On Tue, Mar 13, 2012 at 9:16 PM, Joachim Durchholz <j...@durchholz.org> wrote: > Am 13.03.2012 17:24, schrieb Sergiu Ivanov: > >> Apparently, operator precedence parsing together with some additional >> processing of the string may be enough. This may be a good start. > > > For the umteenth time, NO string processing. You'll be going straight to > ambiguity hell if you do that - you'll end with a parser that misparses > strings just because you weren't careful with the string substitutions. > > If you want to substitute, do it on the AST.
Yes, sure. Sorry, didn't think before hitting Send :-( >> For more complicated expressions (like "limit sin(x)/x when x goes to >> 0"), I think that operator precedence will not be enough. > > > In fact this kind of stuff is easy. Just make "limit", "when", and "goes to" > bind less tightly than operators. > > What's hard is unary minus. Or anything where you want the same symbol on > different precedence levels depending on syntactic context. I see; I did a really poor job thinking over the previous message, sorry. >> In that >> >> case I'd go for a generator of one of the more powerful parsers (LR >> for example). > > > Did you ever write a nontrivial LR grammar? > Fixing the inevitable shift-reduce conflicts is hard. Really hard. > Fixing the conflicts also means changing the grammar. Which means that the > grammar will produce parse trees that do not reflect the original grammar > anymore. > Adding another rule to an LR grammar tends to break it. All over the place. > Basically, you need to restart the conflict-fixing process. > > We'd need an LR grammar expert, or live with the suffering. I see. My course on compilers was seriously biased to simple explanations of simple algorithms; my practical experience is the weakest part of my knowledge in this area. Whenever I know that conflicts can be fixed, I rarely bother to see how hard it can to fix them practically. Sergiu -- You received this message because you are subscribed to the Google Groups "sympy" group. To post to this group, send email to sympy@googlegroups.com. To unsubscribe from this group, send email to sympy+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/sympy?hl=en.
