Method prepend

include/boost/parser/parser.hpp:6397–6413

    template<typename ParserTuple>
    template<typename Parser>
    constexpr auto or_parser<ParserTuple>::prepend(
        parser_interface<Parser> parser) const noexcept
    {
        // If you're seeing this as a compile- or run-time failure, you've
        // tried to put an eps parser at the beginning of an
        // alternative-parser, such as "eps | (int_ | double_)".  This is not
        // what you meant.  Since eps always matches any input, "eps | (int_ |
        // double_)" is just an awkward spelling for "eps".  To fix this this,
        // put the eps as the last alternative, so the other alternatives get
        // a chance.  Possibly, you may have meant to add a condition to the
        // eps, like "eps(condition) | (int_ | double_)", which also is
        // meaningful, and so is allowed.
        BOOST_PARSER_ASSERT(!detail::is_unconditional_eps<Parser>{});
        return parser_interface{
            or_parser<decltype(detail::hl::prepend(parsers_, parser.parser_))>{
                detail::hl::prepend(parsers_, parser.parser_)}};
    }

    template<typename ParserTuple>
    template<typename Parser>