Finch usage to transfer custom headers

model.scala

// Let change finch example to specify custom http-headers (let call them CTPPHeader) per models


object model {
  trait ToJsonWithHeaders {
    def toJson: Json  
    // provide specific headers per model file
    def getTemplate :String
  }

  case class User(val id: Long, val name: String) extends ToJsonWithHeaders {
    override def toJson = User.encodeJson(this)
    // this would be used later to generate specific http headers: 'x-template' -> 'user'
    override def getTemplate = "user"
  }

  // Some code parts are omitted which are the same with finch-example


  implicit class SeqToJson[A](s: Service[A, Seq[ToJsonWithHeaders]]) extends Service[A, ToJsonWithHeaders] {
    private[server] def seqToJson(seq: Seq[ToJsonWithHeaders]) = new ToJsonWithHeaders {
      def toJson: Json = Json.array(seq.map { _.toJson }: _*)
      // Needs to specify templates anyway, so follow the approach when 
      // 'template' is for single model entity, 'templateSeq' for the Seq[entity], and 'empty' for the empty collection
      def getTemplate = if(seq.isEmpty) "empty" else seq(0).getTemplate+"Seq"
    }

    def apply(req: A): Future[ToJsonWithHeaders] = s(req) map seqToJson
  }
}

server.scala

  def okWithCustomHeaders[A, B <:ToJsonWithHeaders](req:B): ResponseBuilder = 
     Ok.withHeaders( "x-template" -> req.getTemplate)

  class TurnIntoHttpWithHeaders[A<:ToJsonWithHeaders](val e: EncodeResponse[A]) extends Service[A, HttpResponse] {
    def apply(req: A): Future[HttpResponse] = okWithCustomHeaders(req)(req)(e).toFuture
  }

  /**
   * A service that converts an encoded object into HTTP response with status ''OK'' using an implicit
   * [[io.finch.response.EncodeResponse EncodeResponse]].
   */
  object TurnIntoHttpWithHeaders {
    def apply[A<:ToJsonWithHeaders](implicit e: EncodeResponse[A]): Service[A, HttpResponse] = new TurnIntoHttpWithHeaders[A](e)
  }

  // EncodeResponse for Json implicit conversion 
  implicit val encodeToJson = EncodeResponse[ToJsonWithHeaders]("text/html") { _.toJson.toString}

  import endpoint._

  val api: Service[AuthRequest, ToJsonWithHeaders] = restEndpoint

  import model._
  val backend: Service[HttpRequest, HttpResponse] =
     handleExceptions ! authorize ! (api !  TurnIntoHttpWithHeaders[ToJsonWithHeaders])