The start of an HList implementation from my nescala unconf session. There are two errors that are left to the reader as an exercise. Hint: it's mostly right but a couple types are incorrect.

hlist.scala

import HList._

sealed abstract class HList
case object HNil extends HList
final case class HCons[H, T <: HList](head: H, tail: T) extends HList

object HList {
  type HNil = HNil.type
  type ::[H, T <: HList] = HCons[H, T]

  implicit final class HListOps[L <: HList](l: L) {
    def filterNot[A](implicit f: FilterNot[A, L]): f.Out =
      f(l)
  }
}

trait FilterNot[A, L <: HList] {
  type Out <: HList

  def apply(l: L): Out
}

object FilterNot extends LowPriorityFilterNotInstances {
  implicit def filterNotHNil[A]: Aux[A, HNil, HNil] = new FilterNot[A, HNil] {
    type Out = HNil

    def apply(l: HNil): HNil = HNil
  }

  implicit def filterNotIsType[A, H <: A, T <: HList](implicit f: FilterNot[A, T]): Aux[A, H :: T, f.Out] = new FilterNot[A, H :: f.Out] {
    type Out = f.Out

    def apply(l: H :: T): f.Out = f(l.tail)
  }
}

trait LowPriorityFilterNotInstances {
  type Aux[A, L <: HList, O <: HList] = FilterNot[A, L]{ type Out = O }

  implicit def filterNotIsntType[A, H, T <: HList](implicit f: FilterNot[A, T]): Aux[A, H :: T, H :: f.Out] = new FilterNot[A, H :: f.Out] {
    type Out = H :: f.Out

    def apply(l: H :: T): H :: f.Out = HCons(l.head, f(l.tail))
  }
}