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

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Corey Huinker <corey.huinker@gmail.com>
Cc: Fabien COELHO <coelho@cri.ensmp.fr>
Cc: Greg Stark <stark@mit.edu>
Cc: Erik Rijkers <er@xs4all.nl>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Daniel Verite <daniel@manitou-mail.org>
Cc: Jim Nasby <Jim.Nasby@bluetreble.com>
Cc: PostgreSQL <pgsql-hackers@postgresql.org>
Cc: pgsql-hackers-owner@postgresql.org
Subject: Re: \if, \elseif, \else, \endif (was Re: PSQL commands: \quit_if, \quit_unless)
Date: Wed, 22 Feb 2017 16:45:54 -0500
Message-ID: <18672.1487799954@sss.pgh.pa.us> (raw)
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Corey Huinker <corey.huinker@gmail.com> writes:
> On Wed, Feb 22, 2017 at 4:00 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> One thing I'm wondering is why the "active_branch" bool is in "pset"
>> and not in the conditional stack.  That seems, at best, pretty grotty.
>> _psqlSettings is meant for reasonably persistent state.

> With the if-stack moved to MainLoop(), nearly all the active_branch checks
> could be against a variable that lives in MainLoop(), with two big
> exceptions: GetVariable() needs to know when NOT to expand a variable
> because it's in a false-block, and get_prompt will need to know when it's
> in a false block for printing the '@' prompt hint or equivalent, and pset
> is the only global around I know of to do that.

Dunno, that sounds a lot like an "if the only tool I have is a hammer,
then this must be a nail" argument.  pset should not accrete every single
global variable in psql just because it's there.  Actually, there's a
pretty fair amount of stuff in it already that should not be there by any
reasonable interpretation of what it's for.  Inventing a PsqlFileState or
similar struct might be a good idea to help pull some of that cruft out of
pset and get it back to having a reasonably clearly defined purpose of
holding "current settings".

So I think that if you're intent on this being a global variable, it might
as well be a standalone global variable.  I was wondering more about
whether we shouldn't be passing the condition-stack top pointer around
to places that need to know about conditional execution.  get_prompt would
be one if we decide that the prompt might need to reflect this (a question
that still seems undecided to me --- I think we'd be better off with "this
command was ignored" warning messages).  I'm failing to follow why
GetVariable would need to care.

			regards, tom lane


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  Cc: tgl@sss.pgh.pa.us, corey.huinker@gmail.com, coelho@cri.ensmp.fr, stark@mit.edu, er@xs4all.nl, robertmhaas@gmail.com, daniel@manitou-mail.org, Jim.Nasby@bluetreble.com, pgsql-hackers-owner@postgresql.org
  Subject: Re: \if, \elseif, \else, \endif (was Re: PSQL commands: \quit_if, \quit_unless)
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