How to parse reserved words correctly in boost spirit

Question

I'm trying to parse a sequence of the syntax: < direction > < type > < name >. For example:

in float foo

where the direction can be either in, out, or in_out. I've succeeded in parsing correct text by using a qi::symbols class to convert the direction keywords to an enum.

However, the problem shows when I don't have correct text. Take the example:

int foo

The symbol table parser will except the 'in' part of the 'int' type and so the results will be:

direction: in
type: t
name: foo

And the error is not detected. What's the best way to be able to parse the in, out and in_out reserved words and ensure that they are followed by a non-identifier character so that the 'int' part of the previous text fails?

Thanks

Answer 1

In addition to the "manual" approach suggested by Mike you can

1. use a convenience wrapper rule
2. use the distinct parser direetive from the Spirit Repository

1. Use a convenience wrapper

I just remembered, I once came up with this quick and dirty helper:

static const qi::rule<It, qi::unused_type(const char*)> kw 
      = qi::lit(qi::_r1) >> !qi::alnum;

Which you could use like (using +"lit" to decay the array-ref into const char*):

stmt = 
         kw(+"if") >> '(' >> expr >> ')' >> block
     >> -(kw(+"else") >> block)
     ;

You can make it considerably more convenient

template <std::size_t N>
static auto kw(char const (&keyword)[N]) -> qi::rule<Iterator> {
    // qi::lit has problems with char arrays, use pointer instead.
    return qi::lit(+keyword) >> !qi::alnum;
}

So you can

kw_if   = kw("if");
kw_then = kw("then");
kw_else = kw("else");
kw_and  = kw("and");
kw_or   = kw("or");
kw_not  = kw("not");

2. Use the distinct directive from the Spirit Repository

In addition to the "manual" approach suggested by Mike you can use the distinct parser directive from the Spirit Repository:

• http://www.boost.org/doc/libs/1_55_0/libs/spirit/repository/test/qi/distinct.cpp

int main()
{
    using namespace spirit_test;
    using namespace boost::spirit;

    {
        using namespace boost::spirit::ascii;

        qi::rule<char const*, space_type> r;
        r = distinct::keyword["description"] >> -lit(':') >> distinct::keyword["ident"];

        BOOST_TEST(test("description ident", r, space));
        BOOST_TEST(test("description:ident", r, space));
        BOOST_TEST(test("description: ident", r, space));
        BOOST_TEST(!test("descriptionident", r, space));
    }

    return boost::report_errors();
}