[boost] Spirit skip parser questions.

5 Apr 2004

      I am trying to write a parser which skipps white space and C, Java style  
comments. To get familiar with spirit, I did some experiments, here is a 
question.

First, I write a simple grammar to accept some non-white space chars.
////////////////////////////////////////////////////////////////////////////////
struct FOO_GRAMMAR : public grammar<FOO_GRAMMAR>
{
	template <typename ScannerT>
	struct definition
	{
		typedef rule<ScannerT> rule_t;

		definition(ABNF_GRAMMAR const & self)
		{
			r_LETTER =
				range_p('a','z')
				| range_p('A','Z')
				| '.';
			r = (+r_LETTER);
		}

		rule_t	r, r_LETTER;
		rule_t const& start() const { return r; }

	};
};
////////////////////////////////////////////////////////////////////////////////

I used FOO_GRAMMAR with space_p as skip parser:
//////////////////////////////////////////////////////////////////////////////
FOO_GRAMMAR g;
std::string line = "This is a test parser.";
parse_info<> info = parse(line.begin(),line.end(), g, space_p);
////////////////////////////////////////////////////////////////////////////////
This works perfectly.

Then, I tried to wrap the skip parser in a skip grammar, since eventually I 
need a more complicated parser. So I wrote the skip grammar as:
///////////////////////////////////////////////////////////////////////////////
struct SKIP_GRAMMAR : public grammar<SKIP_GRAMMAR>
{
	template <typename ScannerT>
	struct definition
	{
		typedef rule<ScannerT> rule_t;

		definition(SKIP_GRAMMAR const & self)
		{
			rule_t r_SKIP = space_p;
		}

		rule_t	r_SKIP;
		rule_t const& start() const { return r_SKIP; }
	};
};
/////////////////////////////////////////////////////////////////////////

I tried to parse the same sentence as before:
///////////////////////////////////////////////////////////////////////////
FOO_GRAMMAR g;
SKIP_GRAMMAR skip;
std::string line = "This is a test parser.";
parse_info<> info = parse(line.begin(),line.end(), g, skip);
//////////////////////////////////////////////////////////////////////////

This time the parsing failed.

Can anyone tell me why? I thought grammars are just like rules.

And any suggestion on building a parser skipping certain pattern? I just can 
figure it out by reading the doc.

Thank you.

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[boost] Spirit skip parser questions.

Jia Pu