Bison 语法/解析器的多个起点

Question

OK, so I have a complete (and working) Bison grammar.

The thing is I want to be able to set another starting point ( %start ) if I wish.

How is this doable, without having to create a separate grammar/parser?

Answer 1

I'm going to try to put together a version of yacc that does this. There is one complication that makes this not as trivial as it seems: the question of what constitutes an "end" symbol. The kind of place where this is of greatest use is in processing chunks in mid-stream (Knuth's TeX processor for [c]Web does this, for instance). Along these lines, another example where this can be used is in providing a unified parser for both the pre-processing layer and language layer and in processing individual macros themselves as entire parsing units (as well as being able to account for which macro bodies are common syntactic units like "expression" or "statement" and which are not).

In those kinds of applications, there is no natural "end" symbol to mark off the boundary of a segment for parsing. Normally, the LR method requires this in order to recognize when to take the "accept" action. Otherwise, you have accept-reduce (and even accept-shift) conflicts to contend with!