How do I get hold of exceptions thrown in a Scala Future?

Question

I've been working up my answer to Is there a standard Scala function for running a block with a timeout?, and have run into a problem if an exception is thrown in a Future.

  def runWithTimeout[T](timeoutMs: Long)(f: => T) : Option[T] = {
    awaitAll(timeoutMs, future(f)).head.asInstanceOf[Option[T]]
  }

So that

runWithTimeout(50) { "result" } should equal (Some("result"))
runWithTimeout(50) { Thread.sleep(100); "result" } should equal (None)

But if I throw an exception in my block it doesn't leak, but is swallowed - so that the following fails with "..no exception was thrown"

intercept[Exception] {
    runWithTimeout(50) { throw new Exception("deliberate") }
}.getMessage should equal("deliberate")

Syserr has a stack trace with the message

<function0>: caught java.lang.Exception: deliberate

but I can't find where in the Scala runtime that is printed.

Apart from wrapping f in another block which catches exceptions and propagates them if thrown, is there any way to persuade awaitAll and/or Future to throw?

Answer 1

Short answer: no.

Exceptions don't do what you want when you're working in a threaded context, because you want to know about the exception in the caller, and the exception happens in the future's thread.

Instead, if you want to know what the exception was, you should return an Either[Exception,WhatYouWant]--of course, you have to catch that exception within the future and package it up.

scala> scala.actors.Futures.future{
  try { Right("fail".toInt) } catch { case e: Exception => Left(e) }
}
res0: scala.actors.Future[Product with Serializable with Either[Exception,Int]] = <function0>

scala> res0()   // Apply the future
res1: Product with Serializable with Either[Exception,Int] =
      Left(java.lang.NumberFormatException: For input string: "fail")