I. Oppenheim wrote: > On Mon, 7 Jul 2003, Bert Van Vreckem wrote: > > >> I. Oppenheim wrote: >> >>> If the ABC community as a whole could provide a standard >>> complying parser, not every developer that wishes to use ABC >>> would have to reinvent the wheel again and again. The current >>> situation is in nobody's interest. >> >> We're already there: using Henrik's BNF spec, a developer can >> generate a parser with tools like lex and yacc. > > > Unfortunately, it is not trivial at all to convert a BNF definition > into a working parser. > > 1/ the parser should be able to ignore future ABC extentions in a > graceful way, whatever they may be; 2/ the parser should be able to > understand (rather than ignore) non-standard legacy code;
It should also have an option that would disallow anything non-standard, such as gcc's -ansi switch.
> 1/ How can Lex know whether a "C" it scans is a note name, or the > first letter of a staff label? Maybe it's even the beginning of the > composer field! > > 2/ How can Yacc know whether "||" is one double bar symbol, or two > single bar symbols next to each other? > > 3/ How can Lex/Yacc know whether "E:|" is a (mid-tune) field that > starts with a pipe, or a note with a repeat sign next to it? > > I do not know whether Lex and Yacc are the proper tools to deal with > ABC.
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.
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.
In the third case, if "E:|" makes sense as a body field, then whitespace would be required to distinguish it from a note followed by a repeat, or perhaps better, do not allow a-g and A-G as body fields.
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