Usage of qw(:const) with use statement

Question

Sorry for a very basic question, but I'm beginner level at Perl and could not find a suitable explanation on SO (or anywhere else!) for this question. I understand I can write a few examples and try to decipher this but I can really use some knowledge from the experts.

I'm going through some code where the developer has sourced libraries using:

use libExample qw(:const)

Now from what I understand this means sourcing the constants from libExample but would really like to know how this works.

1. Why can't I simply say: use libExample qw(const) (Trying to understand relevance of :)

2. Is there something we can/should write in libExample.pm itself to make other developers utilizing this library to mention such options in place of const that is.

Thanks!

Answer 1

The syntax use Foo qw(:const) is using the EXPORT_TAGS feature in Exporter.

When you set up your library module, you usually have a bunch of functions or class variables. Then you configure Exporter by telling it what to export by default

package Foo;
use Exporter;

our @EXPORT = qw( frobnicate );

sub frobnicate { ... }

or when they are asked for.

OUR @EXPORT_OK = qw( frobnicate barnicate );

sub barnicate { ... }

But you can also tell it to group things together, so the user of your library does not need to list all the methods. Consider this example.

package Foo;
use Exporter;

our @EXPORT_OK qw(monday tuesday wednesday thursday friday saturday sunday);

sub monday { ... }
sub tuesday { ... }
sub wednesday { ... }
sub thursday { ... }
sub friday { ... }
sub saturday { ... }
sub sunday { ... }

Now if I wanted all the working days, I'd have to do this:

use Foo qw(monday tuesday wednesday thursday friday);

That's one long line. Instead, it would be super useful if those could be grouped. Well, they can be. If you do this instead in your library:

package Foo;
use Exporter;

our %EXPORT_TAGS = ( 
    working_days => [ 
      qw(monday tuesday wednesday thursday friday)
    ],
    weekend_days => [
      qw(saturday sunday)
    ]
);

# ...

We can then use it with one tag instead of five function names:

use Foo qw(:working_days);

Note that this is equivalent:

use Foo ':working_days';

Answer 2

use libExample qw(:const)

will pick all names in $EXPORT_TAGS{const} anonymous array and will import them to current namespace.

Whereas

use libExample qw(const)

will pick const and will import it in current namespace.

There are also other variants:

[!]name         This name only
[!]:DEFAULT     All names in @EXPORT
[!]:tag         All names in $EXPORT_TAGS{tag} anonymous array
[!]/pattern/    All names in @EXPORT and @EXPORT_OK which match

Please go through Exporter documentation for more details on the topic.