Mastering Modern Perl: Writing Efficient and Maintainable Code/

...

/

The UNIVERSAL Package

The UNIVERSAL Package

Learn about the built-in UNIVERSAL package, the ancestor of all other packages.

We'll cover the following...

Perl’s built-in UNIVERSAL package is the ancestor of all other packages—it’s the ultimate parent class in an object-oriented sense. UNIVERSAL provides a few methods for its children to use, inherit, or override.

The VERSION() method

The VERSION() method returns the value of the $VERSION variable of the invoking package or class. If we provide a version number as an optional parameter, the method will throw an exception if the queried $VERSION is not equal to or greater than the parameter.

Given a HowlerMonkey module of version 1.23, its VERSION() method behaves as follows:

Perl
package HowlerMonkey {
    our $VERSION = 1.23;
    use Moose
    #...
}
my $hm = HowlerMonkey->new;
say HowlerMonkey->VERSION; # prints 1.23
say $hm->VERSION; # prints 1.23
say $hm->VERSION( 0.0 ); # prints 1.23
say $hm->VERSION( 1.23 ); # prints 1.23
say $hm->VERSION( 2.0 ); # exception!

There’s little reason to override VERSION().

The DOES() method

The DOES() method supports the use of roles in programs. Pass it an invocant and the name of a role, and the method will return true if the ...