(NOTA: Dividir deSem forma: tentando restringir os elementos da HList por seu tipo )

Pergunta 2 - Restrição própria usando o Coproduct

O que eu realmente queria fazer é escrever uma nova restrição usando o Coproduct.

trait CPConstraint[L <: HList, CP <: Coproduct] extends Serializable
object CPConstraint {
  import shapeless.ops.coproduct.Selector._

  def apply[L <: HList, CP <: Coproduct](implicit cpc: CPConstraint[L, CP]): CPConstraint[L, CP] = cpc

  type <*<[CP <: Coproduct] = {  // TODO: just invented a symbol ... what would be an appropriate one?
    type λ[L <: HList] = CPConstraint[L, CP]
  }

implicit def hnilCP[HN <: HNil, CP <: Coproduct]: CPConstraint[HN, CP] = new CPConstraint[HN, CP] {}
implicit def hlistCP[H, T <: HList, CP <: Coproduct](implicit ev: coproduct.Selector[CP, H], cpct: CPConstraint[T, CP]): CPConstraint[H :: T, CP] = new CPConstraint[H :: T, CP] {}

}

object testCPConstraint {
  import shapeless.ops.coproduct.Selector._
  import CPConstraint._

  type CPType = Long :+: String :+: CNil

  implicit val selLong = implicitly[Selector[CPType, Long]]
  implicit val selString = implicitly[Selector[CPType, String]]

  def acceptCP[L <: HList : <*<[CPType]#λ](l: L) = true

  val hlLong: ::[Long, HNil] = 1L :: HNil
  val hlString: ::[String, HNil] = "blabla" :: HNil
  val hlMixed: ::[String, ::[Long, HNil]] = "blabla" :: 1L :: HNil
  val hlMixedRev: ::[Long, ::[String, HNil]] = 1L :: "blabla" :: HNil
  val hlInvalid: ::[Double, HNil] = 1.0d :: HNil

  implicit val scpcEmpty: CPConstraint[HNil, CPType] = implicitly[CPConstraint[HNil, CPType]]

  implicit val scpcEmptyLong1: CPConstraint[::[Long,HNil], CPType] = new CPConstraint[::[Long,HNil], CPType] {}

// valor implícito scpcEmptyLong2: CPConstraint [hlLong.type, CPType] = novo CPConstraint [hlLong.type, CPType] {} // linha acima caberia no implícito ausente - POR QUE ???

  implicit val cpcLong = implicitly[CPConstraint[hlLong.type, CPType]]

  val validEmpty = acceptCP(HNil: HNil)
  val validLong = acceptCP(1l :: HNil)
  val validMixed = acceptCP("blabla" :: 1l :: HNil)

  val invalid = acceptCP(1.0d :: HNil) // should fail due to missing evidence
}