Spot the difference

gistfile1.scala

import scala.concurrent._
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration._

def fut(name: String, waitTime: Duration, result: Int): Future[Int] = {
  Future {
    println(s"$name started")
    Thread.sleep(waitTime.toMillis)
    println(s"$name finished")
    result
  }
}

val f1 = for {
  a <- fut("a", 500.milliseconds, 123)
  b <- fut("b", 200.milliseconds, 321)
} yield a + b

val res1 = Await.result(f1, 1.second)

//-------

val cFut = fut("c", 500.milliseconds, 234)
val dFut = fut("d", 200.milliseconds, 432)

val f2 = for {
  c <- cFut
  d <- dFut
} yield c + d

val res2 = Await.result(f2, 1.second)