Improvements in repeat directive with variable factor for X3

Question

I need to parse a sequence of elements where there is a first number telling how many elements must be parsed next.

As a simplification of what I need: [3 10 20 30] should be parsed as showed next:

- Number of elements: 3
- Vector of elements: {10, 20, 30}

Being used to Qi and its "repeat directive + phoenix" syntax I tried the same for X3, failing in compiling. I have being looking in web for the same problem, and i find next link in stack overflow: Boost Spirit X3 cannot compile repeat directive with variable factor

I am shocked of how something so elegantly solved in Qi can be so ugly and cumberson in X3 (personal opinion, please nobody got offended). Of course I get the reasons about phoonix abandoned due to c++14 replacing it.

But I wonder is there is any further improvement in X3 about this subject due to this post is from 2015. I have been looking but nothing found. Any advise?

NOTE- No code included due that is same case/code that the posted link.

Thanks.

Answer 1

Usually it means that there was no PR for that feature (or it was but has some issues). The repeat also has design problems. For example you can parse {10 20 30} with it, but not {10, 20, 30} (requires a kind of list parser).

I cannot agree with that Qi has an elegant way of doing it because you have to use a rule with local variable or pass a reference to an external value. The natural way seems to be repeat(len_parser)[item_parser], but it has additional design issues with skippers (or skippers has design issues that limits complex directives flexibility).

Fortunately the Spirit X3 is much simpler in writing own parser combinators.

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

namespace x3e {

namespace x3 = boost::spirit::x3;

template <typename LenParser, typename Subject>
struct vlrepeat_directive : x3::unary_parser<Subject, vlrepeat_directive<LenParser, Subject>>
{
    using base_type = x3::unary_parser<Subject, vlrepeat_directive<LenParser, Subject>>;
    static bool const handles_container = true;

    vlrepeat_directive(LenParser const& lp_, Subject const& subject)
        : base_type(subject), lp(lp_) {}

    template<typename Iterator, typename Context, typename RContext, typename Attribute>
    bool parse(Iterator& first, Iterator const& last
      , Context const& context, RContext& rcontext, Attribute& attr) const
    {
        static_assert(x3::traits::has_attribute<LenParser, Context>::value, "must syntesize an attribute");

        Iterator iter = first;
        typename x3::traits::attribute_of<LenParser, Context>::type len;
        if (!lp.parse(iter, last, context, rcontext, len))
            return false;

        for (; len; --len) {
            if (!x3::detail::parse_into_container(
                    this->subject, iter, last, context, rcontext, attr))
                return false;
        }

        first = iter;
        return true;
    }

    LenParser lp;
};

template <typename LenParser>
struct vlrepeat_gen
{
    template <typename Subject>
    vlrepeat_directive<LenParser, typename x3::extension::as_parser<Subject>::value_type>
    operator[](Subject const& p) const
    {
        return { lp, x3::as_parser(p) };
    }

    LenParser lp;
};

template <typename Parser>
vlrepeat_gen<Parser> vlrepeat(Parser const& p)
{
    static_assert(x3::traits::is_parser<Parser>::value, "have to be a parser");
    return { p };
}

}

template <typename LenParser, typename Subject, typename Context>
struct boost::spirit::x3::traits::attribute_of<x3e::vlrepeat_directive<LenParser, Subject>, Context>
    : build_container<typename attribute_of<Subject, Context>::type> {};

And use it:

#include <iostream>
#include <vector>

int main()
{
    namespace x3 = boost::spirit::x3;

    auto s = "5: 1 2 3 4 5", e = s + std::strlen(s);
    std::vector<int> v;
    if (phrase_parse(s, e, x3e::vlrepeat(x3::uint_ >> ':')[x3::int_], x3::space, v)) {
        std::cout << "Result:\n";
        for (auto x : v)
            std::cout << x << '\n';
    }
    else
        std::cout << "Failed!\n";
}

Output:

Result:
1
2
3
4
5

https://wandbox.org/permlink/K572K0BMEqA8lMJm

(it has a call to detail::parse_into_container which is not a public API)