first code block use future promise second only use future

future.scala

import scala.concurrent.{ future, promise }
import scala.concurrent.ExecutionContext.Implicits.global
val p = promise[String]
val f = p.future
val producer = future {
  println("producing")
  Thread.sleep(1000)
  val r = "produced"
  p success r
  Thread.sleep(1000)
  println("producer ends")
}
val consumer = future {
  println("listening to producer")
  f onSuccess {
    case r => println("got result from producer:" + r)
  }
  Thread.sleep(2000)
  println("consumer ends")
}