Scala Futures + For Comprehension/Seq's

gistfile1.scala

// DEMO CODE

import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global

def time[A](label: String, block: => Future[A]) = {
  val t0 = System.nanoTime()
  block.onComplete {
    case _ => {
      val t1 = System.nanoTime()
      println(s"[$label] elapsed time: " + ((t1 - t0) / 1000000000) + "s")
    }
  }
}

def test1 = {
  for {
    f1Result <- Future { Thread.sleep(3000); 1 }
    f2Result <- Future { Thread.sleep(3000); 2 }
    f3Result <- Future { Thread.sleep(3000); 3 }
  } yield (f1Result, f2Result, f3Result)
}

def test2 = {
  val fut1 = Future { Thread.sleep(3000); 1 }
  val fut2 = Future { Thread.sleep(3000); 2 }
  val fut3 = Future { Thread.sleep(3000); 3 }

  for {
    f1Result <- fut1
    f2Result <- fut2
    f3Result <- fut3
  } yield (f1Result, f2Result, f3Result)
}

def test3 = {
  // Seq[Future] => Future[Seq]
  Future.sequence(Seq(
    Future { Thread.sleep(3000); 1 },
    Future { Thread.sleep(3000); 2 },
    Future { Thread.sleep(3000); 3 }
  ))
}

time("test1", test1)
time("test2", test2)
time("test3", test3)


// OUTPUT:

// [test2] elapsed time: 3s
// [test3] elapsed time: 3s
// [test1] elapsed time: 9s