Map a Future for both Success and Failure

Since Scala 2.12 you can use transform to map both cases:

future.transform {
      case Success(_) => Try("OK")
      case Failure(_) => Try("KO")
}

You also have transformWith if you prefer to use a Future instead of a Try. Check the documentation for details.

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
  }
}

Both map and flatMap variants:

implicit class FutureExtensions[T](f: Future[T]) {
  def mapAll[Target](m: Try[T] => Target)(implicit ec: ExecutionContext): Future[Target] = {
    val promise = Promise[Target]()
    f.onComplete { r => promise success m(r) }(ec)
    promise.future
  }

  def flatMapAll[Target](m: Try[T] => Future[Target])(implicit ec: ExecutionContext): Future[Target] = {
    val promise = Promise[Target]()
    f.onComplete { r => m(r).onComplete { z => promise complete z }(ec) }(ec)
    promise.future
  }
}