On May 31, 2011, at 2:30 PM, Waldemar Horwat wrote: > I would not want to use anything like a PEG to standardize a grammar. Here's > why: > > PEG being unambiguous by construction simply means that it resolves all > ambiguities by picking the earliest rule. This turns all rules following the > first one into negative rules: X matches Z only if it DOESN'T match a Y or a > Q or a B or .... You could pick the same strategy to disambiguate an LR(1) > grammar, and it would be equally bad. > > Negative rules are the bane of grammars and behind the majority of the > problems with the C++ grammar, including the examples I listed earlier. They > make a grammar non-understandable because the order of the rules is subtly > significant and makes it hard to reason about when an X matches a Z; a > language extension might expand the definition of Y to make an X no longer > match a Q, and you wouldn't know it just by looking at a grammar with > negative rules. In a positive-rule-only grammar you'd discover the problem > right away because the grammar wouldn't compile.
Thanks -- you've made this point before and I've agreed with it. It helps to restate and amplify it, I think, because my impression is that not many people "get it". PEG users may be happy with their JS parsers at any given point in the language's standard version-space, of course. It still could be that we use LL(1) or another positive-rule-only grammar, of course, but we can hash that out separately. > Negative rules also interact badly with both semicolon insertion and > division-vs-regexp lexer disambiguation. One might naively think that > semicolon insertion would be an ideal match for negative rules: You first > try to parse > > tokens-on-line1 > tokens-on-line2 Heh; this doesn't pass the first rule of ASI fight-club: there's no insertion is there is no error. /be _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
