Proposed Future API

Future.scala

/**
 * A future is a possibly asynchronous, functional computation producing a value of type `T`, or a failure
 * of type `E`.
 */
trait Future[+E, +T] {
  def swap: Future[T, E]

  def foreach[U] (f: T => U): Future[E, T]

  def getOrElse[T1 >: T] (default: => T1): Future[E, T1]

  def flatMap[E1, T1] (f: (T) => Future[E1, T1]): Future[E1, T1]

  def map[T1] (f: (T) => T1): Future[E, T1]

  def leftMap[E1] (f: E => E1): Future[E1, T]

  def filter[E1 >: E] (predicate: T => Boolean) (implicit identity: ZeroIdentity[E1]): Result[E1, T]

  /**
   * Evaluate this future to completion, passing the result to `f`.
   *
   * @param f The callback function to which the result will be supplied.
   */
  def subscribe[U] (f: (Result[E, T]) => U): Unit
}