Why do I need to use andThen in order to pattern match Futures?

Question

I found out that in order to pattern match Future fur Success/Failure, I need to use andThen (or onComplete, onSuccess...) and cannot use map. Why is that?

What I wanted to do (simplified, I am matching for Success and so on as well):

val f1 = Future(throw new Exception("Oops"))
f1 map { case Failure(e) => ??? }

Gives:

error: constructor cannot be instantiated to expected type;
 found   : scala.util.Failure[T]
 required: Nothing
       f1 map { case Failure(e) => ??? }

What I ended up doing:

val f1 = Future(throw new Exception("Oops"))
f1 andThen { case Failure(e) => ??? }

I would like to understand why map cannot be used here.

Answer 1

The answer is in the signature of map: it takes a A => B and returns a Future[B]. If you will, you can look at a Future as follows:

type Future[A] = Async[Either[Throwable, A]]

Future#map, Future#flatMap and Future.apply view this "stack" of types as a single big thing with a hole (Future is basically a special cased monad transformer). When you map/flatMap on a Future, you are only operating on the inner A.