How to generically extract field names with Shapeless?

问题

Given a case class A I can extract its field names with Shapeless using the following snippet:

val fieldNames: List[String] = {
  import shapeless._
  import shapeless.ops.record.Keys

  val gen = LabelledGeneric[A]
  val keys = Keys[gen.Repr].apply
  keys.toList.map(_.name)
}

This works all nice, but how can I implement this in a more generic manner, so that I can conveniently use this technique for arbitrary classes, like

val fields: List[String] = fieldNames[AnyCaseClass]

Is there a library that already does this for me?

回答1:

Something like this maybe, slightly modified version of this example:

import shapeless._
import shapeless.ops.record._
import shapeless.ops.hlist.ToTraversable

trait FieldNames[T] {
  def apply(): List[String]
}

implicit def toNames[T, Repr <: HList, KeysRepr <: HList](
  implicit gen: LabelledGeneric.Aux[T, Repr],
  keys: Keys.Aux[Repr, KeysRepr],
  traversable: ToTraversable.Aux[KeysRepr, List, Symbol]
): FieldNames[T] = new FieldNames[T] {
  def apply() = keys().toList.map(_.name)
}

def fieldNames[T](implicit h : FieldNames[T]) = h()

来源：https://stackoverflow.com/questions/46525541/how-to-generically-extract-field-names-with-shapeless