restricting Spray api response types

ouch.scala

// Wanted to ensure complete() was only called with a certain response type, to give guarantees about api response structure
// (no accidental "returned a string when it should be json!" or "didn't actually give a body when I should have")
trait CustomRouteDirectives extends spray.routing.directives.RouteDirectives {
  import spray.httpx.marshalling.ToResponseMarshallable
  import spray.httpx.marshalling.ToResponseMarshallable.{isMarshallable => _}
  import spray.httpx.marshalling.ToResponseMarshaller
  import spray.httpx.marshalling.ToResponseMarshaller.{
    eitherMarshaller,
    futureMarshaller,
    liftMarshaller,
    liftMarshallerConversion,
    optionMarshaller,
    tryMarshaller,
    fromStatusCode => _
  }

  /* And filling out implicit defs to convert to the restricted ToResponseMarshaller[T] types... */
  //implicit def fromEitherM: ToRespon
  // well. this is exactly the point where I threw my hands up and said not worth it (for now)

  class RestrictedToResponseMarshaller[T](marshaller: ToResponseMarshaller[T])
  object RestrictedToResponseMarshaller {
    implicit def fromToResponseMarshaller[T](marshaller: ToResponseMarshaller[T])(implicit restrictor: ToResponseMarshaller[T] => RestrictedToResponseMarshaller[T]) =
      restrictor(marshaller)
  }

  implicit def fromStatusCode = ()

  implicit def toMarshaller[T](value: T)(implicit marshaller: RestrictedToResponseMarshaller[T]) =
    ToResponseMarshallable.isMarshallable(value)(marshaller)

  override def complete: (=> ToResponseMarshallable) => StandardRoute =
    response => new StandardRoute {
      def apply(ctx: RequestContext): Unit = ctx.complete(
        response
      )
    }
}