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

On Thu, Jan 26, 2017 at 3:55 PM, Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr> wrote:

>
> Hello Daniel,
>
> A comment about control flow and variables: in branches that are not
>> taken, variables are expanded nonetheless, in a way that can be surprising.
>> Case in point:
>>
>> \set var 'ab''cd'
>> -- select :var;
>> \if false
>> select :var ;
>> \else
>> select 1;
>> \endif
>>
>> To avoid that kind of trouble, would it make sense not to expand
>> variables in untaken branches?
>>
>
> Hmmm. This case is somehow contrived (for a string, :'var' could be used,
> in which case escaping would avoid the hazard), but I think that what you
> suggest is a better behavior, if easy to implement.
>
> --
> Fabien.
>

Good question, Daniel. Variable expansion seems to be done via
psql_get_variable which is invoked via callback, which means that I might
have to move branch_block_active into PsqlSettings. That's slightly
different because the existing boolean is scoped with MainLoop(), but
there's no way to get to a new MainLoop unless you're in an executing
branch, and no way to legally exit a MainLoop with an unbalanced if-endif
state. In short, I think it's better behavior. It does prevent someone from
setting a variable to '\endif' and expecting that to work, like this:

select case
when random() < 0.5 then '\endif'
else E'\else\n\echo fooled you\n\endif'
end as contrived_metaprogramming
\gset

\if false
:contrived_metaprogramming

I'm sure someone will argue that preventing that is a good thing. Unless
someone sees a good reason not to move that PsqlSettings, I'll make that
change.