Object-Oriented Programming in Perl

My first and last brush with Perl OO programming was back in 2008. I was in the Netherlands with a couple of colleagues, getting "KT" for a set of small e-commerce applications whose support was being outsourced to India. We were horrified to see that the code was object-oriented, with many layers of inheritance, which made it extremely difficult to learn. As part of the Knowledge Transfer, we were asked to create installers and use them to get the applications running on a new server. But we would inevitably run into errors, which would send us hunting from file to file and class to class to try and find the ultimate parent class. Even the Dutch team that had been supporting the code for a few years did not know the nitty-gritty of the inheritance - they  had only learnt enough to support the applications on a day to day basis.

Having learnt C++ and Java earlier, I had realized then that Perl is not the best language for object-oriented programming. But the fact that it supports OO means that you need to know the basics, just in case you face a situation like I did. There are many online tutorials available on the subject but the best ones come straight from the horse's mouth, viz: the perlootut tutorial and  the perlobj language reference. After going through these, you may feel confident enough to create your own class and then test it with a simple script. I first created a very basic class called Muzak to represent the songs from my music collection. Each Muzak object has up to 4 attributes - a song name (mandatory), artist name, album title and year. It has 1 method - artist() - that returns the artist name. I put the class in a Perl module called Muzak.

Muzak.pm

package Muzak;

# Constructor

sub new {

    my $class = shift;

    my ( $song, $artist, $album, $year ) = @_;

    

    if ($song eq "") {

        die "Muzak object requires at least Song Name attribute";

    }

    my $self = bless {

        song => $song,

        artist => $artist,

        album => $album,

        year => $year,

    }, $class;

    return $self;

}

# Method: Get artist

sub artist {

    my $self = shift;

    return $self->{artist};

}

1;

Note that the "1;" at the end of the .pm file is critical. If you miss it, the Perl interpreter will throw an error while testing the script, which would look like "Muzak.pm did not return a true value at test_muzak.pl line 6". Also note that the check for non-blank song name is also not mandatory to create a working class - it is just a logical check that I added.

test_muzak.pl

use Muzak;

use feature qw(say);

# Create new Muzak object with all 4 paramaters

my $muzak1 = Muzak->new('Aqualung', 'Jethro Tull', 'Aqualung', '1971');

# Print the song artist name

say "Artist1 is " . $muzak1->artist();

# Create new Muzak object with only 1 parameter

my $muzak2 = Muzak->new('Bouree');

# Try printing the song artist name

say "Artist2 is " . $muzak2->artist();

# Try creating new Muzak object with 0 parameters

my $muzak3 = Muzak->new();

# Try printing the song artist name

say "Artist3 is " . $muzak3->artist();

There's a lot more to explore in Perl OO programming. I can write code that is much more complicated, like those Dutch applications (which were created, if I remember correctly, by a UK firm). More importantly, I can write code that is more useful than just playing around with song details. But that's another day, another class, another blog post