fix-gadt11.scala

val nestedQuery = queryPath(
                    "oh",
                    queryPath(
                      "my",
                      queryString
                    )
                  )

// a trick to fold into String, this is interesting as it shows that
// generalized type constructor is super powerful; it can represent a
// more specialized type easily
type JustString[A] = String

// important part: convert each layer of query into a string
val print: HAlgebra[QueryF, JustString] = new HAlgebra[QueryF, JustString] {
  override def apply[A](fa: QueryF[JustString, A]): JustString[A] = {
    fa match {
      case QueryStringF                 => "as[String]"
      case QueryBoolF                   => "as[Bool]"
      case q: QueryPathF[JustString, A] => s"${q.path}.${q.next}"
    }
  }
}

hCata(print, nestedQuery)
// res44: JustString[String] = "oh.my.as[String]"