[SOLVED] Spray json format for Unit type

Spray json format for Unit type

I'm writing method with the following signature:

def foo[A: Marshaller, B: Marshaller](f: A => B) = {...}

The catch is that A could be Unit. It makes sense that there should be an already-existing json format for Unit that converts to and from the empty string, and it also makes sense that it should be trivial to implement such a format even if it doesn't exist. How can I define or import a json format for Unit like I do for case classes as follows:

implicit val myFormat = jsonFormat4(myCaseClassWithFourFields)

Solution

To my knowledge no predefined json format for Unit exists. But you can write your own json format:

import spray.json._
import DefaultJsonProtocol._

implicit object UnitJsonFormat extends JsonFormat[Unit] {
  def write(u: Unit) = JsObject()
  def read(value: JsValue): Unit = value match {
      case JsObject(fields) if fields.isEmpty => Unit
  }
}

Using it:

scala> println("").toJson

res0: spray.json.JsValue = {}

scala> res0.convertTo[Unit]

scala>

Update: I'm not sure what you are expecting the json to look like for Unit, please clarify.