Wrapper policy for doubles

Henri Menke wrote:
> You can have a look at Compound Attribute Rules [1] and find that in
>
> (a | b)    a: A, b: Unused --> (a | b): optional<A>
>
> so you will always end up with an inferred attribute.  Therefore I 
> suggest that you alter the grammar slightly to
>
> *item_parser % x3::omit['{' >> *(x3::char_ - '}') >> '}']

Hi Henri,
A belated thanks. And I'm getting the hang of '%'. From there, I'm 
trying to understand more about using parsers inside parsers. I had a 
challenge to parse dollar amounts into doubles, so I came up with this. 
I just don't know if there is a more elegant way to do it. I was able to 
keep semantic action out of the bigger picture with this method. It did 
give me a better understanding of attributes/values.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#include <iostream>
#include <boost/spirit/home/x3.hpp>

using namespace boost::spirit::x3;

auto sa = [](auto &ctx) {
     _pass(ctx) = parse(begin(_attr(ctx)), end(_attr(ctx)), double_, 
_val(ctx));
};
auto const mstr = rule<struct money_def, std::string>("mstr") = 
-lit('$') >> *char_("0-9.") % ',';
auto const money = rule<struct money_def, double>("money") = mstr[sa];

int main()
{
     std::vector<double> tv;
     std::string ds("$1,234 $45,675 89.99");

    parse(begin(ds), end(ds), money % ' ', tv);

     for (auto& i : tv)
         std::cout << i << std::endl;

     return 0;
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~