mapper from an HList to another

HListMapper.scala

  trait HListMapper[In <: HList, Out <: HList] {
    def apply(in: In): Out
  }

  object HListMapper {

    implicit def caseHNil: HListMapper[HNil, HNil] = (in: HNil) => HNil

    implicit def caseHList[H1, H2, T1 <: HList, T2 <: HList]
    (
      implicit
      headMapper: H1 => H2,
      tailMapper: HListMapper[T1, T2]
    ): HListMapper[H1 :: T1, H2 :: T2] = (in: H1 :: T1) => headMapper(in.head) :: tailMapper(in.tail)
  }