On Fri, 11 Mar 2005, Frank Heckenbach wrote: > Laurence Finston wrote: > > > I've found that I prefer using Flex and Bison separately rather than > > together. If you're just starting out learning Bison, I think you > > might find it easier to write your own `yylex()' function rather than > > using Flex. > > Actually why? For lexing identifiers, numbers, the usual stuff, it's > one line in flex, and loops, sometimes switches and manual > loop-ahead in C. > And for similar-beginning tokens (e.g. `&', `&&', > `&=' in C), IMHO it gets more readable in flex (one entry per item, > can be group topically -- arithmetics, comparisons, ...) than in > manual code (where they have to be handled together after reading > the first `&').
> So I'm interested why you find the manual way easier. I had problems generating a reentrant scanner using Flex, and I wasn't able to find out what was going wrong. There was some discussion about this on this list at the time, about a year ago. In my scanner, I use what I call the "category code" approach rather than regular expressions. It's the way Knuth used for TeX and Metafont. I'm pretty sure that any scanner written in one of these ways could be reformulated using the other one. However, I find that the category code approach allows for finer control. At least one other person has had problems with regular expressions and posted a question to this list since I've been a subscriber. It might not be possible to implement a couple of things that my scanner does in a straightforward way with Flex. Therefore, I think that using Flex might artificially limit the kinds of things people do with Bison. I don't recall the details, since it's been awhile since I've worked on this part of my program. Nor do I think that the power of Flex is needed for the kind of scanning Bison needs. I like to use Flex when I can implement functionality in its actions, rather than just passing tokens to Bison. If someone wants to learn Bison, I think it might simplify the task if he or she could just concentrate on Bison, rather than having to learn Flex at the same time. I also think that implementing `yylex()' oneself might give one a better insight into what it does and what Bison's requirements are. This might help one to understand how to use Flex well in combination with Bison, if one later decides to use it. I try to avoid using `switch' (I just don't like it), and I don't have to examine the lookahead token too often. Otherwise, I like doing the sort of somewhat low-level programming you describe. Clearly, this is a matter of taste, and I certainly don't think there's anything wrong with using Bison in combination with Flex, if someone prefers to do this. I might even do it myself, if I ever really needed regular expressions when scanning. However, I thought it was worthwhile to point out that it isn't necessarily the best way under all circumstances. Laurence _______________________________________________ Help-bison@gnu.org http://lists.gnu.org/mailman/listinfo/help-bison
