Boost::spirit::qi defining a calculator for nullaries

Question

I'm trying to write a parser for math expressions where named variables are nullaries in boost::spirit (version 1_51_0), to which I'm completely new. I define typedef boost::function<double()> Value and my rules will be declared like so: qi::rule<Iterator, Value()> expression, term, others, ...;

I define binary operators on nullaries with this macro

#define BINARY_FUNCTOR(name, op)                        \
struct name                                             \
{                                                       \
  name(Value x, Value y): x_(x), y_(y) {}               \
  double operator()() { return x_() op y_(); }          \
  Value x_, y_;                                         \
}; 

and have ADD, SUB, etc. From the examples I've seen, I'd expect the rules to be defined like this:

expression = term
             >> *( (lit('+') >> term[ADD(_val, _1)])
                 | (lit('-') >> term[SUB(_val, _1)])
                 );

but that doesn't seem to be the right syntax, as I get an error

boost/spirit/home/support/action_dispatch.hpp:162: error: no match for call to ‘(const<unnamed>::SUB) (boost::function<double ()()>&, boost::spirit::context<boost::fusion::cons<boost::function<double ()()>&, boost::fusion::nil>, boost::fusion::vector0<void> >&, bool&)’
SRParser.cpp:38: note: candidates are: double<unnamed>::SUB::operator()()

which looks to me like _1 isn't quite what I expect it to be, namely the Value associated with the next term. What's the right syntax to define a rule like this?

Answer 1

The parser expression looks okay.

What you are confused over is on constructing the AST. Apparently, you have decided to use Semantic Actions to do so, but your effort is too sketchy for me to see just how (or even decide what sample you are basing this on).

In essence: What is it that you want to do with the instances of 'ADD'/'SUB' that you seem to magically "will" into your rules?

Right now, you just use the instance directly as a semantic action. This leads to the error message shown, which tells you, directly, that the instance is not valid as a semantic action.

I assume you really wanted to use Phoenix assignment to assign the binary operation to the exposed attribute, instead. This looks like:

expression = term
     >> *( (lit('+') >> term[ _val = phx::construct<ADD>(_val, _1)])
         | (lit('-') >> term[ _val = phx::construct<SUB>(_val, _1)])
         );

You'll see this matches much more closely with traditional expression grammars.

For fun I adapted a complete expression parser based on your Value type and created this working demonstration: http://liveworkspace.org/code/3kgPJR$0

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

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

typedef std::function<double()> Value;

#define BINARY_FUNCTOR(name, op)                        \
struct name                                             \
{                                                       \
  name(Value x, Value y): x_(x), y_(y) {}               \
  double operator()() { return x_() op y_(); }          \
  Value x_, y_;                                         \
}; 

BINARY_FUNCTOR(ADD, +)
BINARY_FUNCTOR(SUB, -)
BINARY_FUNCTOR(MUL, *)
BINARY_FUNCTOR(DIV, /)

struct LIT
{
  LIT(double x): x_(x) {}
  double operator()() { return x_; }
  double x_;
}; 

struct NEG
{
  NEG(Value x): x_(x) {}
  double operator()() { return -x_(); }
  Value x_;
}; 


template <typename It, typename Skipper = qi::space_type>
    struct parser : qi::grammar<It, Value(), Skipper>
{
    parser() : parser::base_type(expression)
    {
        using namespace qi;
        expression =
            term                    [_val = _1]
            >> *( ('+' >> term  [_val = phx::construct<ADD>(_val, _1)])
                | ('-' >> term  [_val = phx::construct<SUB>(_val, _1)])
                );

        term =
            factor                [_val = _1]
            >> *( ('*' >> factor  [_val = phx::construct<MUL>(_val, _1)])
                | ('/' >> factor  [_val = phx::construct<DIV>(_val, _1)])
                );

        factor =
            double_               [_val = phx::construct<LIT>(_1)]
            |   '(' >> expression [_val = _1] >> ')'
            |   ('-' >> factor    [_val = phx::construct<NEG>(_1)])
            |   ('+' >> factor    [_val = _1]);

        BOOST_SPIRIT_DEBUG_NODE(expression);
        BOOST_SPIRIT_DEBUG_NODE(term);
        BOOST_SPIRIT_DEBUG_NODE(factor);
    }

  private:
    qi::rule<It, Value(), Skipper> expression, term, factor;
};

Value doParse(const std::string& input)
{
    typedef std::string::const_iterator It;
    parser<It, qi::space_type> p;
    Value eval;

    auto f(begin(input)), l(end(input));

    if (!qi::phrase_parse(f,l,p,qi::space,eval))
        std::cerr << "parse failed: '" << std::string(f,l) << "'\n";
    if (f!=l) 
        std::cerr << "trailing unparsed: '" << std::string(f,l) << "'\n";

    return eval;
}

int main()
{
    auto area = doParse("2 * (3.1415927 * (10*10))");
    std::cout << "Area of a circle r=10: " << area() << "\n";
}

It will print

Area of a circle r=10: 628.319