Re: PSQL commands: \quit_if, \quit_unless

>
> My 0,02 €, which is just a personal opinion:
>
> I think that handling manually "!/not" would be worth the effort rather
> than having two commands, especially if the boolean expression syntax may
> be extended some day and the negative if would become obsolete.
>
> If there is a negative condition syntax, I would slightly prefer \ifnot to
> \if_not or worse \unless. I would disaprove strongly of \unless because it
> looses the clear symmetry with a closing \fi.
>
> --
> Fabien.

Pavel had previously written this patch
http://ftp.pgpi.org/pub/databases/postgresql/projects/pgFoundry/epsql/epsql/epsql-8.5-develop/psql-enhanced-macros.diff
which I leave here as a point of reference.

Obviously, there's a lot more in there than we'd need, and it's back quite
a few versions.

I agree that the boolean tests available should be *very* simple, and all
of the weight of complex calculation should be put in SQL, like we do with
\gset

I propose those be:

\if STRING : true if STRING evaluates to true via ParseVariableBool, empty
means false
\ifnot STRING: true if STRING evaluates to false via ParseVariableBool,
empty means false
\ifdef psql_var: true if psql_var is defined
\ifndef psql_var: true if psql_var is not defined, helpful for when --set
var=val was omitted and should be defaulted

A full compliment of \elseif \elseifnot \elseifdef \elseifndef, each
matching the corresponding \if* above. I'm ok with these being optional in
the first revision.

\else - technically we could leave this out as well
\endif

Then seems like we need an if-state-stack to handle nesting. At any given
point, the mode could be:

1. None

psql currently isn't in an if-then structure, no non-conditional statements
are skipped. any conditional commands other than \if* are an error.
For that matter, the script can always add a new level to the stack with an
\if* command.

2. If-Then

psql is currently executing statements in an \if* branch that evaluated to
true. valid conditional commands are: \if*, \else* \endif

3. If-Skip

psql is currently in a block that evaluated to false, and will still parse
commands for psql-correctness, but will skip them until it encounters an
\endif or \else*

4. Else-Then

Just like If-Then, but encountering an \else* command would be an error.

5. Else-Skip

Just like If-Skip, but encountering an \else* command would be an error.

The only data structure we'd need is the stack of enums listed above. Any
commands would check against the stack-state before executing, but would
otherwise be parsed and checked as they are now. The new \commands would
manipulate that stack.

Does that seem work-able?