future_one_by_one.scala

import scala.concurrent.Future
import scala.concurrent.ExecutionContext

implicit val ec = ExecutionContext.global

val ns = Seq(1,2,3,4,5,6,7,8,9,10)

def makeF(n:Int):Future[Int] = Future.successful(n)

def sum(numbers:Seq[Int]):Future[Int] = {
  def go(t:(Seq[Int], Int)):Future[(Seq[Int], Int)] = {
    println(t._2)
    t._1 match {
      case Seq() => Future.successful(t)
      case Seq(h) => makeF(h).map(n => (Seq(), t._2+n))
      case h::tail => makeF(h).flatMap(s => go((tail, s+t._2)))
    }
  }
	go((numbers, 0)).map(t => t._2)
}

sum(ns).map{n=>println(n); n}