تحليل النحو الخالي من السياق

Question

أعرف أن محلل محلل من القاعدة من أسفل إلى أعلى من المحللين من أعلى إلى أسفل لأنه يمكن أن يقبل قواعد اللغة اليسرى العديدية، ما يمكن أن يكون أسبابا أخرى نفضل تحليل القمة من أعلى إلى أسفل؟

Answer 1

Theoretically speaking, the LL(k) grammars are always strict subsets of the LR(k) grammars for any k, so deterministic predictive bottom-up parsers can accept a strictly greater set of grammars than than deterministic predictive top-down parsers. This also means that any LL(k) grammar is also LR(k).

Also, a tricky proof shows that any deterministic CFL (a CFL accepted by a deterministic push down automaton) has an LR(1) grammar, which means that LR grammars correspond precisely to those languages that have efficient stack-based parsing algorithms.

That said, if you allow for more general parsing algorithms like Unger's algorithm, Earley's algorithm, or the CYK algorithm, then top-down and bottom-up methods exist for parsing arbitrary CFGs. These algorithms can be much slower than the predictive methods, though, so they typically aren't used for programming languages.

Hope this helps!

Answer 2

We have bottom-up parsers generators like byson. Using them is much simpler then writing parsers manually.
Also, recursive descent parsers make all operations right-associative by default, which is incorrect for arithmetics. Turning them back to left-associative requires additional steps in parsing.