Why does Future.onSuccess require a partial function

Question

I am trying to chain together some basic functions using Futures returned from a slick action and I'm hitting some pretty trivial stumbling blocks.

Both the andThen and onSuccess methods require a PartialFunction passed as a parameter. My understanding is probably quite flawed but after reading about anonymous functions it seems like andThen needs to know your anonymous function with cater for any Success or Failure input.

Given onSuccess already only caters for the Success case why does it still need to be a PartialFunction?

This block of code my indicate the problem I am having:

val db = Database.forConfig("h2mem1")

try {
  val f = db.run(setupCommands)
    .onSuccess { println(_) }

  Await.ready(f, 10.seconds )
}
finally db.close

I get a compile error:

[error]  found   : Unit => Unit
[error]  required: PartialFunction[Unit,?]
[error]         .onSuccess { println(_) }

Answer 1

They did it so you can pattern match on the result, though I agree that it seems needless, I don't really use onSuccess and prefer to map and flatMap my futures:

  val f = Future.successful("test")

  f.onSuccess({
    case "test" => println("Worked")
    case x: String => println(s"Kind of worked: $x")
  })

In the case of more advanced data types I could see this being more useful:

  val fOpt = Future.successful(Option("Test"))

  fOpt.onSuccess({
    case Some(x) => println(x)
    case None => println("None")
  })

Really this is probably just coming from the actor api since when you ask an actor you don't know the return type, you need to pattern match on it since it's Any:

  val actor:ActorRef = ???

  val fAny = actor ? "asking"

  fAny.onSuccess({
    case x:String => println(s"Something String $x")
    case x:Int => println(s"Something Int $x")
    case x => println(s"Something else $x")
  })