Re: \if, \elseif, \else, \endif (was Re: PSQL commands: \quit_if, \quit_unless)

On Thu, Feb 9, 2017 at 3:13 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> > I (still) think this is a bad design. Even if you've got all the
> > messages just right as things stand today, some new feature that comes
> > along in the future can change things so that they're not right any
> > more, and nobody's going to relish maintaining this.
>
> FWIW, I tend to agree that this is way overboard in terms of the amount of
> complexity going into the messages. Also, I do not like what seems to
> be happening here:
>
> >> $ psql test < test2.sql -v ON_ERROR_STOP=0
> >> unrecognized value "error" for "\if <expr>": boolean expected
> >> new \if is invalid, ignoring commands until next \endif
>
> IMO, an erroneous backslash command should have no effect, period.
> "It failed but we'll treat it as if it were valid" is a rathole
> I don't want to descend into. It's particularly bad in interactive
> mode, because the most natural thing to do is correct your spelling
> and issue the command again --- but if psql already decided to do
> something on the strength of the mistaken command, that doesn't work,
> and you'll have to do something or other to unwind the unwanted
> control state before you can get back to what you meant to do.
>
> regards, tom lane
>

One way around this is to make the small change: commands with invalid
expressions are ignored in interactive mode.

Another way around it would be to ignore branching commands in interactive
mode altogether and give a message like "branching commands not supported
in interactive mode". That'd get rid of a lot of complexity right there. I
for one wouldn't miss it. The only use I saw for it was debugging a script,
and in that case the user can be their own branching via selective
copy/paste.

Do either of those sound appealing?