Re: PSQL commands: \quit_if, \quit_unless

Hello,

>> I think it's really time we seriously considered adding some flow
>> control logic, though.
>
> Yeah, maybe. I'd be interested to see a fully worked out proposal
> for that.

I agree that designing a fuller proposal before including individual parts
would be great and result in a more consistent result.

In order to bootstrap the discussion, I suggest the following:

- boolexpr is a simple "boolean" (t, non 0 int, non empty string.. as
proposed by Corey and Pavel) or !/not boolexp ; it could be extended if
necessary, but I would try to avoid that, as

- actual more complex expressions could be left to the server through SQL
which simplifies the client a lot by avoiding an expression language
altogether

- then having a conditional block is very versatile and can be adapted to
many use cases... maybe all

- \quit CODE, or I would prefer \exit CODE, could be used to exit while
controlling the status

It could look like (although I do not like gset in this context, but
anyway):

SELECT ... AS has_foo_extension \gset
SELECT ... AS has_bla_extension \gset
\if :has_foo_extension
...
\elif :has_bla_extension
...
\else -- no foo nor bla extension
\echo please install foo or bla extension
\exit 1
\fi -- extension
...
SELECT ... AS has_xxx_feature \gset
\if ! :has_xxx_feature
\echo "feature xxx is needed, aborting"
\exit 2
\fi
...

--
Fabien