Thread: 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

> -----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/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
>
>
> ---------------------------(end of broadcast)---------------------------
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>

"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_expra_expr NOT BETWEEN b_expr AND b_expra_expr BETWEEN SYMMETRIC b_expr AND b_expra_expr
NOTBETWEEN SYMMETRIC b_expr AND b_expra_expr BETWEEN ASYMMETRIC b_expr AND b_expra_expr NOT BETWEEN ASYMMETRIC b_expr
ANDb_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