Re: [vox-tech] perl redux - references

[Micah Cowan <micah@coMAPSwanbox.com>]

On Sun, Apr 08, 2001 at 09:40:48AM -0700, Peter Jay Salzman wrote:
> this stuff is way above my head.
> 
> here is a program (that works) which reads a text file with lines of the form
> 
>       James = "William"
>       Leonard = "DeForest"
>       Montgomery = "James"
> 
> and sets the variable $James to "William", and so on.  note that line 3 is
> commented out.
> 
> 1             #!/usr/bin/perl -w
> 2             use diagnostics;
> 3             #my $James;
> 4
> 5             open("FP", "< config");
> 6             while(<FP>) {
> 7                     chomp;
> 8                     /^(\w+)\s+=\s+\"(.*)\"/;
> 9                     $$1 = $2;
> 10            }
> 11            close(FP);
> 12            print ": $James\n";
> 
> the only error message is that this code produces is that "$James" might to a
> typo because it appears only once in the code.  fair enough.  so i uncomment
> line 3.   NOW the error message is that $James is uninitialized.
> 
> i don't completely grok scoping in perl, but this seems ridiculous.   why would
> perl complain that the variable is uninitialized?
> 
> pete

You asked for warnings, you got 'em :)

Warnings tend to be helpful only when you're trying to write "kosher"
code (a la "use strict").  If you're /trying/ to be kludgey, they'll
just get in the way ;).

The code you've written works, as you've pointed out, but your code
essentially "tricks" Perl into instantiating $James, so you're kinda
on your own.

Believe it or not, IIRC, the $James defined on line 3 and the $James
you refer to via $$1 are completely different $James'es.  I'd guess
you didn't notice that the output is empty when you uncomment line 3?
The $$1 creates a *global* $James, aka $main::James or $::James,
whereas the "my" creates a lexically-scoped local.  So in all of your
"normal" uses of $James (in the print statement), you are referring to
the lexically-scoped $James, whose name obscures the visibility of the
global one, unless you specifically refer to $::James.  However, when
you /set/ James via $$1, it goes directly to the global name table.

To get rid of warnings, use "our" instead of "my" (won't work on some
older Perls - like, say, the one I'm using to test it right now from
work :(  ), or "use vars qw($James);", etc.  Then the declaration
will correctly refer to the global $James.

HTH,
Micah