import scalaz._, Scalaz._
import play.api.libs.json._

object Bla extends App {
  def defaultRead[T](v: T)(implicit rt: Reads[T]) = new Reads[T] {
    def reads(js: JsValue) = {
      println("applying default value to " + js)
      JsSuccess(js.asOpt[T].getOrElse(v))
    }
  }
  val t = (__ \ "foo").read[JsString](defaultRead(JsString("hello")))

  val json1 = Json.parse("""{"foo": 3}""")
  val json2 = Json.parse("""{}""")

  json1.transform(t) |> println
  json2.transform(t) |> println
}