Scala puzzler for-expression pitfall

scala-puzzler-for-expression.sc

/*
 Using a for-expression (not comprehension!!) works different then expected!
 When running with lists, see below, the 'println' is run 4 times as expected.
 But when using Futures, the second failing Future is not 'run' and the result is still a success!
 Only when 1st Future is a failure, the result is a failure.
 BUT !!! In case of Future's the 'println' is NEVER run!! Why??
 
 Note: this is tested using Twitter Futures. Don't know about Scala Future.
 */

// With Twitter Futures.
import com.twitter.util.{Future => TFuture, Await}
// Try changing Future results and see what happens.
val l1: TFuture[Int] = TFuture.value(1)
val l2: TFuture[Int] = TFuture.exception(new RuntimeException("2"))
val x = for(
  _ <- l1;
  _ <- l2
) {
  println("here") // Is never run when using Futures!!
}
x.onFailure(_ => println("Fail"))

x.isDefined

Await.ready(x)

// This is what the for-expression is turned into.
// Now it is more clear, the second 'l2.foreach' is just not doing anything if it is a failed Future.
val r = l1.foreach { _ =>
  l2.foreach { _ => () }
}

// With lists
val ll1 = List(1,2)
val ll2 = List(2,3)
val z = for(
  _ <- ll1;
  _ <- ll2
) {
  println("here")
}

// Scala Future
import scala.concurrent.{Future => SFuture}

val sc1: SFuture[Int] = SFuture.successful(1)
val sc2: SFuture[Int] = SFuture.successful(2)
val y = for(
  _ <- sc1;
  _ <- sc2;
) {
  println("Ok?")
  ""
}

// y is not a Future, but Unit !?!?!
// TFuture.foreach: TFuture[Unit], because TFuture.onSuccess: TFuture[Unit]
// SFuture.foreach: Unit         , because SFuture.onSuccess: Unit