Exagon Exagon - 3 months ago 24
C++ Question

parsing identifiers except keywords

I am struggeling writing a identifier parser, which parses a alphanum string which is not a keyword.
the keywords are all in a table:

struct keywords_t : x3::symbols<x3::unused_type> {
keywords_t() {
add("for", x3::unused)
("in", x3::unused)
("while", x3::unused);
}
} const keywords;


and the parser for a identifier should be this:

auto const identifier_def =
x3::lexeme[
(x3::alpha | '_') >> *(x3::alnum | '_')
];


now i try to combine these so an identifier parser fails on parsing a keyword.
I tried it like this:

auto const identifier_def =
x3::lexeme[
(x3::alpha | '_') >> *(x3::alnum | '_')
]-keywords;


and this:

auto const identifier_def =
x3::lexeme[
(x3::alpha | '_') >> *(x3::alnum | '_') - keywords
];


it works on most inputs but if a string starts with a keyword like like
int, whilefoo, forbar
the parser fails to parse this strings.
how can i get this parser correct?

jv_ jv_
Answer

Your problem is caused by the semantics of the difference operator in Spirit. When you have a - b Spirit does the following:

  • check whether b matches:
    • if it does, a - b fails and nothing is parsed.
    • if b fails then it checks whether a matches:
      • if a fails, a - b fails and nothing is parsed.
      • if a succeeds, a - b succeeds and parses whatever a parses.

In your case (unchecked_identifier - keyword) as long as the identifier starts with a keyword, keyword will match and your parser will fail. So you need to exchange keyword with something that matches whenever a distinct keyword is passed, but fails whenever the keyword is followed by something else. The not predicate (!) can help with that.

auto const distinct_keyword = x3::lexeme[ keyword >> !(x3::alnum | '_') ];

Full Sample (Running on Coliru):

//#define BOOST_SPIRIT_X3_DEBUG
#include <iostream>
#include <boost/spirit/home/x3.hpp>

namespace parser {
    namespace x3 = boost::spirit::x3;

    struct keywords_t : x3::symbols<x3::unused_type> {
        keywords_t() {
            add("for", x3::unused)
                    ("in", x3::unused)
                    ("while", x3::unused);
        }
    } const keywords;

    x3::rule<struct identifier_tag,std::string>  const identifier ("identifier");

    auto const distinct_keyword = x3::lexeme[ keywords >> !(x3::alnum | '_') ];
    auto const unchecked_identifier = x3::lexeme[(x3::alpha | x3::char_('_')) >> *(x3::alnum | x3::char_('_'))];


    auto const identifier_def = unchecked_identifier - distinct_keyword;

    //This should also work:
    //auto const identifier_def = !distinct_keyword >> unchecked_identifier


    BOOST_SPIRIT_DEFINE(identifier);

    bool is_identifier(const std::string& input)
    {
        auto iter = std::begin(input), end= std::end(input);

        bool result = x3::phrase_parse(iter,end,identifier,x3::space);

        return result && iter==end;
    }
}



int main() {

    std::cout << parser::is_identifier("fortran") << std::endl;
    std::cout << parser::is_identifier("for") << std::endl;
    std::cout << parser::is_identifier("integer") << std::endl;
    std::cout << parser::is_identifier("in") << std::endl;
    std::cout << parser::is_identifier("whileechoyote") << std::endl;
    std::cout << parser::is_identifier("while") << std::endl;
}
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