One pass over collection to calculate average

avg.scala

def sumLength(xs: List[Int]) = xs.foldLeft(None:Option[(Int,Int)]){
  case (None, x) => Some(x, 1)
  case (Some((s:Int, l:Int)), x) => Some((s + x, l + 1 ))
}
def avg(xs: List[Int]): Option[Int] = sumLength(xs).map{case (sum, length) => sum / length}

val avg1 = avg(List(1, 2, 3, 4, 6, 7, 8, 9))
val avg2 = avg(List(1, 2, 3, 4, 5, 6, 7, 8, 9))
val avg3 = avg(List.empty[Int])