import scala.Predef._

def addNaturals(nats: List[Int]): Int = {

    // assert(nats forall (_ >= 0), "List contains negative numbers")

    require(nats forall (_ >= 0), "List contains negative numbers")

    nats.foldLeft(0)(_ + _)

          } ensuring (_ >= 0) // 这里的"_"是函数的返回值

scala> addNaturals(List(-1, -2, 2, 3))

java.lang.IllegalArgumentException: requirement failed: List contains negative numbers

  at scala.Predef$.require(Predef.scala:224)

  at addNaturals(<console>:50)

  ... 56 elided

scala> addNaturals(List(1, 2, 3, 4, 10))

res15: Int = 20