Wrapper-type Play JSON format automatic derivation using shapeless

wrapperjsonformat.scala

import play.api.libs.json._
import shapeless.ops.hlist.IsHCons
import shapeless.{ Generic, HList, HNil }

/**
 * Typeclass for automatically deriving a [play.api.libs.json.Format] instance for a type `A`.
 * Instances will exist if A is a single-member case class containing a Formattable field
 */
trait WrapperJsonFormat[A] {
  def jsonFormat: Format[A]
}

object WrapperJsonFormat {
  /**
   * Summoner method
   */
  def apply[A](implicit instance: WrapperJsonFormat[A]): WrapperJsonFormat[A] = instance

  /**
   * Instance constructor method
   */
  def make[A](format: Format[A]): WrapperJsonFormat[A] = new WrapperJsonFormat[A] {
    override def jsonFormat: Format[A] = format
  }

  def make[A](reads: Reads[A], writes: Writes[A]): WrapperJsonFormat[A] = make(Format(reads, writes))

  implicit def implyAllWrappers[A, R <: HList, H](
    implicit
    gen: Generic.Aux[A, R],
    isHCons: IsHCons.Aux[R, H, HNil],
    formatH: Format[H]
  ): WrapperJsonFormat[A] = {
    make[A](
      reads = Reads { jsValue =>
        val asH = jsValue.as[H](formatH)
        /* Can I avoid this? How do I prove the value (asString :: HNil) <: R? */
        val asRepr = (asH :: HNil).asInstanceOf[R]
        val a: A = gen.from(asRepr)
        JsSuccess(a)
      },
      writes = Writes { a =>
        val asR = gen.to(a)
        val x: H = isHCons.head(asR)
        formatH.writes(x)
      }
    )
  }

}

// Usage
case class UserId(value: String)
implicit val format: Format[UserId] = WrapperJsonFormat[UserId].jsonFormat