Re: User friendly compiler-compiler tools [comp.compilers #7671]

|> [re building expression grammars]
|> > If you think it's necessary, add a few rules for implicit
|> > parenthesizing; when in doubt, require explicit parenthesizing.
|>
|> Current operator-precedence parser generator tools such as yacc make
|> this unnecessarily difficult, because they require a total ordering of
|> operator precedences. Future such tools should allow a partial ordering.


If I remember my math education, partial ordering implies
transitivity. I.e., by stating explicit precedences like


'+' < '='
'=' < '&&'


you would get an implicit precedence


'+' < '&&'


which I did not learn in school and is not intuitive to everyone; in
other word, this precedence is probably not a good idea.


As long as there are only a few independent cases, normal BNF is nice
enough. You can simply add the rules for implicit parenthesizing to
the 'expr' rules in the grammar above.
E.g., for the precedence '=' < '&&' (without the associativity of '&&')


expr: expr1 < expr1 && expr1
expr: expr1 && expr1 < expr1
expr: expr1 < expr1 && expr1 < expr1


We can of course apply some factoring and get


expr: comparison && comparison


comparison: expr1 < expr1
comparison: expr1


As soon as the cases interact, things become more complex: The
classical hierarchical scheme with implied transitive precedences has
to be avoided. There also is the problem of LALR (or LL) conflicts.


- anton
--
M. Anton Ertl
anton@mips.complang.tuwien.ac.at
http://www.complang.tuwien.ac.at/anton/home.html
--

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