Better way to define global variables

[govindkailas]

Hi All, I have defined some system specific variables in a .pm file and wanted to use those variables in all the perl scripts which use the .pm. This is how my Utilities.pm looks like

package Utilities;
use Exporter;
@EXPORT = ($SYS_DIR);

our $SYS_DIR="/path/to/sys";
1;

And here is the .pl file

use Utilities;
use strict;
print "This is the value for SYS_DIR=$SYS_DIR";

[choroba]

You do not want to export the value of the variable, but the variable itself. Use

@EXPORT = qw($SYS_DIR);

Also, the Exporter's documantation states a different way how to make a package use it. Change the other line to

use Exporter 'import';

[Athanasius]

Ok, you now know how to do this. Here’s why you shouldn’t! From Exporter#What-Not-to-Export:

Do not export variable names. Just because Exporter lets you do that, it does not mean you should.

  @EXPORT_OK = qw( $svar @avar %hvar ); # DON'T!

Exporting variables is not a good idea. They can change under the hood, provoking horrible effects at-a-distance, that are too hard to track and to fix. Trust me: they are not worth it.

To provide the capability to set/get class-wide settings, it is best instead to provide accessors as subroutines or class methods instead.

Hope that helps,

[kcott]

G'day govindkailas,

It's generally not a good idea to export by default (i.e. with @EXPORT) — see the Exporter documentation sections: Selecting What to Export and What Not to Export. Instead, consider using @EXPORT_OK and %EXPORT_TAGS.

Here's an example of Utilities.pm that does that:

package Utilities;

use strict;
use warnings;

use Exporter qw{import};

my @sys_exports = qw{$SYS_DIR $SYS_OTHER};
my @util_exports = qw{$UTIL_THIS $UTIL_THAT};

our @EXPORT_OK = (@sys_exports, @util_exports);
our %EXPORT_TAGS = (SYS => [@sys_exports], UTIL => [@util_exports]);

our $SYS_DIR = "sys_dir";
our $SYS_OTHER = "sys_other";
our $UTIL_THIS = "util_this";
our $UTIL_THAT = "util_that";

1;

Here's some sample runs showing how to use it.

No namespace pollution unless specifically requested:

$ perl -Mstrict -Mwarnings -E '
    use Utilities;                     
    say $SYS_DIR;
'
Global symbol "$SYS_DIR" requires explicit package name at -e line 3.
Execution of -e aborted due to compilation errors.

Request a specific item:

$ perl -Mstrict -Mwarnings -E '
    use Utilities qw{$SYS_DIR};        
    say $SYS_DIR;
'
sys_dir

Request multiple items using a tag:

$ perl -Mstrict -Mwarnings -E '
    use Utilities qw{:SYS};            
    say $SYS_DIR;
'
sys_dir

Mix specific and tag requests:

$ perl -Mstrict -Mwarnings -E '
    use Utilities qw{:UTIL $SYS_OTHER};
    say $SYS_OTHER;
    say $UTIL_THAT;
'
sys_other
util_that

[davies]

If you are using global variables, this can cause problems unless you're sure what you are doing. I frequently do for things that genuinely are global, like an instance of Excel, but almost always I want them to be WORMs (write once, read many). When I ran into a similar problem, BrowserUK gave me some very helpful advice at Re^7: Strict isn't strict enough. I don't know how much use the rest of the thread will be to you, but tobyink's suggestion at Re^2: Strict isn't strict enough (stop it) and Tye's at Re^3: Strict isn't strict enough (stop it) might also be relevant to your situation.

Regards,

John Davies

[MidLifeXis]

In addition to what choroba said, I am getting the same error as you are (although you didn't state it in your question, just in the CB): Global symbol "$SYS_DIR" requires explicit package name. Obviously I am missing something in the documentation. Adding strictures/warnings does not seem to help identify the problem.

Update: The key was explicitly importing the import method with the use Exporter 'import' line. I missed it in my initial reading. Apply choroba's changes and you should be set.

[hdb]

Any chance you have another Utilities.pm lying around? Try:

use strict;
use Utilities;

print $INC{'Utilities.pm'},"\n";
print "This is the value for SYS_DIR=$SYS_DIR";

and see whether you get the correct path.

I have no problems with your code after making choroba's proposed changes.