> I don't think so :?xxx is good idea, because for me :xxx is related to
> content, not to metadata
Hmmm. Indeed it is not. I do not see it as an issue, but it is a good
point.
Consider perl "defined $x" or "exists $f{k}". $x/$f{k} should be contents,
but it is not, the dereferencing is suspended by "defined/exists" Yuk,
but simple and effective.
Also with CPP: "#define x 1, #ifdef x", somehow "x" should be the value,
not the name, but yet again it is not dereferenced.
Now consider python: "if 'varname' in locals():" at least it is
consistent, but I cannot say it looks better in the end:-)
So playing around with a value vs metadata is a frequent trick to keep the
syntax simple, even if the logic is not all there as you point out.
> and Robert's tip of using bash syntax is more logical for me (to have
> syntax for default and custom message).
There is no way to simply test for definition in bash, which is exactly
what is needed...
A second issue with sh-like proposal is that it needs a boundary thing,
i.e. bash uses braces ${name<operator>value}. If it was the beginning of
psql I would suggest to consider ${name} stuff, but now I'm not sure that
such a thing can be introduced like ":{xxx}" ? Maybe that can be done.
However it does not change the issue that sh does not allow to test
whether a variable is defined, which is the thought for feature. Providing
a default value or erroring out is not the same thing.
Another question to address: how do you handle ' and " escaping? Pg
:'name' and :"name" solutions are somewhat horrible, but they are there
which show that it was needed. I'm not sure how to translate that with
braces in pg. Maybe :{'name'} and :{"name"}? Hmmm...
Or ":{name}", but then what happens if I write ':{n} x :{m}', should the
lexer interpolate and escape them inside the strings? That is the sh
solution, but I'm not sure it should be done now in psql.
> I understand well so it is subjective - and more, don't think so this
> point is significant.
Well, depending on the syntax things can be done or not, eg test the
variable definition server-side, not only client side. Hence the
discussion:-)
> We should to have this functionality.
Yes.
--
Fabien.
