In the previous example of the Moose series we had an attribute called "year" that was accepting a number. What if we really want to have an attribute called birthday, that contains a real date. Instead of handling the dates by ourself we would like to use a module that already handles them nicely. For example, we would like to use the DateTime module.

Let's see the class itself in the lib/Person.pm file: 

package Person;
use Moose;

has 'name'     => (is => 'rw');
has 'birthday' => (isa => 'DateTime', is => 'rw');

1;

It has an attribute called birthday, but instead of it being declared as isa => 'Int', we declare it as isa => 'DateTime'.

This expression defines that the 'birthday' attribute must be an instance of the 'DateTime' Perl class. So the constrains are not only the various internally declared types such as Int, but they can be any Perl class that we can load.

So when we call the setter of the 'birthday' attribute we have to pass it a DateTime object. We can do that by creating the object right in the setter: 

$student->birthday( DateTime->new( year => 1988, month => 4, day => 17) );

Here we call the constructor of the DateTime class providing year, month, and day.

When we call the getter, it will return the DateTime object, and with say DateTime will stringify to some human-readable format of the date. 

say $student->birthday;

perl -Ilib script/person.pl 

1988-04-17T00:00:00

In the last expression of the script we called the setter again, but this time we just passed a number. 

$student->birthday(1988);

This will throw an exception like this one: 

Attribute (birthday) does not pass the type constraint because: 
    Validation failed for 'DateTime' with value 1988
       at accessor Person::birthday (defined at lib/Person.pm line 5) line 4
    Person::birthday('Person=HASH(0x2143928)', 1988)
       called at script/person.pl line 14

This happens because the value passed to the birthday setter now needs to be a DateTime object and not just any number.

The full script/person.pl file: 

use strict;
use warnings;
use v5.10;

use Person;
use DateTime;

my $student = Person->new( name => 'Foo' );

$student->birthday( DateTime->new( year => 1988, month => 4, day => 17) );

say $student->birthday;

$student->birthday(1988);