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

On Fri, Mar 17, 2017 at 11:42 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr> writes:
> >> I also fear that there are corner cases where the behavior would still
> >> be inconsistent. Consider
> >>
> >> \if ...
> >> \set foo `echo \endif should not appear here`
>
> > In this instance, ISTM that there is no problem. On "\if true", set is
> > executed, all is well. On "\if false", the whole line would be skipped
> > because the if-related commands are only expected on their own line, all
> > is well again. No problem.
>
> AFAICS, you misunderstood the example completely, or else you're proposing
> syntax restrictions that are even more bizarre and unintelligible than
> I thought before. We cannot have a situation where the syntax rules for
> backslash commands inside an \if are fundamentally different from what
> they are elsewhere; that's just going to lead to confusion and bug
> reports.
>
> regards, tom lane
>

I think Fabien was arguing that inside a false block there would be no
syntax rules beyond "is the first non-space character on this line a '\'
and if so is it followed with a if/elif/else/endif?". If the answer is no,
skip the line. To me that seems somewhat similar to Tom's suggestion that a
false branch just keeps consuming text until it encounters a \conditional
or EOF.