import $ivy.`com.chuusai::shapeless:2.3.3`
import shapeless._

trait Fields[T] {
  def fields: List[String]
  override def toString: String = fields.mkString(", ")
}
object Fields extends LabelledProductTypeClassCompanion[Fields] {
  def apply[T](fs: List[String]): Fields[T] = new Fields[T] {
    override def fields: List[String] = fs
  }

  implicit val string: Fields[String] = apply[String](Nil)
  implicit def anyVal[T <: AnyVal]: Fields[T] = apply[T](Nil)

  object typeClass extends LabelledProductTypeClass[Fields] {
    override def product[H, T <: HList](name: String, ch: Fields[H], ct: Fields[T]) : Fields[H :: T] =
      Fields(name :: ct.fields)

    override def emptyProduct : Fields[HNil] = Fields(Nil)

    override def project[F, G](instance: => Fields[G], to: F => G, from: G => F) = Fields(instance.fields)
  }
}

case class Foo(bar: String, baz: Boolean)

println(implicitly[Fields[Foo]].fields)