How to return Unit from a scala function?

Question

I'm trying to make a function return Unit (this is to implement an RxScala observer), but when I add () to the end of it, I get an error "Application does not take parameters". Here's my code:

val client3MessageStreamObserver: Observable[Message] = client3.messageStream()
client3MessageStreamObserver.subscribe(
  m => println("Unexpected message received by client3"),
  // callback for handling exceptions
  t =>
    println("Ex client3: " + t)
// want to make this line work (which it doesn't) which is why 
// I need to be able to return Unit. 
//        client3TestPromise.success(true)
    ()     // error after adding Unit literal here.
)

Why do I get this error after adding () and how can I get rid of it? If I leave it out I get an error saying "Type mismatch: Expected (Throwable) => Unit, actual: (Throwable) => Any)".

Answer 1

Try this:

val client3MessageStreamObserver: Observable[Message] = client3.messageStream()
client3MessageStreamObserver.subscribe(
  m => println("Unexpected message received by client3"),
  t => println("Ex client3: " + t)
 () => ()
)

The third function onCompleted is a function Unit => Unit. So, the parameter is () and then in the return we can explicitly return () or any method returning a () such as println.