SYB from Json

jsnull-be-gone.scala

import shapeless._, poly._

object SYBMain extends App {

  // Play JSON boils down to:
  sealed trait JsValue

  case object JsNull extends JsValue

  case class JsObject  (fields: Seq[(String, JsValue)]) extends JsValue
  case class JsNumber  (value : BigDecimal)             extends JsValue
  case class JsArray   (value : Seq[JsValue] = List())  extends JsValue
  case class JsString  (value : String)                 extends JsValue
  case class JsBoolean (value : Boolean)                extends JsValue

  // This tree contains JsNull values. They must go.
  val tree : JsValue =
    JsObject(Seq(
      ("cat" -> JsNumber(4)),
      ("dog" -> JsNumber(4)),
      ("car" -> JsNull),
      ("nested" ->
        JsObject(Seq(
          "train"    -> JsNull,
          "squirrel" -> JsNumber(4)
          )) ),
      ("array" -> JsArray(Seq(JsString("rabbit"), JsNull, JsString("bird"))))
      ))

  // SYB currently does Lists out of the box. This gives us Seq:
  object ShapelessDataSeqOps {
    implicit def seqDataT[F <:  Poly, T, U](implicit ft : Case1.Aux[F, T, U]): DataT[F, Seq[T], Seq[U]] =
      new DataT[F, Seq[T], Seq[U]] {
        def gmapT(t : Seq[T]) = t.map(ft)
    }
  }

  import ShapelessDataSeqOps._

  object fix extends Poly1 {
    implicit def removeFromObj = at[Seq[(String,JsValue)]](
      seq => seq.filter{ case (k, JsNull) => false ; case _ => true }
    )
    implicit def removeFromArray = at[Seq[JsValue]](seq => seq.filter{ case JsNull => false; case _ => true })
  }

  println(
    everywhere(fix)(tree)
  )

  // Result:
  // JsObject(List((cat,JsNumber(4)), (dog,JsNumber(4)), (nested,JsObject(List((squirrel,JsNumber(4))))), (array,JsArray(List(JsString(rabbit), JsString(bird))))))

}

Or:

  object fix extends Poly1 {
    implicit def removeFromObj   = at[Seq[(String,JsValue)]](_.filter(withoutNull))
    implicit def removeFromArray = at[Seq[JsValue]](_.filter(withoutNull))

    private def withoutNull[T](node: T): Boolean = node match {
      case (_, JsNull) => false
      case JsNull      => false
      case _           => true
    }
  }