Implicits in Unapply

implicits.scala

// probably could be used for some cool type state stuff

sealed trait HList
case class HCons[A, R <: HList](head: A, tail: R) extends HList {
  def ::[B](b: B): B :+: A :+: R = HCons(b, this)
}
case object HNil extends HList {
  def ::[A](a: A): A :+: HNil = HCons(a, this)
}
type HNil = HNil.type
type :+:[A, R <: HList] = HCons[A, R]

sealed trait In[E, L <: HList] {}
object In {
  implicit def here[E, L <: HList]: E In (E :+: L) = ???
  implicit def there[E, R, L <: HList](implicit rest: E In L): E In (R :+: L) = ???
}

object IntElem {
  def unapply[L <: HList](l: L)(implicit f: Int In L): Option[Int] = ???
}

val l = true :: 4 :: "foo" :: HNil

l match {
  case IntElem(n) => ???
}