/ 
Moose-2.0002
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/ Moose::Cookbook::Basics::Recipe4

NAME

Moose::Cookbook::Basics::Recipe4 - Subtypes, and modeling a simple Company class hierarchy

VERSION

version 2.0002

SYNOPSIS

package Address;
use Moose;
use Moose::Util::TypeConstraints;

use Locale::US;
use Regexp::Common 'zip';

my $STATES = Locale::US->new;
subtype 'USState'
    => as Str
    => where {
           (    exists $STATES->{code2state}{ uc($_) }
             || exists $STATES->{state2code}{ uc($_) } );
       };

subtype 'USZipCode'
    => as Value
    => where {
           /^$RE{zip}{US}{-extended => 'allow'}$/;
       };

has 'street'   => ( is => 'rw', isa => 'Str' );
has 'city'     => ( is => 'rw', isa => 'Str' );
has 'state'    => ( is => 'rw', isa => 'USState' );
has 'zip_code' => ( is => 'rw', isa => 'USZipCode' );

package Company;
use Moose;
use Moose::Util::TypeConstraints;

has 'name' => ( is => 'rw', isa => 'Str', required => 1 );
has 'address'   => ( is => 'rw', isa => 'Address' );
has 'employees' => ( is => 'rw', isa => 'ArrayRef[Employee]' );

sub BUILD {
    my ( $self, $params ) = @_;
    foreach my $employee ( @{ $self->employees || [] } ) {
        $employee->employer($self);
    }
}

after 'employees' => sub {
    my ( $self, $employees ) = @_;
    foreach my $employee ( @{ $employees || [] } ) {
        $employee->employer($self);
    }
};

package Person;
use Moose;

has 'first_name' => ( is => 'rw', isa => 'Str', required => 1 );
has 'last_name'  => ( is => 'rw', isa => 'Str', required => 1 );
has 'middle_initial' => (
    is        => 'rw', isa => 'Str',
    predicate => 'has_middle_initial'
);
has 'address' => ( is => 'rw', isa => 'Address' );

sub full_name {
    my $self = shift;
    return $self->first_name
        . (
        $self->has_middle_initial
        ? ' ' . $self->middle_initial . '. '
        : ' '
        ) . $self->last_name;
}

package Employee;
use Moose;

extends 'Person';

has 'title'    => ( is => 'rw', isa => 'Str',     required => 1 );
has 'employer' => ( is => 'rw', isa => 'Company', weak_ref => 1 );

override 'full_name' => sub {
    my $self = shift;
    super() . ', ' . $self->title;
};

DESCRIPTION

This recipe introduces the subtype sugar function from Moose::Util::TypeConstraints. The subtype function lets you declaratively create type constraints without building an entire class.

In the recipe we also make use of Locale::US and Regexp::Common to build constraints, showing how constraints can make use of existing CPAN tools for data validation.

Finally, we introduce the required attribute option.

In the Address class we define two subtypes. The first uses the Locale::US module to check the validity of a state. It accepts either a state abbreviation of full name.

A state will be passed in as a string, so we make our USState type a subtype of Moose's builtin Str type. This is done using the as sugar. The actual constraint is defined using where. This function accepts a single subroutine reference. That subroutine will be called with the value to be checked in $_ (1). It is expected to return a true or false value indicating whether the value is valid for the type.

We can now use the USState type just like Moose's builtin types:

has 'state'    => ( is => 'rw', isa => 'USState' );

When the state attribute is set, the value is checked against the USState constraint. If the value is not valid, an exception will be thrown.

The next subtype, USZipCode, uses Regexp::Common. Regexp::Common includes a regex for validating US zip codes. We use this constraint for the zip_code attribute.

subtype 'USZipCode'
    => as Value
    => where {
           /^$RE{zip}{US}{-extended => 'allow'}$/;
       };

Using a subtype instead of requiring a class for each type greatly simplifies the code. We don't really need a class for these types, as they're just strings, but we do want to ensure that they're valid.

The type constraints we created are reusable. Type constraints are stored by name in a global registry. This means that we can refer to them in other classes. Because the registry is global, we do recommend that you use some sort of pseudo-namespacing in real applications, like MyApp.Type.USState.

These two subtypes allow us to define a simple Address class.

Then we define our Company class, which has an address. As we saw in earlier recipes, Moose automatically creates a type constraint for each our classes, so we can use that for the Company class's address attribute:

has 'address'   => ( is => 'rw', isa => 'Address' );

A company also needs a name:

has 'name' => ( is => 'rw', isa => 'Str', required => 1 );

This introduces a new attribute option, required. If an attribute is required, then it must be passed to the class's constructor, or an exception will be thrown. It's important to understand that a required attribute can still be false or undef, if its type constraint allows that.

The next attribute, employees, uses a parameterized type constraint:

has 'employees' => ( is => 'rw', isa => 'ArrayRef[Employee]' );

This constraint says that employees must be an array reference where each element of the array is an Employee object. It's worth noting that an empty array reference also satisfies this constraint.

Parameterizable type constraints (or "container types"), such as ArrayRef[`a], can be made more specific with a type parameter. In fact, we can arbitrarily nest these types, producing something like HashRef[ArrayRef[Int]]. However, you can also just use the type by itself, so ArrayRef is legal. (2)

If you jump down to the definition of the Employee class, you will see that it has an employer attribute.

When we set the employees for a Company we want to make sure that each of these employee objects refers back to the right Company in its employer attribute.

To do that, we need to hook into object construction. Moose lets us do this by writing a BUILD method in our class. When your class defined a BUILD method, it will be called immediately after an object construction, but before the object is returned to the caller (3).

The Company class uses the BUILD method to ensure that each employee of a company has the proper Company object in its employer attribute:

sub BUILD {
    my ( $self, $params ) = @_;
    foreach my $employee ( @{ $self->employees || [] } ) {
        $employee->employer($self);
    }
}

The BUILD method is executed after type constraints are checked, so it is safe to assume that if $self->employees has a value, it will be an array reference, and that the elements of that array reference will be Employee objects.

We also want to make sure that whenever the employees attribute for a Company is changed, we also update the employer for each employee.

To do this we can use an after modifier:

after 'employees' => sub {
    my ( $self, $employees ) = @_;
    foreach my $employee ( @{ $employees || [] } ) {
        $employee->employer($self);
    }
};

Again, as with the BUILD method, we know that the type constraint check has already happened, so we know that if $employees is defined it will contain an array reference of Employee objects..

The Person class does not really demonstrate anything new. It has several required attributes. It also has a predicate method, which we first used in recipe 3.

The only new feature in the Employee class is the override method modifier:

override 'full_name' => sub {
    my $self = shift;
    super() . ', ' . $self->title;
};

This is just a sugary alternative to Perl's built in SUPER:: feature. However, there is one difference. You cannot pass any arguments to super. Instead, Moose simply passes the same parameters that were passed to the method.

A more detailed example of usage can be found in t/000_recipes/moose_cookbook_basics_recipe4.t.

CONCLUSION

This recipe was intentionally longer and more complex. It illustrates how Moose classes can be used together with type constraints, as well as the density of information that you can get out of a small amount of typing when using Moose.

This recipe also introduced the subtype function, the required attribute, and the override method modifier.

We will revisit type constraints in future recipes, and cover type coercion as well.

FOOTNOTES

(1)

The value being checked is also passed as the first argument to the where block, so it can be accessed as $_[0].

(2)

Note that ArrayRef[] will not work. Moose will not parse this as a container type, and instead you will have a new type named "ArrayRef[]", which doesn't make any sense.

(3)

The BUILD method is actually called by Moose::Object->BUILDALL, which is called by Moose::Object->new. The BUILDALL method climbs the object inheritance graph and calls any BUILD methods it finds in the correct order.

AUTHOR

Stevan Little <stevan@iinteractive.com>

COPYRIGHT AND LICENSE

This software is copyright (c) 2011 by Infinity Interactive, Inc..

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.