Generic Derivation of `JsonValue` from shapeless-book - my attempt

Json.scala

package net

import shapeless.{::, Generic, HList, HNil, Lazy}

sealed trait JsonValue
case class JsonObject(fields: List[(String, JsonValue)]) extends JsonValue
case class JsonArray(items: List[JsonValue])             extends JsonValue
case class JsonString(value: String)                     extends JsonValue
case class JsonNumber(value: Double)                     extends JsonValue
case class JsonBoolean(value: Boolean)                   extends JsonValue
case object JsonNull                                     extends JsonValue

trait JsonEncoder[A] {
  def encode(value: A): JsonValue
}
object JsonEncoder {

  def instance[A](f: A => JsonValue): JsonEncoder[A] =
    new JsonEncoder[A] {
      override def encode(x: A): JsonValue = f(x)
    }

  implicit val doubleEncoder: JsonEncoder[Double] =
    instance[Double](JsonNumber)

  implicit val stringEncoder: JsonEncoder[String] =
    instance[String](JsonString)

  implicit val boolEncoder: JsonEncoder[Boolean] =
    instance[Boolean](JsonBoolean)

  implicit val nullEncoder: JsonEncoder[Null] =
    instance[Null](_ => JsonNull)

  implicit val hnilEncoder: JsonEncoder[HNil] =
    instance[HNil](_ => JsonObject(Nil))

  implicit def hlistEncoder[H, T <: HList](
    implicit hEncoder: Lazy[JsonEncoder[H]],
    tEncoder: JsonEncoder[T]
  ): JsonEncoder[H :: T] =
    instance[H :: T] {
      case h :: t => JsonObject( List(hEncoder.value.encode(h), tEncoder.encode(t) ) )
    }

  implicit def genericEncoder[A, R](
                                     implicit gen: Generic[A] { type Repr = R },
                                     enc: Lazy[JsonEncoder[R]]): JsonEncoder[A] =
    instance[A] { a =>
      enc.value.encode( gen.to(a) )
    }

}