Ambiguous variant and boost spirit x3

Question

Trying to tweak the boost spirit x3 calc example to parse functions that can take functions as arguments. However it does not compile.

namespace client{ namespa         


        

Answer 1

I spotted this old question. X3 has evolved a bit in the mean time and I though I'd answer it now anyways.

I suspected that the main issue might have been with (missing) (implicit) constructors on the variant members.

Anyhow, here's a live demo with a more lightweight grammar:

namespace grammar_def {
    using namespace x3;

    rule<class funct, ast::fnc> const funct("function");

    auto const ts        = lexeme [ '#' >> uint_ >> '#' ];
    auto const fname     = lexeme [ '@' >> raw [ alpha >> *(alnum | '_') ] ];
    auto const expr      = ts|funct;

    auto const funct_def = fname >> '(' >> -expr % ',' >> ')';

    BOOST_SPIRIT_DEFINE(funct)
}

I also added some output streaming helpers. Note how I changed the id type to std::string for simplicity (it's hard/impossible to overload operator<< for vector<char> without invading namespace std):

namespace client { namespace ast {
    static std::ostream& operator<<(std::ostream& os, ts const& v) {
        using namespace boost::fusion;
        return os << tuple_open("") << tuple_close("") << tuple_delimiter("") << as_vector(v);
    }

    static std::ostream& operator<<(std::ostream& os, fnc const& v) {
        using namespace boost::fusion;
        return os << tuple_open("") << tuple_close("") << tuple_delimiter("") << as_vector(v);
    }
    template<typename T>
    static std::ostream& operator<<(std::ostream& os, std::vector<T> const& v) {
        os << "("; for (auto& el : v) os << (&el==&v[0]?"":", ") << el; return os << ")";
    }
} }

Demo

This has more (optional) plumbing to allow for richer debug information:

Live On Coliru

//#define BOOST_SPIRIT_X3_DEBUG
#include <iostream>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/include/as_vector.hpp>
#include <boost/fusion/include/io.hpp>
#include <boost/spirit/home/x3.hpp>

namespace client { namespace ast {
    struct ts;
    struct fnc;

    //using string = std::vector<char>;
    using string = std::string; // for easier printing/debugging

    struct ts {
        unsigned int id;
        ts(unsigned id=0):id(id) {}
    };

    typedef boost::variant<ts, boost::recursive_wrapper<fnc> > node;

    struct fnc {
        string id;
        std::vector<node> args;
    };
} }

BOOST_FUSION_ADAPT_STRUCT(client::ast::ts, id)
BOOST_FUSION_ADAPT_STRUCT(client::ast::fnc, id, args)

//namespace std { static ostream& operator<<(ostream&os, vector<char> const& v) { return os.write(&v[0], v.size()); } }

namespace client { namespace ast {
    static std::ostream& operator<<(std::ostream& os, ts const& v) {
        using namespace boost::fusion;
        return os << tuple_open("") << tuple_close("") << tuple_delimiter("") << as_vector(v);
    }

    static std::ostream& operator<<(std::ostream& os, fnc const& v) {
        using namespace boost::fusion;
        return os << tuple_open("") << tuple_close("") << tuple_delimiter("") << as_vector(v);
    }
    template<typename T>
    static std::ostream& operator<<(std::ostream& os, std::vector<T> const& v) {
        os << "("; for (auto& el : v) os << (&el==&v[0]?"":", ") << el; return os << ")";
    }
} }

namespace client {
    namespace x3 = boost::spirit::x3;
    namespace grammar_def {
        using namespace x3;

        x3::rule<class funct, ast::fnc> const funct("function");

        auto const ts     // = x3::rule<class ts, ast::ts> {"timeseries"}
                             = lexeme [ '#' >> uint_ >> '#' ];
        auto const fname  // = x3::rule<class fname, ast::string> {"function_name"}
                             = lexeme [ '@' >> raw [ alpha >> *(alnum | '_') ] ];
        auto const expr   // = rule<struct expr_, ast::node > {"expr"}
                             = ts|funct;

        auto const funct_def = fname >> '(' >> -expr % ',' >> ')';

        BOOST_SPIRIT_DEFINE(funct)
    }

    auto const& grammar = x3::skip(x3::space) [grammar_def::funct];
}

#include <iostream>

int main() {
    std::string const s {
        "@pow( #1#, \n"
            "     @trunc(\n"
            "           @pi ()\n"
            "   ) )"};
    std::cout << "Parsing '" << s << "'\n";

    auto f = s.begin();
    client::ast::fnc parsed;

    if (parse(f, s.end(), client::grammar, parsed)) {
        std::cout << "Parse succeeded: " << parsed << "\n";
    } else {
        std::cout << "Parse failed\n";
    }

    if (f != s.end())
        std::cout << "Remaining unparsed input: '" << std::string(f, s.end()) << "'\n";
}

Prints:

Parsing '@pow( #1#, 
     @trunc(
           @pi ()
   ) )'
Parse succeeded: pow(1, trunc(pi()))