Better workaround for implicits and ordering issues, while serializing to JSON with Spray?

Question

I'm using the Spray JSON library to serialize our case classes into JSON. The problem is that we have some mutually recursive definitions. I am working from the example for serializing sealed traits here: http://www.cakesolutions.net/teamblogs/2012/11/30/spray-json-and-adts/

Here is a simple example that works. Please note the definition of C:

import spray.json._
import DefaultJsonProtocol._

sealed trait BaseTrait
sealed trait SpecializedTrait extends BaseTrait

case class A(argA: BaseTrait, foo: Int) extends SpecializedTrait
case class B(argB: BaseTrait, foo: Int) extends SpecializedTrait

case class C(foo: Int) extends BaseTrait

object BaseTrait {
  implicit val cJsonFormat = jsonFormat1(C)

  implicit object BaseTraitJsonFormat extends RootJsonFormat[BaseTrait] {
    override def write(obj: BaseTrait): JsValue = throw new IllegalStateException("Not Implemented")
    override def read(v: JsValue): BaseTrait = throw new IllegalStateException("Not Implemented")
  }
}

object SpecializedTrait {
  implicit val aJsonFormat = jsonFormat2(A)
  implicit val bJsonFormat = jsonFormat2(B)

  implicit object SpecializedTraitJsonFormat extends RootJsonFormat[SpecializedTrait] {
    override def write(obj: SpecializedTrait): JsValue = throw new IllegalStateException("Not Implemented")
    override def read(v: JsValue): SpecializedTrait = throw new IllegalStateException("Not Implemented")
  }
}

When you change the definition of "C" to the original mutually recursive definition, .

...
case class C(argC: SpecializedTrait, foo: Int) extends BaseTrait

object BaseTrait {
  implicit val cJsonFormat = jsonFormat2(C)

...

Now obviously, you can't just say no, I can't have mutually recursive data structures. It seems the compiler is stumbling because of resolution rules for implicit objects.

Is there a workaround for this? I got around it by changing the objects to anonymous class declarations, i.e.

import spray.json._
import DefaultJsonProtocol._

sealed trait BaseTrait
sealed trait SpecializedTrait extends BaseTrait

case class A(argA: BaseTrait, foo: Int) extends SpecializedTrait
case class B(argB: BaseTrait, foo: Int) extends SpecializedTrait

case class C(argC: SpecializedTrait, foo: Int) extends BaseTrait

object BaseTrait {
  implicit val cJsonFormat : RootJsonFormat[C] = jsonFormat2(C)

  implicit val baseTraitFormat : RootJsonFormat[BaseTrait] = new RootJsonFormat[BaseTrait] {
    def write(obj: BaseTrait): JsValue = throw new IllegalStateException("Not Implemented")
    def read(v: JsValue): BaseTrait = throw new IllegalStateException("Not Implemented")
  }
}

object SpecializedTrait {
  implicit val aJsonFormat : RootJsonFormat[A] = jsonFormat2(A)
  implicit val bJsonFormat : RootJsonFormat[B] = jsonFormat2(B)

  implicit val specializedTraitFormat : RootJsonFormat[SpecializedTrait] = new RootJsonFormat[SpecializedTrait] {
    override def write(obj: SpecializedTrait): JsValue = throw new IllegalStateException("Not Implemented")
    override def read(v: JsValue): SpecializedTrait = throw new IllegalStateException("Not Implemented")
  }
}

That last snippet works, note the change from "implicit object" to "implicit val" and the subsequent anonymous class.

Dave Swartz Dave Swartz · Accepted Answer · 2014-03-25T23:49:21

Define the implicits in order and put them into one object.

import spray.json._
import DefaultJsonProtocol._

sealed trait BaseTrait
sealed trait SpecializedTrait extends BaseTrait

case class A(argA: BaseTrait, foo: Int) extends SpecializedTrait
case class B(argB: BaseTrait, foo: Int) extends SpecializedTrait
case class C(argC: SpecializedTrait, foo: Int) extends BaseTrait

object SomeProtocol {
  implicit object BaseTraitJsonFormat extends RootJsonFormat[BaseTrait] {
    override def write(obj: BaseTrait): JsValue = throw new IllegalStateException("Not Implemented")
    override def read(v: JsValue): BaseTrait = throw new IllegalStateException("Not Implemented")
  }
  implicit object SpecializedTraitJsonFormat extends RootJsonFormat[SpecializedTrait] {
    override def write(obj: SpecializedTrait): JsValue = throw new IllegalStateException("Not Implemented")
    override def read(v: JsValue): SpecializedTrait = throw new IllegalStateException("Not Implemented")
  }
  implicit val aJsonFormat = jsonFormat2(A)
  implicit val bJsonFormat = jsonFormat2(B)
  implicit val cJsonFormat = jsonFormat2(C)
}

Better workaround for implicits and ordering issues, while serializing to JSON with Spray?

1 Answers