class RequireAuthentication(a: Authenticator) extends Filter[...] {
  def apply(
    request: Request,
    continue: Service[AuthenticatedRequest, HttpResponse]
  ) = {
      a.authenticate(request) flatMap {
        case AuthResult(OK, passport) =>
          continue(AuthenticatedRequest(request, passport))
        case AuthResult(Error(code)) =>
          Future.exception(new RequestUnauthenticated(code))
    }
  }
}