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

Corey Huinker <corey(dot)huinker(at)gmail(dot)com> writes:
> Reading this, I started to wonder "so how did I get that impression?" and I
> found this from Feb 9:

> 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.

Yeah, it's not the greatest thing for interactive usage, but as we
already discussed, this feature needs to be optimized for scripting not
interaction --- and even a bit of thought shows that the current behavior
is disastrous for scripting. If your only suggestion for getting sane
behavior in a script is "set ON_ERROR_STOP", you've failed to provide
useful error handling.

One point here is that we need to distinguish problems in the expression,
which could arise from changing variable values, from some other types of
mistakes like \elif with no preceding \if. When you see something like
that you pretty much have to treat it as a no-op; but I don't think that's
a problem for scripting usage.

We could imagine resolving this tension by treating failed \if expressions
differently in interactive and noninteractive cases. But I fear that cure
would be worse than the disease.

regards, tom lane