Advanced Perl Programming

Importing Symbols

Sometimes, you may want to selectively import symbols into your namespace, just for typing efficiency. For example, you might want to say sqrt instead of math::sqrt or deposit instead of BankAccount::deposit. The use statement allows you to specify an optional list of function names to be imported:

use BankAccount ('withdraw', 'deposit');
withdraw();  # Can now call function without fully qualifying it.

For its part, the module has to be ready to export these names (and only them) to whoever uses it. It should also have a policy for what it should do if the user does not specify a list at all. Both these tasks are handled for you by a standard module called Exporter. The BankAccount package can be implemented as shown next:

package BankAccount;
use Exporter;
@ISA = ('Exporter');     # Inherit from Exporter
@EXPORT_OK = ('withdraw', 'deposit');

sub deposit  { .... }
sub withdraw { .... }

This code loads the Exporter module and arranges to inherit from that module, using the @ISA array. For now, take it on faith that this works; we will study inheritance shortly. The @EXPORT_OK array states which symbols are fine to export. The user of this module can in turn specify a list of one or more symbols specified in @EXPORT_OK to the use statement. If the user says,

use BankAccount ('deposit');

the deposit function can be called without fully qualifying the name, in contrast to withdraw(). To tell the Exporter module not to export any symbols into your namespace, leave the list ...