Shapeless: 如何表达 <: Generix.Aux 的案例类类型参数

Question

我有以下代码基本上遍历case class 的字段并使用Poly 将它们映射到相同类型并使用ToList[HL, Out]

为简单起见，我们可以假设Poly 这样做：

object Schema extends Poly1 {
  implicit def caseInt = at[Int](_ => "I'm an int")
  implicit def caseString = at[String](_ => "Oh boy a string")
}

def infer[V1 <: Product, Out <: HList, MapperOut <: HList](v1: V1)(
  implicit gen: Generic.Aux[V1, Out],
    map: Mapper.Aux[Schema.type, Out, MapperOut],
    to: ToList[MapperOut, String]
): List[String] = to (gen to v1 map Schema)

这一切都非常简单，非常适合简单的场景：

case class Test(id: Int, text: String)
val list = infer(Test(2, "text"))
// List("I'm an int", "Oh boy a string")

现在去公共汽车不运行的地方：

class Automagical[T <: Product with Serializable : TypeTag] {
  def instance: T 
  // The typetag bit is needed for something else
  def convert: List[String] = infer(instance)
}

遗憾的是，对上述的任何调用都失败了：

could not find implicit value for parameter gen: shapeless.Generic.Aux[T,Out]

奖金

如何通过根本不需要T 的实例来改进infer 方法？显然类型推断很好，但我确实需要以某种方式从HList[Lub] 实现List[String] 并映射一些东西。

鉴于我只关心类型，是否有可能仅通过知道要被多映射编码为HList? 的类型来派生List[String] 的具体实例

类似：

 def infer[V1 <: Product, Out <: HList, MapperOut <: HList]()(
  implicit gen: Generic.Aux[V1, Out],
    map: Mapper.Aux[Schema.type, Out, MapperOut],
    to: ToList[MapperOut, String]
): List[String] = {
  // Magically build an HList with samples from its types.
  // Or an alternative approach that gives the same outcome
  val reifiedInstance = reify[Out]
  to (reifiedInstance map Schema)
}

Answer 1

为确保convert 可以访问infer 的隐式参数，它们必须在Automagical 实例创建时出现：

abstract class Automagical[T <: Product with Serializable : TypeTag,
    Out <: HList, MapperOut <: HList]
  (implicit
    gen: Generic.Aux[T, Out],
    map: Mapper.Aux[Schema.type, Out, MapperOut],
    to: ToList[MapperOut, String]) {
  def instance: T
  // The typetag bit is needed for something else
  def convert: List[String] = infer(instance)
}