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

Tom Lane wrote:

> Ah, I see why *that* wants to know about it ... I think. I suppose you're
> arguing that variable expansion shouldn't be able to insert, say, an \else
> in a non-active branch? Maybe, but if it can insert an \else in an active
> branch, then why not non-active too? Seems a bit inconsistent.

Are we sold on the idea that conditionals should be implemented
by meta-commands, rather than for example terminal symbols of
a new grammar on top of the existing?

To recall the context, psql variables are really macros that may
contain meta-commands, and when they do they're essentially
injected and executed at the point of interpolation. That's more
or less what started this thread: demo'ing how we could exit
conditionally by injecting '\q' or nothing into a variable, and
saying that even if doable it was pretty weird, and it would be
better to have real conditional structures instead.

But when conditional structures are implemented as
meta-commands, there's the problem that this structure
can be generated on the fly too, which in a way is no less weird.
While I think that the introduction of conditionals in
psql is great, I'm getting doubtful about that part.
Are there popular script languages or preprocessors
that accept variables/macros instead of symbols to structure
the flow of instructions? I can't think of any myself.

Best regards,
--
Daniel Vérité
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