Re: Making tab-complete.c easier to maintain

On Thu, Oct 22, 2015 at 02:36:53PM -0700, Tom Lane wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> > On Wed, Oct 21, 2015 at 8:54 PM, Thomas Munro
> > <thomas(dot)munro(at)enterprisedb(dot)com> wrote:
> >> (Apologies for sending so many versions. tab-complete.c keeps moving
> >> and I want to keep a version that applies on top of master out there,
> >> for anyone interested in looking at this. As long as no one objects
> >> and there is interest in the patch, I'll keep doing that.)
>
> > I don't want to rain on the parade since other people seem to like
> > this, but I'm sort of unimpressed by this. Yes, it removes >1000
> > lines of code, and that's not nothing. But it's all mechanical code,
> > so, not to be dismissive, but who really cares? Is it really worth
> > replacing the existing notation that we all know with a new one that
> > we have to learn? I'm not violently opposed if someone else wants to
> > commit this, but I'm unexcited about it.
>
> What I would like is to find a way to auto-generate basically this entire
> file from gram.y.

I've been hoping we could use a principled approach for years. My
fondest hope along that line would also involve catalog access, so it
could correctly tab-complete user-defined things, but I have the
impression that the catalog access variant is "much later" even if
autogeneration from gram.y is merely "soon." I'd love to be wrong
about that.

> That would imply going over to something at least
> somewhat parser-based, instead of the current way that is more or less
> totally ad-hoc. That would be a very good thing though, because the
> current way gives wrong answers not-infrequently, even discounting cases
> that it's simply not been taught about.

Indeed.

> I have no very good idea how to do that, though. Bison does have a
> notion of which symbols are possible as the next symbol at any given
> parse point, but it doesn't really make that accessible. There's a lack
> of cooperation on the readline side too: we'd need to be able to see the
> whole query buffer not just the current line.

This may be on point:

http://stackoverflow.com/questions/161495/is-there-a-nice-way-of-handling-multi-line-input-with-gnu-readline

I suspect we might have to stop pretending to support alternatives to
libreadline if we went that direction, not that that would necessarily
be a bad idea.

Cheers,
David.
--
David Fetter <david(at)fetter(dot)org> http://fetter.org/
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