On Mon, 7 Jul 2003, T.M. Sommers wrote:
These are not problems with lex and yacc specifically; any program parsing abc will have to face them. They are ambiguities, or potential ambiguities, in the language itself.
In the first case above, there should be no problem: the composer field is only in the header and starts a line, and the staff labels are in the V field also in the header.
Contrary to what one would expect, this is more problematic to deal with in Lex/Yacc than in other parsers. The point is that Lex/Yacc do not have facilities for context dependent tokenizing (are we in a header or outside) nor a clean interface to exchange information between the scanner and the parser. So one would have to resort to global flags, which would make the parser among other things thread-unsafe.
flex has context dependent tokenizing; it calls the contexts 'start conditions'. Since flex is free, and runs just about everywhere, I don't see why a reference implementatin could not use it.
In the second case (which is really a lexing issue), either disallow two single bars next to each other (does such a construct even make sense?), or require whitespace between them.
Well,
bar: '|' { $$.type = T_REG_BAR } | '|' '|' { $$.type = T_DBL_BAR_1 } ;
Is all it takes to create a shift/reduce conflict in Yacc.
'|' and '||' should be tokens, and distinguished in the lexer. C manages to distinguish between them without any problems.
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