Re: Variable substitution in psql backtick expansion

2017-04-17 10:58 GMT+02:00 Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>:

>
> 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 have same thinks. We can disallow nesting - it can be acceptable limit.
The :{xxx:operator} can be used for more things - default, check, user
input, ...

necessary escaping can be done in next line

>
>
> 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:-)

It depends if variables are declared or defined by value. In psql there are
defined by value. So some tests if var is defined or not is necessary.

>
>
> We should to have this functionality.
>>
>
> Yes.
>
> --
> Fabien.
>