computed symbolic references

valentin has asked for the wisdom of the Perl Monks concerning the following question:

Hi, my question is two folded:
1. What is the difference between ${$x."::VERSION"} and ${"$x::VERSION"}?
2. Why is $y undefined in the first example?
3. Why does perl think $y is a filehandle in the second example?

perl -MFile::Find -we '$x='File::Find'; print ${ $y = "$x::VERSION" } 
+print $y'
[download]

perl -MFile::Find -we '$x='File::Find'; print ${ $y = $x."::VERSION" }
+ print $y'
[download]

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Re: computed symbol references by gellyfish (Monsignor) on Jul 13, 2004 at 13:16 UTC
The first is appending '::VERSION' to the value of $x whereas the second is going to look for the variable $VERSION in the package 'x' - it will not expand $x separately. Because there is no variable $x::VERSION defined Because you missed the semicolon after the first print statement. /J\	[reply]
Re: computed symbol references by broquaint (Abbot) on Jul 13, 2004 at 13:23 UTC
The first concatenates `$x` and the string `::VERSION` then symbolically dereferences, the second symbolically dereferences `$x::VERSION` `$y` is undefined because `$x::VERSION` is undefined, perhaps you meant `${ $y = ${ $x . '::VERSION' } }` because you're passing it as the first argument to `print` which is expected to be a filehandle if it is not preceded by a comma If you're interested in dynamically accessing the `$VERSION` package variable then check out `UNIVERSAL::VERSION`. HTH `_________ broquaint`	[reply]
Re: computed symbol references by dragonchild (Archbishop) on Jul 13, 2004 at 13:21 UTC
Your first question is easy to answer. `package x; VERSION = 'bar'; package Foo; VERSION = '1.01'; package main; $bar = '0.01'; $x = 'Foo'; print ${"$x::VERSION"}, $/; print ${$x . "::VERSION"}, $/; ---- 0.01 1.01` [download] The first goes and gets the thing in $x::VERSION (the $VERSION variable in the x:: namespace), then dereferences it as a symbolic reference in the main:: namespace. The second takes the $x variable in the main:: namespace and uses it to create a variable name that is used as a symbolic reference. In this case, the variable $Foo::VERSION. I'm not sure on the other questions, but you don't have a comma or semi-colon after the ${ ... }. Maybe that's changing your results? ------ We are the carpenters and bricklayers of the Information Age. Then there are Damian modules.... sigh* ... that's not about being less-lazy -- that's about being on some really good drugs -- you know, there is no spoon.* - flyingmoose I shouldn't have to say this, but any code, unless otherwise stated, is untested	[reply] [d/l]
Re: computed symbol references by valentin (Abbot) on Jul 13, 2004 at 13:50 UTC
Thank you all, 1. I was confused by `${"$x::VERSION"}` and `${"${x}::VERSION"}` even if I know the difference, I could not see it. 2. explained by 1. 3. I missed the `;`.	[reply] [d/l] [select]