Conversion from shapeless records to json4s JValues.

shapelessToJson.scala

trait JSONRecords {
  import scala.annotation.implicitNotFound

  import org.json4s._
  import shapeless._
  import shapeless.record._
  import shapeless.ops.hlist._
  import shapeless.ops.record._
  import shapeless.syntax._
  import shapeless.syntax.singleton._

  object toJField extends Poly1 {
    implicit def caseLong[K <: String]         = at[(K, Long)](kv => JObject(JField(kv._1, JInt(kv._2))))
    implicit def caseString[K <: String]       = at[(K, String)](kv => JObject(JField(kv._1, JString(kv._2))))
    implicit def caseOptionString[K <: String] = at[(K, Option[String])](kv => JObject(JField(kv._1, kv._2.fold(JNothing: JValue)(JString(_)))))
    implicit def caseJField[K <: String, V <% JValue] = at[(K, V)](kv => JObject(JField(kv._1, kv._2)))

    @implicitNotFound("\nYour record contains an unsupported key or value type.\nKeys must be a String.\nValues must be a: Long, String, Option[String]\nA quick workaround is to 'import org.json4s.JsonDSL._' in your scope.")
    type Mapper[OutZ <: HList, OutM <: HList] = Mapper.Aux[toJField.type, OutZ, OutM]
  }

  object combine extends Poly {
    implicit def caseJObject = use((f1: JObject, f2: JObject) => JObject(f1.obj ::: f2.obj))
  }

  def toJValue[L <: HList, K <: HList, V <: HList, OutZ <: HList, OutM <: HList](l : L)
    (implicit
      keys: Keys.Aux[L, K],
      values: Values.Aux[L, V],
      zipper: Zip.Aux[K :: V :: HNil, OutZ],
      mapper: toJField.Mapper[OutZ, OutM],
      reducer: LeftReducer[OutM, combine.type]) =
    (l.keys zip l.values).map(toJField).reduceLeft(combine)
}

object examples extends JSONRecords {
  import shapeless._
  import shapeless.syntax.singleton._

  val r =
    ("id"  ->> 1234L) ::
    ("foo" ->> "bar") ::
    HNil
    
  val jvalue = toJValue(r)
  // JObject(JField("id", 1234), JField("foo", "bar))
}

You may find this interesting https://github.com/rrmckinley/jsonless/blob/master/json4s/src/main/scala/json4s.scala