gistfile1.scala

class NotifyingFuture[A] extends Future[A] {
  import NotifyingFuture._

  private var result: Option[A] = None
  private val computations = new ArrayBuffer[A => Unit]

  def setResult(result: A) {
    setResultIfEmpty(result) || {
      throw new ImmutableResult("Result set multiple times: " + result)
    }
  }

  private def isSet(onSet: A => B)(onEmpty: => B)(onWasSet: A => B)(onWasEmpty: => B) {
    result map(onSet) orElse {
      val wasSet = synchronized {
        result map(onSet) getOrElse(onEmpty)
      }
      if (wasSet) onWasSet else onWasEmpty
    }
  }

  def setResultIfEmpty(result: A) = {
    isSet {} {
      this.result = Some(result)
    } {
      false
    } {
      computations foreach(_(result))
      true
    }
  }

  def respond(k: A => Unit) {
    isSet {} {} {
      k(result)
    } {
      computations += k
    }
  }
}

gistfile2.scala

class NotifyingFuture[A] extends Future[A] {
  import NotifyingFuture._

  private var result: Option[A] = None
  private val computations = new ArrayBuffer[A => Unit]

  def setResult(result: A) {
    setResultIfEmpty(result) || {
      throw new ImmutableResult("Result set multiple times: " + result)
    }
  }

  def setResultIfEmpty(result: A) = {
    val didSetResult = synchronized {
      result map(false) getOrElse {
        this.result = Some(result)
        true
      }
    }
    if (didSetResult) computations foreach(_(result))
    didSetResult
  }

  def respond(k: A => Unit) {
    result map(k) orElse {
      val isDefined = synchronized {
        result.isDefined || { computations += k; false }
      }
      if (isDefined) k(result.get)
    }
  }
}