scala – 无形：尝试按类型限制HList元素

我第一次尝试使用现有的LUBConstraints失败,因为缺少证据(参见下面的代码块).任何暗示为什么？空列表不是有效的长列表吗？没有元素违反约束.

import shapeless.ops.coproduct
import shapeless.{::,:+:,Coproduct,HNil,HList}

object testLUBConstraints {
  import shapeless.LUBConstraint._

  // !!! see comment on question - this satisfies the implicit below!!! 
  // implicit val hnilLUBForLong = new LUBConstraint[HNil.type,Long] {}

  def acceptLong[L <: HList : <<:[Long]#λ](l: L) = true
  val validLong = acceptLong(1l :: HNil)

  val validEmpty = acceptLong(HNil)
  //  => WHY??? Error: could not find implicit value for evidence parameter of type shapeless.LUBConstraint[shapeless.HNil.type,Long]
  //  MY EXPECTATION WAS: 'implicit def hnilLUB[T] = new LUBConstraint[HNil,T] {}' defined within LUBConstraint companion should provide so

  // val invalid = acceptLong(1.0d :: HNil) -> fails due to missing evidence (as expected)
}

任何帮助赞赏.

问题2 – 使用Coproduct自己的约束
(分成一个单独的问题：Shapeless: own HList constraint using Coproduct)

问题3 – 按参数类型限制案例类
(分成一个单独的问题：Shapeless: restricting case class types)