete _ }
    this
  }

  /** Completes the promise with a value.
   *
   *  @param v    The value to complete the promise with.
   *
   *  $promiseCompletion
   */
  def success(v: T): this.type = complete(Success(v))

  /** Tries to complete the promise with a value.
   *
   *  $nonDeterministic
   *
   *  @return    If the promise has already been completed returns `false`, or `true` otherwise.
   */
  def trySuccess(value: T): Boolean = tryComplete(Success(value))

  /** Completes the promise with an exception.
   *
   *  @param t        The throwable to complete the promise with.
   *
   *  $allowedThrowables
   *
   *  $promiseCompletion
   */
  def failure(t: Throwable): this.type = complete(Failure(t))

  /** Tries to complete the promise with an exception.
   *
   *  $nonDeterministic
   *
   *  @return    If the promise has already been completed returns `false`, or `true` otherwise.
   */
  def tryFailure(t: Throwable): Boolean = tryComplete(Failure(t))
}



object Promise {

  /** Creates a promise object which can be completed with a value.
   *  
   *  @tparam T       the type of the value in the promise
   *  @return         the newly created `Promise` object
   */
  def apply[T](): Promise[T] = new impl.Promise.DefaultPromise[T]()

  /** Creates an already completed Promise with the specified exception.
   *  
   *  @tparam T       the type of the value in the promise
   *  @return         the newly created `Promise` object
   */
  def failed[T](exception: Throwable): Promise[T] = new impl.Promise.KeptPromise[T](Failure(exception))

  /** Creates an already completed Promise with the specified result.
   *  
   *  @tparam T       the type of the value in the promise
   *  @return         the newly created `Promise` object
   */
  def successful[T](result: T): Promise[T] = new impl.Promise.KeptPromise[T](Success(result))
  
}