ignore generated recipes

[gitmo/Moose.git] / lib / Moose / Cookbook / Basics / Recipe4.pod

=pod

=head1 NAME

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

=head1 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 ) = @_;
      if ( @{ $self->employees } ) {
          foreach my $employee ( @{ $self->employees } ) {
              $employee->company($self);
          }
      }
  }

  after 'employees' => sub {
      my ( $self, $employees ) = @_;
      if ($employees) {
          foreach my $employee ( @{$employees} ) {
              $employee->company($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;
  };

=head1 DESCRIPTION

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

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

Finally, we introduce the C<required> attribute option.

The the C<Address> class we define two subtypes. The first uses the
L<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 C<USState> type
a subtype of Moose's builtin C<Str> type. This is done using the C<as>
sugar. The actual constraint is defined using C<where>. This function
accepts a single subroutine reference. That subroutine will be called
with the value to be checked in C<$_> (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 C<USState> type just like Moose's builtin types:

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

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

The next C<subtype>, C<USZipCode>, uses
L<Regexp::Common>. L<Regexp::Common> includes a regex for validating
US zip codes. We use this constraint for the C<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 C<MyApp.Type.USState>.

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

Then we define our C<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 C<Company> class's
C<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, C<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
C<required> attribute can still be false or C<undef>, if its type
constraint allows that.

The next attribute, C<employees>, uses a I<parameterized> type
constraint:

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

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

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

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

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

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

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

  sub BUILD {
      my ( $self, $params ) = @_;
      if ( $self->employees ) {
          foreach my $employee ( @{ $self->employees } ) {
              $employee->company($self);
          }
      }
  }

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

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

To do this we can use an C<after> modifier:

  after 'employees' => sub {
      my ( $self, $employees ) = @_;
      if ($employees) {
          foreach my $employee ( @{$employees} ) {
              $employee->company($self);
          }
      }
  };

Again, as with the C<BUILD> method, we know that the type constraint
check has already happened, so we can just check for definedness on the
C<$employees> argument.

The B<Person> class does have demonstrate anything new. It has several
C<required> attributes. It also has a C<predicate> method, which we
first used in L<recipe 3|Moose::Cookbook::Basics::Recipe3>.

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

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

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

A more detailed example of usage can be found in
F<t/000_recipes/004_recipe.t>.

=head1 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 C<subtype> function, the C<required>
attribute, and the C<override> method modifier.

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

=head1 FOOTNOTES

=over 4

=item (1)

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

=item (2)

Note that C<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.

=item (3)

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

=back

=head1 AUTHORS

Stevan Little E<lt>stevan@iinteractive.comE<gt>

Dave Rolsky E<lt>autarch@urth.orgE<gt>

=head1 COPYRIGHT AND LICENSE

Copyright 2006-2009 by Infinity Interactive, Inc.

L<http://www.iinteractive.com>

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

=cut