typeInferanceProblem.scala

sealed trait TaskComapped[L <: HList, M <: HList]
object TaskComapped{
  implicit def nil: TaskComapped[HNil, HNil] = new TaskComapped[HNil, HNil]{}
  implicit def cons[Head, Tail <: HList, ResultsTail <: HList](implicit tc: TaskComapped[Tail, ResultsTail]) =
    new TaskComapped[Task[Head] :: Tail, Head :: ResultsTail]{}
}

abstract class Task[R](value: R) {
  def destination: Int
}

implicit class hlist2Relation[DL <: HList, RL <: HList](dependencies: DL)(implicit ev: TaskComapped[DL, RL]) {
  def dependenciesOf[R](f: RL => Task[R]) = println("something")
}

val a = new Task("abs") {
  def destination: Int = 9
}

val aHList: ::[Task[String] {def destination: Int}, HNil] = a :: HNil

val aD = (a :: HNil) dependenciesOf { case s :: HNil =>
  new Task(s.length) {
    def destination: Int = 5
  }
}

val b: Task[String] = new Task("abs") {
  def destination: Int = 9
}

val bHList: ::[Task[String], HNil] = b :: HNil

val bD = (b :: HNil) dependenciesOf { case s :: HNil =>
  new Task(s.length) {
    def destination: Int = 5
  }
}