A utility that transcodes UTF string is needed #124

#ifndef XP_PARSER_STRING_HPP
#define XP_PARSER_STRING_HPP

#include "xp/config.hpp"

#include <iris/x4/core/action_context.hpp>
#include <iris/x4/attribute/as.hpp>

#include <iris/unicode/string.hpp>

#include <concepts>
#include <string>

namespace xp::parser {

namespace detail {

struct as_native_string_fn
{
    [[nodiscard]] static constexpr auto operator()(auto const& parser)
    {
        return x4::as<std::string>(
            x4::as<std::u32string>(parser).on_match([](auto&& ctx) {
                x4::_as_var(ctx) = iris::unicode::transcode<char>(x4::_attr(ctx));
            })
        );
    }
};

template<class ToCharT = char32_t, class FromCharT = char32_t>
struct as_string_fn
{
    [[nodiscard]] static constexpr auto operator()(auto const& parser)
    {
        if constexpr (std::same_as<FromCharT, ToCharT>) {
            return x4::as<std::basic_string<ToCharT>>(
                parser
            );

        } else {
            return x4::as<std::basic_string<ToCharT>>(
                x4::as<std::basic_string<FromCharT>>(parser).on_match([](auto&& ctx) {
                    x4::_as_var(ctx) = iris::unicode::transcode<ToCharT>(x4::_attr(ctx));
                })
            );
        }
    }
};

} // detail

[[maybe_unused]] inline constexpr detail::as_native_string_fn as_native_string{};

template<class ToCharT = char32_t, class FromCharT = char32_t>
[[maybe_unused]] inline constexpr detail::as_string_fn<ToCharT, FromCharT> as_string{};

} // xp::parser

#endif