sealedDecoder.scala

import cats.syntax.functor._

sealed trait Foo
@ElementCodec
case class Bar(a: Int) extends Foo
@ElementCodec
case class Qux(b: String) extends Foo

object Foo {
    implicit val fooDecoder: ElementDecoder[Foo] = ElementDecoder[Foo] {
      def decodeAsElement(c: Cursor, localName: String, namespaceUri: Option[String]): ElementDecoder[Foo] = {
        if (c.isStartElement) {
          ElementDecoder.errorIfWrongName[String](c, localName, namespaceUri).getOrElse {
            val idx = c.getAttributeIndex("http://namespace.com", "type")
            if (idx > -1) {
              c.getAttributeValue(idx) match {
                case "bar" => ElementDecoder[Bar].decodeAsElement(c, localName, namespaceUri).widen
                case "qux" => ElementDecoder[Qux].decodeAsElement(c, localName, namespaceUri).widen
                case _     => new FailedDecoder(c.error("Unexpected 'type' value"))
              }
            } else {
              new FailedDecoder(c.error("Missing 'type' attribute"))
            }
          }
        } else {
          new FailedDecoder(c.error("Wrong state"))
        }
        
      }
    
    def result(history: List[String]): Either[DecodingError, Foo] =
      Left(DecodingError("Decoding not complete", history))

    val isCompleted: Boolean = false
  }
}