Retrieving AST from boost::spirit parser

After I've read the tutorials on boost::spirit, I quite liked it because of the parser combinator syntax. Making a parser is so easy.

Unfortunately, the tutorials were not as exact on the matter of getting a complex data structure out of the parser. I am trying to get to the Kaleidoscope AST.

Anyway, here be my AST code:

#ifndef __AST_HPP__
#define __AST_HPP__

#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/variant/recursive_variant.hpp>
#include <boost/lexical_cast.hpp>
#include <boost/variant/apply_visitor.hpp>
#include <string>
#include <vector>

namespace ast {

struct add;
struct sub;
struct mul;
struct div;
struct func_call;
template<typename OpTag> struct binary_op;

typedef boost::variant<double, std::string, boost::recursive_wrapper<binary_op<
        add>>, boost::recursive_wrapper<binary_op<sub>>,
        boost::recursive_wrapper<binary_op<mul>>, boost::recursive_wrapper<
                binary_op<div>>, boost::recursive_wrapper<func_call>>
        expression;

template<typename OpTag>
struct binary_op {
    expression left;
    expression right;

    binary_op(const expression & lhs, const expression & rhs) :
        left(lhs), right(rhs) {
    }
};

struct func_call {
    std::string callee;
    std::vector<expression> args;

    func_call(const std::string func, const std::vector<expression> &args) :
        callee(func), args(args) {
    }
};

struct prototype {
    std::string name;
    std::vector<std::string> args;

    prototype(const std::string &name, const std::vector<std::string> &args) :
        name(name), args(args) {
    }
};

struct function