sameLength(5)

gistfile1.scala

class Vec(list: List[Double]) {
  case class Value(list: List[Double])
  val value = Value(list)

  def sameLength(x: Vec#Value): Option[Value] = {
    if (x.list.length == list.length) Some(Value(x.list))
    else None
  }

  def innerProd(x: Value): Double = {
    x.list.zip(value.list).map{case (a, b) => a*b}.sum
  }
}

val v1 = new Vec(List(1,2,3))
val v2 = new Vec(List(10,20,30))

v1.innerProd(v2.value)
v1.innerProd(v1.sameLength(v2.value).get)