Boost.Spirit.Qi: dynamically create "difference" parser at parse time

Question

A "difference" parser can be created by the binary -(minus) operator:

rule = qi::char_ - qi::lit("}}")

or even compound differences:

rule = qi::char_ - qi::lit("}}") - qi::lit("]]")

But how could I generate the whole result of the difference parser at the parse time?
I'm guessing it might be some kind of form like below:

phoenix::function<difference_parser_impl> difference_parser;
rule = qi::lazy(difference_parser(qi::char_, {"}}", "]]"}));

Here, the {..., ..., ...} part would actually be a stl container, but it is not the point; I can handle that part.

I have found the template qi::difference<Left, Right> -- but I couldn't find out how to use it.

Answer 1

It seems to me you're not looking for a dynamic "difference" expression so much, but rather a dynamic "variadic alternative (a|b|c...)" expression:

expr - a - b - c is equivalent to expr - (a|b|c)

You could then easily achieve the difference using either:

expr - orCombine(alternatives)

or

!orCombine(alternatives) >> expr

Now, getting this done has many rough edges, which I'll explain first. Luckily, there is a simpler way, using qi::symbols, which I'll demonstrate right after that.

The tricky stuff

If you want, you can "generate" alternative parser expressions on-demand, with a fair bit of wizardry. I showed how to do this in this answer:

• Generating Spirit parser expressions from a variadic list of alternative parser expressions

But

1. it is fraught with pitfalls (as proto expressions don't lend themselves to copying well)¹

2. it conveniently used variadics in order to avoid intermediate storage (note the deepcopy_ to ward of Undefined Behaviour):

   template<typename ...Expr>
   void parse_one_of(Expr& ...expressions)
   {
       auto parser = boost::fusion::fold(
                   boost::tie(expressions...),
                   qi::eps(false),
                   deepcopy_(arg2 | arg1)
               );
   

   Seeing how you have a need for truly dynamic composition of the alternative parser, I don't see how this could be adapted to your needs without an explosion of complexity and opportunity for subtle error (believe me, I already tried).

So, instead I recommend a tried & true approach that "abuses" an existing "dynamic" parser:

Simplify using qi::symbols

This idea borrows losely from the well-famed "Nabialek Trick". It uses qi::symbols, and consequently has excellent runtime performance characteristics².

With no further ado, this is an example of how you could use it, starting from a vector of string literals:

template <typename It, typename Skipper = qi::space_type>
    struct parser : qi::grammar<It, std::string(), Skipper>
{
    parser() : parser::base_type(start)
    {
        static const std::vector<std::string> not_accepted { "}}", "]]" };

        using namespace qi;
        exclude = exclusions(not_accepted);
        start = *(char_ - exclude);

        BOOST_SPIRIT_DEBUG_NODE(start);
    }

  private:
    qi::rule<It, std::string(), Skipper> start;

    typedef qi::symbols<char, qi::unused_type> Exclude;
    Exclude exclude;

    template<typename Elements>
    Exclude exclusions(Elements const& elements) {
        Exclude result;

        for(auto& el : elements)
            result.add(el);

        return result;
    }
};

A full working sample of this is here: http://coliru.stacked-crooked.com/view?id=ddbb2549674bfed90e3c8df33b048574-7616891f9fd25da6391c2728423de797 and it prints

parse success
data: 123
trailing unparsed: ']] 4'

Full code

For future reference:

#include <boost/spirit/include/qi.hpp>

namespace qi    = boost::spirit::qi;

template <typename It, typename Skipper = qi::space_type>
    struct parser : qi::grammar<It, std::string(), Skipper>
{
    parser() : parser::base_type(start)
    {
        static const std::vector<std::string> not_accepted { "}}", "]]" };

        using namespace qi;
        exclude = exclusions(not_accepted);
        start = *(char_ - exclude);

        BOOST_SPIRIT_DEBUG_NODE(start);
    }

  private:
    qi::rule<It, std::string(), Skipper> start;

    typedef qi::symbols<char, qi::unused_type> Exclude;
    Exclude exclude;

    template<typename Elements>
    Exclude exclusions(Elements const& elements) {
        Exclude result;

        for(auto& el : elements)
            result.add(el);

        return result;
    }
};

int main()
{
    const std::string input = "1 2 3]] 4";
    typedef std::string::const_iterator It;
    It f(begin(input)), l(end(input));

    parser<It> p;
    std::string data;

    bool ok = qi::phrase_parse(f,l,p,qi::space,data);
    if (ok)   
    {
        std::cout << "parse success\n";
        std::cout << "data: " << data << "\n";
    }
    else std::cerr << "parse failed: '" << std::string(f,l) << "'\n";

    if (f!=l) std::cerr << "trailing unparsed: '" << std::string(f,l) << "'\n";
}

---

¹ I believe this problem is about to be removed in the upcoming new version of Spirit (currently dubbed "Spirit X3" for the experimental version)

² It uses Tries to lookup the matches