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Re: [HACKERS] Re: Patch for more readable parse error messages

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Jeroen van Vianen <jeroen(at)design(dot)nl>, pgsql-patches(at)postgreSQL(dot)org, pgsql-hackers(at)postgreSQL(dot)org
Subject: Re: [HACKERS] Re: Patch for more readable parse error messages
Date: 2000-06-09 12:40:21
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Lists: pgsql-hackerspgsql-patches
Here is more information about it.

> Jeroen van Vianen <jeroen(at)design(dot)nl> writes:
> >> Does this work with a non-bison parser?  It looks mighty
> >> bison-dependent to me...
> > I'm not sure, but it probably is flex dependent (but Postgres always needed 
> > flex anyway). I'm not aware of any yacc / byacc / bison dependencies. Don't 
> > know if anybody has been successful building Postgres with another parser 
> > generator.
> Um, you're right of course --- those are lexer not parser datastructures
> you're poking into.  Sorry for my confusion.
> We do in fact work with non-bison parser generators, or did last time
> I tried it (around 6.5 release).  I would not like us to stop working
> with non-bison yaccs, since bison's output depends on alloca() which
> is not available everywhere.
> I'm not sure about the situation with lexers.  We have been saying for
> a long time that flex was required, but since we got rid of the
> scanner's use of trailing context ('/' rules) I think there is a better
> chance that it would work with vanilla lex.  Anyone want to try that
> with current sources?
> > BTW, as we ship flex's output lex.yy.c (as scan.c) and bison's output
> > (gram.c) in the distribution, any user would be able to compile the
> > sources, but if they want to start hacking the .l or .y files, they'll
> > need appropriate tools.
> Right.  I am not aware of any portability problems with flex's output
> as there are with bison's, so it may be that the concern is moot.
> We may just be able to say "use the prebuilt scan.c or get flex; we
> don't care about supporting vendor lexes anymore".
> I do see a potential problem with this patch that's not related to
> portability questions; it is that you're assuming that the lexer's
> furthest penetration into the source text is a good place to point
> at for parser errors.  That may not be true always.  In particular,
> I've been advocating solving some other problems by inserting a
> one-token lookahead buffer between the parser and the lexer.  If that
> happens then you'd be off by (at least) one token in some cases.
> I think the way that this sort of thing is customarily handled in
> "real" compilers is that each token carries along an indication of
> just where it was found in the source, and then error messages can
> finger the right place without making assumptions about synchronization
> between different phases of the scanning/parsing process.  That might
> be more work than we can justify for SQL queries; not sure.
> BTW, I think that the immediate problem of generating a good error
> message for unterminated comments and literals could be solved in other
> ways.  This patch or something like it might be cool anyway, but you
> should bear in mind that printing out a query and then a marker that's
> supposed to line up with something in the query doesn't always work
> all that well.  Consider a query that's several dozen lines long,
> such as a large table definition.  If we had more control over the
> user interface and could highlight the offending token directly,
> I'd be more excited about doing something like this.  (Actually, you
> could partially address that problem by only printing one line's worth
> of query text leading up to the error marker point.  It would still be
> tricky to get it right in the presence of newlines, tabs, etc.)
> 			regards, tom lane
> ************

  Bruce Momjian                        |
  pgman(at)candle(dot)pha(dot)pa(dot)us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

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