serializing polymorphic classes using spray.json getting could not find implicit value for evidence parameter of type

Question

I am trying to produce json

trait Bar
case class Foo(name:String) extends Bar
case class Buz(name:String,age:Int) extends Bar
case class Responsive(id:String ,bars:List[Bar])

when calling

import spray.json._
val foo  = Foo("foo")
val fooRes  = Responsive("abc",List(foo))

println(fooRes.toJson)

I am getting

Cannot find JsonWriter or JsonFormat type class for com.demo.Responsive
          println(s" res = ${fooRes.toJson}")
                                    ^

when I add

implicit val impResponsive = jsonFormat2(Responsive)

I am getting

`could not find implicit value for evidence parameter of type` com.demo.routing.JsonImplicits.JF[List[com.avi.demo.Bar]]
 implicit val impResponsive = jsonFormat2(Responsive)
                                          ^

why am I getting these errors ? how can I solve it ?

Aldo Stracquadanio Aldo Stracquadanio · Accepted Answer · 2015-05-18T09:33:23

The error you get on Responsive is really due to the fact that this class contains a reference to Bar and to the fact you don't have a (de-)serialiser for the Bar trait. More in detail, the type-system knows that you can (de-)serialise instances of Foo, Buz and Responsive because you have the appropriate formats in scope. But how can the type-system know that you can (de-)serialise a generic Bar?

It would be great if spray-json could support this use case making the Bar trait sealed and defining the serialisers for its children, but I am afraid that this wouldn't work either. In the end you will need to write a custom serialiser for the Bar trait. If you want a proof that the error is due to this just add the following to your implicit formats:

implicit val barFormat: RootJsonFormat[Bar] = ???

Everything should compile now, but obviously fail at runtime due to the ???.

serializing polymorphic classes using spray.json getting could not find implicit value for evidence parameter of type

1 Answers