Test.scala

  object tester extends Poly1 {
    implicit def hlistCase[T <: HList](implicit mapper : Mapper[tester.type, T]) = at[T](x => {
      testMap(x)
    })
    implicit def anyCase[T] = at[T](x => 1)
  }

  def testMap[T <: HList](t: T)(implicit mapper : Mapper[tester.type, T]) = t.map(tester)


  val entities = (ComponentA() :: HNil) :: HNil
  println(testMap(entities))
  
  /* 
  error: could not find implicit value for parameter mapper: shapeless.ops.hlist.Mapper[tester.type,shapeless.::[shapeless.::[ComponentA,shapeless.HNil],shapeless.HNil]]
  at: println(testMap(entities))
  */