jsonunion.scala

import io.circe.{Decoder, DecodingFailure}
import io.circe.parser._
import io.circe.generic.auto._
import cats.syntax.xor._


object Test extends  App {

  val unionA = "{\"union\" : { \"a\" : { \"a1\": 1, \"a2\": \"two\" } } }"
  val unionB = "{\"union\" : { \"b\" : { \"b1\": \"type\", \"b2\": 2 } } }"

  sealed trait JsonUnion
  case class typeA(a1: Int, a2: String) extends JsonUnion
  case class typeB(b1: String, b2: Int) extends JsonUnion

  implicit def decoder: Decoder[JsonUnion] = Decoder.instance[JsonUnion] { c =>
    val u = c.downField("union")
      Seq(
        u.downField("a").cursor.map(_.as[typeA]),
        u.downField("b").cursor.map(_.as[typeB])
      ).collectFirst { case Some(s) => s }
        .map(_.right)
        .getOrElse(DecodingFailure("invalid union", List.empty).left)
        .flatMap(identity)
  }


  println(decode(unionA))
  println(decode(unionB))

}