CombiningDecoders.scala

object CirceDecoderCombination {

  sealed trait Base
  case class StringField(field1: String) extends Base
  case class IntField(field2: Int) extends Base
  case class FloatField(field3: Float) extends Base

  import io.circe.{Decoder, Encoder}
  import io.circe.generic.auto._

  object Base {
    import io.circe.syntax._
    implicit val encoder: Encoder[Base] = Encoder.instance {
      case c:StringField => c.asJson
      case c:IntField => c.asJson
      case c:FloatField => c.asJson
    }


    implicit val decoder: Decoder[Base] = {
      import cats.syntax.functor._
      List[Decoder[Base]](
        Decoder[StringField].widen,
        Decoder[IntField].widen,
        Decoder[FloatField].widen
      ).reduceLeft(_ or _)
    }

  }
}
  
class CirceDecoderCombinationSepc extends FlatSpec with Matchers {
      it should "encode and decode a subtype using its aggregate en/decoder" in {
        import io.circe.syntax._
        val value = StringField("1")
        val jason = value.asInstanceOf[Base].asJson
        jason.as[Base] shouldBe Right(value)
      }
}