fix-gadt12.scala

import $ivy.`io.circe::circe-parser:0.10.0`
import io.circe.Decoder

val toDecoder: HAlgebra[QueryF, Decoder] = new HAlgebra[QueryF, Decoder] {
  override def apply[A](fa: QueryF[Decoder, A]): Decoder[A] = fa match {
    case QueryBoolF   => Decoder.decodeBoolean
    case QueryStringF => Decoder.decodeString
    case q: QueryPathF[Decoder, A] =>
      Decoder.instance { cursor =>
        cursor.get(q.path)(q.next)
      }
  }
}

val decoder = hCata(toDecoder, nestedQuery)
val json = parse("""
    {
      "oh": {
        "my": "20202"
      }
    }
""").right.get

decoder.decode(json) // Right("20202")