Tentando ajustar o espírito de impulso x3 calc example para analisar funções que podem assumir funções como argumentos. No entanto, não compila.

namespace client{ namespace ast{
    struct ts;
    struct fnc;
    typedef boost::variant<
    ts,
    boost::recursive_wrapper<fnc>
    > node;
    struct ts{
        unsigned int id;
    };
    struct fnc{
        std::vector<char> id;
        std::vector<node> args;
    };
}}
BOOST_FUSION_ADAPT_STRUCT(
    client::ast::ts,
    (unsigned int, id)
)
BOOST_FUSION_ADAPT_STRUCT(
    client::ast::fnc,
    (std::vector<char>, id)
    (std::vector<client::ast::node>, args)
)
namespace client{
    namespace x3 = boost::spirit::x3;
    namespace calc_grammar{
        using x3::uint_;
        using x3::alpha;
        using x3::alnum;
        using x3::lit;
        using x3::char_;
        x3::rule<class funct, ast::fnc> const funct("function");
        x3::rule<class ts, ast::ts> const ts("timeseries");
        x3::rule<class funct_name, std::vector<char>> const funct_name("function_name");
        auto const funct_def = funct_name >> lit('(') >> -((ts|funct)%lit(',')) >> lit(')');
        auto const ts_def = lit('#') >> uint_ >> lit('#');
        auto const funct_name_def = lit('@') >> alpha >> *(alnum|char_('_'));
        auto const calc = x3::grammar(
            "calc", 
            funct = funct_def, 
            ts = ts_def,
            funct_name = funct_name_def
        );
    }
    using calc_grammar::calc;
}

erro C2665: 'boost :: detail :: variante :: make_initializer_node :: apply :: initializer_node :: initialize': nenhuma das 5 sobrecargas poderia converter todos os tipos de argumento

Há também uma nota para o usuário em variant.hpp

// NOTE TO USER :
// Compile error here indicates that the given type is not 
// unambiguously convertible to one of the variant's types
// (or that no conversion exists).

No entanto, eu não sou o mais sábio ...