Hello Marshall,

You can use types to constrain the values of attributes, see Moose::Manual::Types and Moose::Util::TypeConstraints. For example, you could use an anonymous type to ensure that the value of Month is between 1 and 12.

If you need to perform more complex checking of your arguments, then you can define a BUILD method (see Moose::Manual::Construction).

package Appointment;

use Moose;      # automatically turns on strict and warnings
use Moose::Util::TypeConstraints;

my @freq_valid = (qw/OneTime Daily Monthly Quarterly/);

has 'Month'  => ( is       => 'ro', 
                  isa => subtype( 'Int' => where { $_ > 0 and $_ < 13 
+} ),
                  required => 1,
                );
                  
has 'Day'   => ( is       => 'ro', 
                 isa      => 'Int',
                 required => 1,
               );                
                        
has 'Freq' => (  is       => 'ro', 
                 isa      => 'Str',
                 default  => 'OneTime',
              );

sub BUILD {
    my $self = shift;
    if ($self->Month == 12 and $self->Day == 1){
        die "I'm having a bad Moose day.";
    }
}

1;
[download]

UPDATE: Here is an example of using a subtype to check the values of your Freq attribute.

package Appointment;

use Moose;      # automatically turns on strict and warnings
use Moose::Util::TypeConstraints;
use List::Util qw/any/;

my @freq_valid = (qw/OneTime Daily Monthly Quarterly/);

subtype 'AppointmentFreq'
    => as 'Str'
    => where { my $arg = $_; any{ $_ eq $arg } @freq_valid }
    => message { 'The Freq you provided is not valid' };

has 'Month'  => ( is       => 'ro', 
                  isa => subtype( 'Int' => where { $_ > 0 and $_ < 13 
+} ),
                  required => 1,
                );
                  
has 'Day'   => ( is       => 'ro', 
                 isa      => 'Int',
                 required => 1,
               );                
                        
has 'Freq' => (  is       => 'ro', 
                 isa      => 'AppointmentFreq',
                 default  => 'OneTime',
              );

sub BUILD {
    my $self = shift;
    if ($self->Month == 12 and $self->Day == 1){
        die "I'm having a bad Moose day.";
    }
}

1;
[download]