HList.scala

  trait HList {
    def ::[A](x: A): A :: this.type
  }
  trait ::[+Head, +Tail <: HList] extends HList {
    val head: Head
    val tail: Tail
    def ::[A](x: A): A :: this.type = HCons(x,this)
  }
  final case class HCons[+Head, +Tail <: HList](head: Head, tail: Tail) extends (Head :: Tail) {
    override def toString: String = s"$head :: $tail"
  }
  trait HNil extends HList
  final case object HNil extends HNil {
    def ::[A](x: A): A :: this.type = HCons(x, this)
  }