Map a Future for both Success and Failure

Question

I have a Future[T] and I want to map the result, on both success and failure.

Eg, something like

val future = ... // Future[T]
val mapped = future.mapAll { 
  case Success(a) => "OK"
  case Failure(e) => "KO"
}

If I use map or flatmap, it will only map successes futures. If I use recover, it will only map failed futures. onComplete executes a callback but does not return a modified future. Transform will work, but takes 2 functions rather than a partial function, so is a bit uglier.

I know I could make a new Promise, and complete that with onComplete or onSuccess/onFailure, but I was hoping there was something I was missing that would allow me to do the above with a single PF.

Answer 1

Edit 2017-09-18: As of Scala 2.12, there is a transform method that takes a Try[T] => Try[S]. So you can write

val future = ... // Future[T]
val mapped = future.transform {
  case Success(_) => Success("OK")
  case Failure(_) => Success("KO")
}

For 2.11.x, the below still applies:

AFAIK, you can't do this directly with a single PF. And transform transforms Throwable => Throwable, so that won't help you either. The closest you can get out of the box:

val mapped: Future[String] = future.map(_ => "OK").recover{case _ => "KO"}

That said, implementing your mapAll is trivial:

implicit class RichFuture[T](f: Future[T]) {
  def mapAll[U](pf: PartialFunction[Try[T], U]): Future[U] = {
    val p = Promise[U]()
    f.onComplete(r => p.complete(Try(pf(r))))
    p.future
  }
}