trigger warning from boost spirit parser

Question

How I can add warnings in boost spirit parser.

Edit: ... that could report the issue with position

For example if I have an integer parser:

('0' >> oct)
| int_

I would like to be able to do something like this:

('0' >> oct)
| "-0" --> trigger warning("negative octal values are not supported, it will be interpreted as negative decimal value and the leading 0 will be ignored")
| int_

Answer 1

Q. Can I create my own callback? How?

A. Sure. Any way you'd normally do it in C++ (or look at Boost Signal2 and/or Boost Log)

parser(std::function<bool(std::string const& s)> callback) 
    : parser::base_type(start),
      callback(callback)
{
    using namespace qi;

    start %= 
        as_string[+graph] 
            [ _pass = phx::bind(callback, _1) ]
        % +space
        ;

    BOOST_SPIRIT_DEBUG_NODES((start));
}

As you can see, you can even make the handler decide whether the warning should be ignored or cause the match to fail.

---

UPDATE #1 I've extended the sample to show some of the unrelated challenges you mentioned in the comments (position, duplicate checking). Hope this helps

Here's a simple demonstration: see it Live on Coliru (Word)

UPDATE #2 I've even made it (a) store the source information instead of the iterators, (b) made it "work" with floats (or any other exposed attribute type, really).

Note how uncannily similar it is, s/Word/Number/, basically: Live On Coliru (Number)

#define BOOST_RESULT_OF_USE_DECLTYPE // needed for gcc 4.7, not clang++
#define BOOST_SPIRIT_USE_PHOENIX_V3
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
#include <boost/spirit/include/phoenix_stl.hpp>
#include <functional>

namespace qi  = boost::spirit::qi;
namespace phx = boost::phoenix;

// okay, so you want position reporting (actually unrelated):
#include <boost/spirit/include/support_line_pos_iterator.hpp>
using It = boost::spirit::line_pos_iterator<std::string::const_iterator>;

// AST type that represents a Number 'token' (with source and location
// information)
struct Number 
{ 
    double      value;
    size_t      line_pos;
    std::string source;

    explicit Number(double value = 0.0, boost::iterator_range<It> const& range = {})
        : 
          value(value),
          line_pos(get_line(range.begin())),
          source(range.begin(), range.end())
    {}

    bool operator< (const Number& other) const { return (other.value - value) > 0.0001; }
};

// the exposed attribute for the parser:
using Words    = std::set<Number>;

// the callback signature for our warning; you could make it more like
// `on_error` so it takes the iterators directly, but again, I'm doing the
// simple thing for the dmeo
using Callback = std::function<bool(Number const& s)>;

template <typename It>
    struct parser : qi::grammar<It, Words()>
{
    parser(Callback warning) 
        : parser::base_type(start),
          warning(warning)
    {
        using namespace qi;
        auto check_unique = phx::end(_val) == phx::find(_val, _1);

    word   = 
               raw [ double_ [ _a = _1 ] ] [ _val = phx::construct<Number>(_a, _1) ]
               ;

        start %= 
               - word        [ _pass = check_unique || phx::bind(warning, _1) ]
               % +space
               >> eoi
               ;
    }

  private:
    Callback warning;
    qi::rule<It, Number(), qi::locals<double> > word;
    qi::rule<It, Words()> start;
};

int main(int argc, const char *argv[])
{
    // parse command line arguments
    const auto flags          = std::set<std::string> { argv+1, argv+argc };
    const bool fatal_warnings = end(flags) != flags.find("-Werror");

    // test input
    const std::string input("2.4 2.7 \n\n\n-inf \n\nNaN 88 -2.40001 \n3.14 240001e-5\n\ninf");

    // warning handler
    auto warning_handler = [&](Number const& w) { 
        std::cerr << (fatal_warnings?"Error":"Warning") 
                  << ": Near-identical entry '" << w.source << "' at L:" << w.line_pos << "\n"; 
        return !fatal_warnings;
    };

    // do the parse
    It f(begin(input)), l(end(input));
    bool ok = qi::parse(f, l, parser<It>(warning_handler));

    // report results
    if (ok)   std::cout << "parse success\n";
    else      std::cerr << "parse failed\n";
    if (f!=l) std::cerr << "trailing unparsed: '" << std::string(f,l) << "'\n";

    // exit code
    return ok? 0 : 255;
}

Prints:

Warning: Near-identical entry 'NaN' at L:6
Warning: Near-identical entry '240001e-5' at L:7
parse success