boost::spirit composing grammars from grammars

Question

I have figured out how to use spirit -- i.e., I have written a moderately complex grammar. I always take the approach of growing a program -- one subsystem at a time. I've written the data structures for a complex model which has 4 types at the highest level.

I would like to use the grammar composed from rules approach to parse the top level types one type at a time -- i.e., I want to write 4 grammars with one top level grammar. If this is possible (which I am beginning to doubt), could someone please post a snippet or a reference to a project that does this.

One top level grammar with 50+ (possible a lot more) rules (for proper error handling) does not sound like fun (TMP code is volatile / slow to compile, and provides useless error messages).

Answer 1

simplified from an actual program, Qi should work the same as Karma.

template<class Iter>
struct subgrammar_1
: karma::grammar<Iter, ...>
{
    ...
}

template<class Iter>
struct top_level_grammar
: karma::grammar<Iter, ...>
{
    top_level_grammar() : top_level_grammar::base_type(start)
    {
        start %= r1 | r2;
    }
    karma::rule<Iter, ...> r1;
    subgrammar_1<Iter> r2;
    ...
}