use vars in a require()d file

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

Here is foobarbaz.pl, which sets some default values:

#!/usr/local/bin/perl -w
#
use strict;
use diagnostics;

use vars qw( $qux $quo $zip );

$foo = 1;
$bar = 2;
$baz = 3;
$qux = 4;
$quo = 5;
$zip = 6;

sub hello {
  print "Hello, World!\n"
}
[download]

And here is program.pl:

#!/usr/local/bin/perl -w
#
use strict;
use diagnostics;

use vars qw( $foo $bar $baz );

require 'foobarbaz.pl';

# OK, but need package name for $qux, $quo, $zip
print "$_\n" foreach( $foo, $bar, $baz, $::qux, $::quo, $::zip );

hello(); # fine

#print $qux;     ## XXX gives an error

while (my ($k,$v) = each %:: ) {
  print $k . ":\t\t" . $v . "\n";
}
[download]

In program.pl, when printing the symbol table, the identifiers for $qux, $quo, $zip appear in main's symbol table, but I can only access them by explicitly providing the package name. This throws a warning, however: Name "main::zip" used only once: possible typo at require.pl line 11 (same warning for $qux and $quo).

Normally, when using a separate file to initialize variables, I would declare all the variables in program.pl with use vars (or make the separate file a module that uses Exporter). I was just curious to see the effects of use vars in a require()d file. I do not understand why the identifiers exist in main's symbol table, but appear 'hidden' to an extent. Can anyone enlighten me?

Thanks much!
--sacked

Comment on use vars in a require()d file Select or Download Code

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Re: use vars in a require()d file by chromatic (Archbishop) on Mar 12, 2001 at 05:52 UTC
require fires at run-time. Until then, Perl has no way of knowing that you've declared your variables in the other file. Luckily, a BEGIN block will fix things: `BEGIN { require 'foobarbaz.pl'; }` [download] It's not that the variables are hidden, it's just that Perl doesn't see them in time to avoid the warning, unless you tell the compiler to read the file before it does the warning check.	[reply] [d/l]
Re: use vars in a require()d file by athomason (Curate) on Mar 12, 2001 at 05:58 UTC
From perlman:lib:vars: Unlike pragmas that affect the `$^H' hints variable, the `use vars' and `use subs' declarations are not BLOCK-scoped. They are thus effective for the entire file in which they appear. You may not rescind such declarations with `no vars' or `no subs'. [download] Note the phrase entire file. So, when you declare `$qux`, `$quo`, and `$zip` in `foobarbaz.pl`, perl has already forgetten that they were declared via `use vars` by the time you get around to reading them in `program.pl`. However, the symbol table is more permanent, which is why you can still access them by explicitly specifying package main. Update Whoops, guess you shouldn't always believe the docs... it does look like the behavior follows chromatic's explanation.	[reply] [d/l] [select]
Re (tilly) 2: use vars in a require()d file by tilly (Archbishop) on Mar 12, 2001 at 07:58 UTC
chromatic is right. However the documentation for vars should be updated to make it clear that a vars declaration is actually package scoped, not file scoped. In other words that global has been declared, and will be considered declared in every file you encounter from then on as long as they are all in the same package. Of course if you are following good Perl tradition and writing lots of modules with private packages, the issue won't come up...	[reply]


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