Scala: Future compositions

// 1. Let's try Future composition (suppose all the Futures are independent!)
import java.time.LocalDateTime
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.Future
import scala.io.StdIn
import scala.language.postfixOps

object Main extends App {
  def print(x: Any): Unit = {Thread.sleep(50); println(s"${LocalDateTime.now()}: $x")}

  def f1 = Future {
    print("Future 1 started")
    Thread.sleep(3000)
    print("Future 1 finished")
    5
  }

  def f2 = Future {
    print("Future 2 started")
    Thread.sleep(3000)
    print("Future 2 finished")
    true
  }

  def f3 = Future {
    print("Future 3 started")
    Thread.sleep(3000)
    print("Future 3 finished")
    "Tommy"
  }

  val result = for {
    x <- f1
    y <- f2
    z <- f3
  } yield (x, y, z)

  print("Press ENTER to finish")
  StdIn.readLine()
  print(s"result is $result")
}

        // Output (note seconds!):
        // 2018-02-27T22:24:33.867: Press ENTER to finish
        // 2018-02-27T22:24:33.867: Future 1 started
        // 2018-02-27T22:24:36.924: Future 1 finished
        // 2018-02-27T22:24:36.975: Future 2 started
        // 2018-02-27T22:24:40.025: Future 2 finished
        // 2018-02-27T22:24:40.076: Future 3 started
        // 2018-02-27T22:24:43.126: Future 3 finished

        // 2018-02-27T22:24:46.707: result is Future(Success((5,true,Tommy)))

// 2. Now let's do it simultaneously!
val runF1 = f1 // start Future #1!
val runF2 = f2 // start Future #2!
val runF3 = f3 // start Future #3!
print("Now we can use for-comprehension")
val result = for {
  x <- runF1
  y <- runF2
  z <- runF3
} yield (x, y, z)

        // Output (note seconds!):
        // 2018-02-27T22:27:03.395: Future 1 started
        // 2018-02-27T22:27:03.395: Future 3 started
        // 2018-02-27T22:27:03.395: Future 2 started
        // 2018-02-27T22:27:03.395: Now we can use for-comprehension
        // 2018-02-27T22:27:03.455: Press ENTER to finish
        // 2018-02-27T22:27:06.452: Future 1 finished
        // 2018-02-27T22:27:06.452: Future 2 finished
        // 2018-02-27T22:27:06.452: Future 3 finished

        // 2018-02-27T22:27:07.949: result is Future(Success((5,true,Tommy)))

// 3. Bonus! If all the Futures return the same type (e.g. Int), we can use 'sequence'.
// if the types are different, it still works, but casts to a most common type (e.g. to Any).
object Main extends App {
  def print(x: Any): Unit = {Thread.sleep(50); println(s"${LocalDateTime.now()}: $x")}

  def f1 = Future {
    print("Future 1 started")
    Thread.sleep(3000)
    print("Future 1 finished")
    5
  }

  def f2 = Future {
    print("Future 2 started")
    Thread.sleep(3000)
    print("Future 2 finished")
    6
  }

  def f3 = Future {
    print("Future 3 started")
    Thread.sleep(3000)
    print("Future 3 finished")
    7
  }

  val result: Future[List[Int]] = Future.sequence(List(f1, f2, f3))

  print("Press ENTER to finish")
  StdIn.readLine()
  print(s"result is $result")
}

        //Output:
        //2018-02-27T22:28:27.663: Future 2 started
        //2018-02-27T22:28:27.663: Future 1 started
        //2018-02-27T22:28:27.663: Future 3 started
        //2018-02-27T22:28:27.663: Press ENTER to finish
        //2018-02-27T22:28:30.719: Future 2 finished
        //2018-02-27T22:28:30.719: Future 1 finished
        //2018-02-27T22:28:30.719: Future 3 finished

        //2018-02-27T22:28:31.665: result is Future(Success(List(5, 6, 7)))

// 3. Bonus #2! 'traverse' is more generic variant of 'sequence':
object Main extends App {
  def print(x: Any): Unit = {Thread.sleep(50); println(s"${LocalDateTime.now()}: $x")}

  def f(x: Int) = Future {
    print("Future started")
    Thread.sleep(3000)
    print("Future finished")
    x*x
  }

  val result: Future[List[Int]] = Future.traverse(List(1, 2, 3, 4)) {t => f(t)}

  print("Press ENTER to finish")
  StdIn.readLine()
  print(s"result is $result")
}

        //Output:
        //2018-02-27T22:30:40.698: Future started
        //2018-02-27T22:30:40.698: Future started
        //2018-02-27T22:30:40.698: Future started
        //2018-02-27T22:30:40.698: Press ENTER to finish
        //2018-02-27T22:30:40.698: Future started
        //2018-02-27T22:30:43.756: Future finished
        //2018-02-27T22:30:43.756: Future finished
        //2018-02-27T22:30:43.757: Future finished
        //2018-02-27T22:30:43.758: Future finished

        //2018-02-27T22:30:45.072: result is Future(Success(List(1, 4, 9, 16)))