Error reporting in Boost Spirit

Question

I have the following error handler at the bottom of my parser grammar:

qi::on_error<qi::fail>(
    launch,
    std::cerr << phoenix::val("Paring error in ") << spirit::_4 << std::endl
              << phoenix::construct<std::string>(spirit::_3, spirit::_2)
              << std::endl
    );

The problem is that the parser's input isn't broken-up by new lines beforehand, so the resulting error statement will be all lines in the source code from the error point until the end. Is there a straightforward alternative to

phoenix::construct<std::string>(spirit::_3, spirit::_2)

to only print the one line that the error occurs on? The Phoenix semantics are giving me trouble if I try to just search for '\n'.

Answer 1

We need to create a Phoenix function that can take the Spirit parameters.

// lazy function for error reporting
struct ReportError {
  // the result type must be explicit for Phoenix
  template<typename, typename, typename, typename>
  struct result { typedef void type; };

  // contract the string to the surrounding new-line characters
  template<typename Iter>
  void operator()(Iter first_iter, Iter last_iter,
                  Iter error_iter, const qi::info& what) const {
    std::string first(first_iter, error_iter);
    std::string last(error_iter, last_iter);
    auto first_pos = first.rfind('\n');
    auto last_pos = last.find('\n');
    auto error_line = ((first_pos == std::string::npos) ? first
                        : std::string(first, first_pos + 1))
                      + std::string(last, 0, last_pos);
    auto error_pos = (error_iter - first_iter) + 1;
    if (first_pos != std::string::npos) {
      error_pos -= (first_pos + 1);
    }
    std::cerr << "Parsing error in " << what << std::endl
              << error_line << std::endl
              << std::setw(error_pos) << '^'
              << std::endl;
  }
};

const phoenix::function<ReportError> report_error = ReportError();

Then we just invoke this function in the error handler.

qi::on_error<qi::fail>(
    launch,
    report_error(spirit::_1, spirit::_2, spirit::_3, spirit::_4)
    );