Re: [HACKERS] Variable substitution in psql backtick expansion

> 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.