json.scala

sealed trait Foo

case class Bar(a: Int) extends Foo
case class Baz(b: Int) extends Foo

object FooJson {
  def fromFoo(foo: Foo) = foo match {
    case Bar(a) => FooJson("bar", Some(a), None)
    case Baz(b) => FooJson("baz", None, Some(b))
  }
}

case class FooJson(
  `type`: String,
  a: Option[Int],
  b: Option[Int]
)