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

On Sat, Mar 11, 2017 at 5:45 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

>
> * Whether or not you think it's important not to expand skipped variables,
> I think that it's critical that skipped backtick expressions not be
> executed.
> ​ [...] ​
> I do not think that a skipped \if or \elif
> should evaluate its argument at all.
>
>
​[...]
​

> * I'm not on board with having a bad expression result in failing
> the \if or \elif altogether. It was stated several times upthread
> that that should be processed as though the result were "false",
> and I agree with that.

​+1​

​Oddly, Corey was using you as support for this position...though without
an actual quote:

"""
Tom was pretty adamant that invalid commands are not executed. So in a case
like this, with ON_ERROR_STOP off:

\if false
\echo 'a'
\elif true
\echo 'b'
\elif invalid
\echo 'c'
\endif

Both 'b' and 'c' should print, because "\elif invalid" should not execute.
The code I had before was simpler, but it missed that.
"""
https://www.postgresql.org/message-id/CADkLM%3De9BY_-
PT96mcs4qqiLtt8t-Fp%3De_AdycW-aS0OQvbC9Q%40mail.gmail.com

Also,

Robert made a comment somewhere along the line about users wanting to
simply re-type the intended line if the "invalid" was interactive and due
to a typo. That concern is pretty much limited to just the "\if" situation
- if you typo an "\elif" block you can just type "\elif" again and begin
yet another "\elif" block. I say we live with it and focus on the UX - if
you type \if no matter what happens after you hit enter you are in a
conditional block and will need to \endif at some point. Re-typing the
correct \if command will just make you need another one of them.

David J.