Scope Puzzle

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

Given this script:

my $string       = 'foo';
my $variablename = 'string';
my $temp         = ${$variablename};
print $temp, $/;
[download]

What would you expect it to print?

It prints only $/, because $temp gets nothing from the assignment.

However this:

$string = 'foo';
my $variablename = 'string';
my $temp         = ${$variablename};
print $temp, $/;
[download]

works as expected.

I can't run the first one under strict, because it won't allow the scalar reference.

But when I run it with warnings, the only warning I get is about the uninitialised value in the concatenation.

What am I missing about scope here?

P.S. I really do know why coding this way is a Bad Idea. I was in a big hurry!

($_='kkvvttuu bbooppuuiiffss qqffssmm iibbddllffss')
=~y~b-v~a-z~s; print

Comment on Scope Puzzle Select or Download Code

Replies are listed 'Best First'.
Re: Scope Puzzle by merlyn (Sage) on Feb 17, 2006 at 01:27 UTC
Symbolic references see package variables, and not lexical variables. And are bad, m'kay? -- Randal L. Schwartz, Perl hacker Be sure to read my standard disclaimer if this is a reply.	[reply]
Re: Scope Puzzle by GrandFather (Saint) on Feb 17, 2006 at 01:44 UTC
My understanding was helped by this variant (note the 'our'): `our $string = 'foo'; my $variablename = 'string'; my $temp = ${$variablename}; print $temp, $/;` [download] Prints: `foo` [download] DWIM is Perl's answer to Gödel	[reply] [d/l] [select]
Re: Scope Puzzle by ikegami (Patriarch) on Feb 17, 2006 at 05:41 UTC
There are two kinds of variables in Perl: lexical (`my`) variables and package (`our`) variables. If you don't declare a variable, Perl assumes it's a package variable. Symbolic references (e.g. `${'varname'}`) can only fetch and set the value of package variables. Symbolic references are strongly discouraged. In fact, they won't even work if you use `use strict` as you should. Hashes or arrays usually do the trick. If you're looking to create aliases, check out Data::Alias. (I wrote this earlier, but forgot to post it.)	[reply] [d/l] [select]
Re: Scope Puzzle by chromatic (Archbishop) on Feb 17, 2006 at 06:36 UTC
As an optimization (I suspect) of an implementation detail, `perl` translates lexical variables into array accesses, where each lexical pad is an array. That's not quite exactly correct, but it's close enough to explain why you can look up and rebind global symbols -- those that live in the symbol table -- but not lexicals. `perl` always looks up global symbols (package variables, subroutine names) by name and lexical symbols by lexical pad index. If you examine the optree with B::Concise or a similar module, you'll notice the difference in `gvsv` and `padsv` ops.	[reply] [d/l] [select]
Re^2: Scope Puzzle by Anonymous Monk on Feb 17, 2006 at 08:12 UTC
No, its because it was designed that way %)	[reply]