gistfile1.scala

import shapeless._
import shapeless.Nat._
import shapeless.Record._
import shapeless.Tuples._
import shapeless.TypeOperators._
import shapeless.HList._
import shapeless.UnaryTCConstraint._

trait FilterSubs[L <: HList, U] {
  type Out <: HList
  def apply(l : L) : Out
}
object FilterSubs {
  implicit def hlistFilterSubs[L <: HList, U, Out0 <: HList](implicit aux : FilterSubsAux[L, U, Out0]) = new FilterSubs[L, U] {
    type Out = Out0
    def apply(l : L) : Out0 = aux(l)
  }
}
trait FilterSubsAux[L <: HList, U, Out <: HList] {
  def apply(l : L) : Out
}
object FilterSubsAux {
  import TypeOperators._

  implicit def hlistFilterSubsHNil[L <: HList, U] = new FilterSubsAux[HNil, U, HNil] {
     def apply(l : HNil) : HNil = HNil
  }

  implicit def hlistFilterSubs1[H <: U, L <: HList, U, Out <: HList]
    (implicit aux : FilterSubsAux[L, U, Out], st: H <:< U) = new FilterSubsAux[H :: L, U, H :: Out] {
       def apply(l : H :: L) : H :: Out = l.head :: aux(l.tail)
    }

  implicit def hlistFilterSubs2[H, L <: HList, U, Out <: HList]
    (implicit aux : FilterSubsAux[L, U, Out], nst: H <:!< U) = new FilterSubsAux[H :: L, U, Out] {
       def apply(l : H :: L) : Out = aux(l.tail)
    }
}

object Tests {
	// If you take all strings from a String :: HNil, you should get the same HList back. These shouldn't compile:
	implicitly[FilterAux[String :: HNil, String, HNil]]
	// implicitly[FilterSubsAux[String :: HNil, String, HNil]]

	// These should compile:
	trait Letter
	trait A extends Letter
	trait B extends Letter
	trait Number
	trait One extends Number
	trait Two extends Number
	implicitly[FilterSubsAux[A :: One :: B :: Two :: HNil, Letter, A :: B :: HNil]]
	implicitly[FilterSubsAux[A :: One :: B :: Two :: HNil, Number, One :: Two :: HNil]]
	implicitly[FilterSubsAux[A :: B :: HNil, Number, HNil]]
	implicitly[FilterSubsAux[One :: Two :: HNil, Letter, HNil]]
}