Mastering Perl for Bioinformatics [Electronic resources] نسخه متنی

1.8 Using Modules

So
far, the benefit of modules may seem questionable. You may be
wondering what the advantage is over simple libraries (without
package declarations), since the main result seems
to be the necessity to refer to subroutines in the modules with
longer names!

1.8.1 Exporting Names

There's a way to avoid
lengthy module names and still use the short ones if you place a call
to the special Exporter module in the module code
and modify the use MODULE declaration in the
calling code.

Going back to the first example Geneticcode.pm
module, recall it began with this line:

package Geneticcode;

and included the definition for the hash
genetic_code and the subroutine
codon2aa.

If you add these lines to the beginning of the file, you can export
the symbol names of variables or subroutines in the module into the
namespace of the calling program. You can then use the convenient
short names for things (e.g., codon2aa instead of
Geneticcode::codon2aa). Here's a
short example of how it works (try typing perldoc
Exporter to see the whole story):

package Geneticcode;
require Exporter;
@ISA = qw(Exporter);
@EXPORT_OK = qw(...);         # symbols to export on request

Here's how to export the name
codon2aa from the module only when explicitly
requested:

@EXPORT_OK = qw(codon2aa);    # symbols to export on request

The calling program then has to explicitly request the
codon2aa symbol like so:

use Geneticcode qw(codon2aa);

If you use this approach, the calling program can just say:

codon2aa($codon);

instead of:

Geneticcode::codon2aa($codon);

The Exporter module that's
included in the standard Perl distribution has several other optional
behaviors, but the way just shown is the safest and most useful. As
you'll see, the object-oriented programming style of
using modules doesn't use the
Export facility, but it is a useful thing to have
in your bag of tricks. For more information about exporting (such as
why exporting is also known as "polluting your
namespace"), see the Perl documentation for the
Exporter module (by typing perldoc
Exporter at a command line or by going to the http://www.perldoc.com web page).