Re: LEX and YACC, jeff & mutt

In article <201@lgc.UUCP> arthur@lgc.UUCP (arthur) writes:
>If someone were to ask you for the reason/s why you wrote your SQL II
>(BNF as given by ANSI) interpreter by way of recursive descent
>parsing, as opposed to simply using tools like LEX and YACC, what
>would you say ?....


I would say that I made a mistake.


>I can only think of such obvious reasons as :
>1. better error handling ( of course, you have total control over
> what you will do with silly syntactic or semantic errors - blow
> them out of the system....)


I've always thought that this was true in theory, but false in practice.
In theory, it is possible to give better error handling in recursive
descent parsers, but in practice their error handling is usually worse
(or not better than) YACC parsers.


Two relevant stories:


When I was in school, I was part of a two person team which wrote a
compiler for most of C. It took me about two evenings of reading and
one evening of coding to add basic error handing. I'm sure that if I
had put a week or a month into it, I could have come up with better
error messages and recovery.


When I graduated, I maintained a compiler for an expert system shell,
which was recursive descent. Its error messages were terrible, and
generating them took up a fair about of code.


Specifics:


Recursive descent parsers are much harder to maintain than YACC parsers.
(I've maintained a parser in each, both of which were written by others.)
Recursive descent parsers always seem to grow little "ad hoc" extensions,
and are a maintenance nightmare, while I find a YACC specification to be
a good head start in understanding the code.


YACC makes it easy to restart a parse from an error. This is worth the
price of admission alone! Most recursive descent parsers seem to punt
after the first error.


Given a file with four errors (on lines 23 30, 45 and 108), YACC based
parsers tend to give messages like this:
      syntax error line 23
      syntax error line 23
      syntax error line 30
      syntax error line 31
      syntax error line 45
      syntax error line 108
      6 errors, so no code generated


While recursive descent tend to give error messages like this:
      missing '[' line 23
      parsing aborted


I'd much rather have all the errors, than a little more detail with one
particular error.


Joshua Levy joshua@atherton.com home:(415)968-3718
                                                {decwrl|sun|hpda}!athertn!joshua work:(408)734-9822

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