gistfile1.scala

// problem with making it really optional in the general case ..
// what will be the output for the following ?

val str: Option[String] = None
tojson(str) // what ?

// hence ..
val str = None
fromjson[Option[String]](tojson[Option[String]](str)) should equal(str)

// really optional can be made using specific typeclass instance as follows
case class P(lastName: String, firstName: String, age: Option[Int] = None)
object PProtocol extends DefaultProtocol {
  import dispatch.json._
  import JsonSerialization._
  implicit object PFormat extends Format[P] {
    def reads(json: JsValue): P = json match {
      case JsObject(m) =>
        P(
          fromjson[String](m(JsString("lastName"))), 
          fromjson[String](m(JsString("firstName"))), 
          m.get(JsString("age")).map(fromjson[Option[Int]](_)).getOrElse(None))
      case _ => throw new RuntimeException("JsObject expected")
    }
    def writes(p: P): JsValue =
      p.age.map(a =>
        JsObject(List(
          (tojson("lastName").asInstanceOf[JsString], tojson(p.lastName)),
          (tojson("firstName").asInstanceOf[JsString], tojson(p.firstName)),
          (tojson("age").asInstanceOf[JsString], tojson(a)))))
      .getOrElse(
        JsObject(List(
          (tojson("lastName").asInstanceOf[JsString], tojson(p.lastName)),
          (tojson("firstName").asInstanceOf[JsString], tojson(p.firstName)))))
  }
}

// now None is really optional for class P serialization
import PProtocol._
val p = P("ghosh", "debasish")
println(tojson[P](p)) // prints {"lastName" : "ghosh", "firstName" : "debasish"}
println(fromjson[P](tojson[P](p))) // prints P(ghosh,debasish,None)

fromjson[P](tojson[P](p)) should equal(p)

Please look at my test case (trial-sjson): https://github.com/weiglewilczek/trial-json/blob/master/src/test/scala/SjsonSpec.scala
You won't find any errors there ;-)

To be more general: You can have companion objects for for case classes.

silly me :-( .. Not sure why I thought the companion object will be an error .. Thanks .. will change ..