Typed return value can make the duplicate impl, the boilerplates gone, and more type safe. This is some drill and improvement on sample of "Scala in Depth"

Heterogenous List

object HLists {

  trait HList {
    type ConResult[A] <: HList

    def ::[A](a: A): ConResult[A]
  }

  case class HCol[H, T](h: H, t: T) extends HList {
    override type ConResult[A] = HCol[A, HCol[H, T]]

    override def ::[A](a: A): ConResult[A] = {
      HCol[A, HCol[H, T]](a, this)
    }
  }

  case object HNil {
    type ConResult[A] = HCol[A, HNil.type]

    def ::[A](a: A): ConResult[A] = HCol(a, HNil)
  }

}
import HLists._
val x = "ok"::Boolean::2L::HNil

Try Line 13 with: ConResult[A](a, this)