Scala's “for comprehension” with futures

Question

I am reading through the Scala Cookbook (http://shop.oreilly.com/product/0636920026914.do)

There is an example related to Future use that involves for comprehension.

Answer 1

To elaborate those existing answers here a simple result to demonstrate how for comprehension works.

Its bit lengthy functions yet they worth taking look into it.

A function that give us a range of integers

scala> def createIntegers = Future{
             println("INT "+ Thread.currentThread().getName+" Begin.")
             val returnValue = List.range(1, 256)
             println("INT "+ Thread.currentThread().getName+" End.")
             returnValue
         }
createIntegers: createIntegers: scala.concurrent.Future[List[Int]]

A function that give us a range of chars

scala> def createAsciiChars = Future{
             println("CHAR "+ Thread.currentThread().getName+" Begin.")
             val returnValue = new ListBuffer[Char]
             for (i <- 1 to 256){
                  returnValue += i.toChar
             }
             println("CHAR "+ Thread.currentThread().getName+" End.")
             returnValue
          }
createAsciiChars: scala.concurrent.Future[scala.collection.mutable.ListBuffer[Char]]

Using these function calls within the for comprehension.

scala> val result = for{
                        i <- createIntegers
                        s <- createAsciiChars
                    } yield i.zip(s)
       Await.result(result, Duration.Inf)
result: scala.concurrent.Future[List[(Int, Char)]] = Future()

For these below lines we can make out that all the function calls are synchronous i.e. createAsciiChars function call is not executed until createIntegers completes its execution.

scala> INT scala-execution-context-global-27 Begin.
       INT scala-execution-context-global-27 End.
       CHAR scala-execution-context-global-28 Begin.
       CHAR scala-execution-context-global-28 End.

Making these function createAsciiChars, createIntegers calls outside the for comprehensions will be asynchronous execution.