BETWEEN SYMMETRIC/ASYMMETRIC
Hi All,
As part of my ongoing quest to understand grammar files, I've been trying to
implement BETWEEN SYMMETRIC/ASYMMETRIC.
I've attached my current work. Can someone please look and tell me if I'm
on the right track? With this patch, I get parse errors after BETWEEN if I
go:
SELECT 2 BETWEEN ASYMMETRIC 1 and 3;
or
SELECT 2 BETWEEN SYMMETRIC 1 and 3;
So it doesn't seem to be working - I don't know why!!
Don't look at the NOT BETWEEN stuff - I've not done it yet.
I was forced to put SYMMETRIC and ASYMMETRIC as reserved words - anything
else seemed to give shift/reduce errors. Is there anything I can do about
that?
Chris
*sigh*
I actually attached the diff this time...
Chris
Show quoted text
-----Original Message-----
From: pgsql-hackers-owner@postgresql.org
[mailto:pgsql-hackers-owner@postgresql.org]On Behalf Of Christopher
Kings-Lynne
Sent: Wednesday, 3 April 2002 12:26 PM
To: Hackers
Subject: [HACKERS] BETWEEN SYMMETRIC/ASYMMETRICHi All,
As part of my ongoing quest to understand grammar files, I've
been trying to
implement BETWEEN SYMMETRIC/ASYMMETRIC.I've attached my current work. Can someone please look and tell me if I'm
on the right track? With this patch, I get parse errors after
BETWEEN if I
go:SELECT 2 BETWEEN ASYMMETRIC 1 and 3;
or
SELECT 2 BETWEEN SYMMETRIC 1 and 3;
So it doesn't seem to be working - I don't know why!!
Don't look at the NOT BETWEEN stuff - I've not done it yet.
I was forced to put SYMMETRIC and ASYMMETRIC as reserved words - anything
else seemed to give shift/reduce errors. Is there anything I can do about
that?Chris
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Attachments:
between.txttext/plain; name=between.txtDownload+37-29
"Christopher Kings-Lynne" <chriskl@familyhealth.com.au> writes:
I was forced to put SYMMETRIC and ASYMMETRIC as reserved words - anything
else seemed to give shift/reduce errors. Is there anything I can do about
that?
First thought is "don't try to be cute": forget the opt_asymmetry
clause, and instead spell out six productions
a_expr BETWEEN b_expr AND b_expr
a_expr NOT BETWEEN b_expr AND b_expr
a_expr BETWEEN SYMMETRIC b_expr AND b_expr
a_expr NOT BETWEEN SYMMETRIC b_expr AND b_expr
a_expr BETWEEN ASYMMETRIC b_expr AND b_expr
a_expr NOT BETWEEN ASYMMETRIC b_expr AND b_expr
I have not checked that this will work, but usually the cure for parse
conflicts is to postpone the decision about which production applies.
The reason opt_asymmetry forces SYMMETRIC and ASYMMETRIC to become
reserved is that it requires a premature decision. Given, say
a_expr BETWEEN . SYMMETRIC
(where . means "where we are now" and SYMMETRIC is the current lookahead
token), an LR(1) parser *must* decide whether to reduce opt_asymmetry as
null, or to shift (implying that opt_asymmetry will be SYMMETRIC); it
has to make this choice before it can look beyond the SYMMETRIC token.
If SYMMETRIC might be a regular identifier then this is unresolvable
without more lookahead. The six-production approach avoids this problem
by not requiring any shift/reduce decisions to be made until an entire
clause is available.
On second thought there may be no other way out. Consider
foo BETWEEN SYMMETRIC - bar AND baz
Is SYMMETRIC a keyword (with "-" a prefix operator) or an identifier
(with "-" infix)? This example makes me think that SYMMETRIC has to
become reserved. But I wanted to point out that opt_asymmetry is
certainly a loser based on lookahead distance.
regards, tom lane