[Boost-users] [spirit] working with end_p

From: james.jones_at_[hidden]
Date: 2006-07-07 08:59:53

From: Neil Hunt <boost_at_[hidden]>
> Hello all,
> 
> Given a simple spirit grammar (simplified from my real grammer and 
> untested) such as:
> 
> file = *line;
> line = sensible_line >> ending >> *comment;
> sensible_line = confix_p( '\"', *c_escape_ch_p, '\"' );
> comment = !( ch_p(';') >> ( anychar_p - eol_p ) ) >> ending;
> ending = *blank_p >> ( eol_p | end_p );
> 
> 
> Note: any given line can be legitimately terminated by the end of the 
> file or end of line
> 
> And there can be arbitrary numbers of comment lines between sensible lines.
> 
> However since 'comment' is terminated with 'ending', the parser ends up 
> in an infinite loop matching the kleene star on end_p because it returns 
> zero length (doesn't advance the scanner). At least I think this is the 
> problem.

Indeed. You can fix it by adjusting your grammar thus:

line = sensible_line >> (ending >> comment)* >> ending;
sensible_line ... as before
comment = !( ch_p(';') >> ( anychar_p - eol_p ) );
ending ... as before

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