public function index()
    {
        $this->authorize('readList', User::class);
        $input   = $this->validator->validate('list');
        $params  = $this->processor->process($input)->getProcessedFields();
        $results = $this->userRepo->getUsers(
            $params['filter'],
            $params['orderBy'],
            $params['page'],
            $params['perPage']
        );
        return $this->respondWithSuccess($results, new UserTransformer);
    }