[writing-async-app-in-scala-part-3] map described

MapDescribed.scala

def map[U](f: T => U)
          (implicit ec: ExecutionContext): Future[U] = {
  val promise = Promise()
  if (isCompleted) // in real code this is an atomic operation + pattern matching
    // even if it's completed, callback is submitted to ExecutionContext
    ec.execute(() => {
      f(result)
    })
  else
    registerCallback(f)(ec)
  promise.future
}