DecoderExample.scala

  implicit val ExtendedGreetingDecoder: Decoder[ExtendedGreeting] =
    Decoder.instance { hello: io.circe.HCursor =>
      for {
        language <- hello.downField("language").as[String]
        greeting <- hello.downField("greeting").as[Greeting]
      } yield ExtendedGreeting(greeting, language)
    }