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+.\" ========================================================================
+.\"
+.IX Title "Moose::Cookbook::Basics::Recipe4 3"
+.TH Moose::Cookbook::Basics::Recipe4 3 "2009-05-05" "perl v5.8.7" "User Contributed Perl Documentation"
+.SH "NAME"
+Moose::Cookbook::Basics::Recipe4 \- Subtypes, and modeling a simple \fBCompany\fR class hierarchy
+.SH "SYNOPSIS"
+.IX Header "SYNOPSIS"
+.Vb 3
+\& package Address;
+\& use Moose;
+\& use Moose::Util::TypeConstraints;
+.Ve
+.PP
+.Vb 2
+\& use Locale::US;
+\& use Regexp::Common 'zip';
+.Ve
+.PP
+.Vb 7
+\& my $STATES = Locale::US\->new;
+\& subtype 'USState'
+\& => as Str
+\& => where {
+\& ( exists $STATES\->{code2state}{ uc($_) }
+\& || exists $STATES\->{state2code}{ uc($_) } );
+\& };
+.Ve
+.PP
+.Vb 5
+\& subtype 'USZipCode'
+\& => as Value
+\& => where {
+\& /^$RE{zip}{US}{\-extended => 'allow'}$/;
+\& };
+.Ve
+.PP
+.Vb 4
+\& 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' );
+.Ve
+.PP
+.Vb 3
+\& package Company;
+\& use Moose;
+\& use Moose::Util::TypeConstraints;
+.Ve
+.PP
+.Vb 3
+\& has 'name' => ( is => 'rw', isa => 'Str', required => 1 );
+\& has 'address' => ( is => 'rw', isa => 'Address' );
+\& has 'employees' => ( is => 'rw', isa => 'ArrayRef[Employee]' );
+.Ve
+.PP
+.Vb 8
+\& sub BUILD {
+\& my ( $self, $params ) = @_;
+\& if ( @{ $self\->employees || [] } ) {
+\& foreach my $employee ( @{ $self\->employees } ) {
+\& $employee\->employer($self);
+\& }
+\& }
+\& }
+.Ve
+.PP
+.Vb 8
+\& after 'employees' => sub {
+\& my ( $self, $employees ) = @_;
+\& if ($employees) {
+\& foreach my $employee ( @{$employees} ) {
+\& $employee\->employer($self);
+\& }
+\& }
+\& };
+.Ve
+.PP
+.Vb 2
+\& package Person;
+\& use Moose;
+.Ve
+.PP
+.Vb 7
+\& 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' );
+.Ve
+.PP
+.Vb 9
+\& sub full_name {
+\& my $self = shift;
+\& return $self\->first_name
+\& . (
+\& $self\->has_middle_initial
+\& ? ' ' . $self\->middle_initial . '. '
+\& : ' '
+\& ) . $self\->last_name;
+\& }
+.Ve
+.PP
+.Vb 2
+\& package Employee;
+\& use Moose;
+.Ve
+.PP
+.Vb 1
+\& extends 'Person';
+.Ve
+.PP
+.Vb 2
+\& has 'title' => ( is => 'rw', isa => 'Str', required => 1 );
+\& has 'employer' => ( is => 'rw', isa => 'Company', weak_ref => 1 );
+.Ve
+.PP
+.Vb 4
+\& override 'full_name' => sub {
+\& my $self = shift;
+\& super() . ', ' . $self\->title;
+\& };
+.Ve
+.SH "DESCRIPTION"
+.IX Header "DESCRIPTION"
+This recipe introduces the \f(CW\*(C`subtype\*(C'\fR sugar function from
+Moose::Util::TypeConstraints. The \f(CW\*(C`subtype\*(C'\fR function lets you
+declaratively create type constraints without building an entire
+class.
+.PP
+In the recipe we also make use of Locale::US and Regexp::Common
+to build constraints, showing how constraints can make use of existing
+\&\s-1CPAN\s0 tools for data validation.
+.PP
+Finally, we introduce the \f(CW\*(C`required\*(C'\fR attribute option.
+.PP
+The the \f(CW\*(C`Address\*(C'\fR class we define two subtypes. The first uses the
+Locale::US module to check the validity of a state. It accepts
+either a state abbreviation of full name.
+.PP
+A state will be passed in as a string, so we make our \f(CW\*(C`USState\*(C'\fR type
+a subtype of Moose's builtin \f(CW\*(C`Str\*(C'\fR type. This is done using the \f(CW\*(C`as\*(C'\fR
+sugar. The actual constraint is defined using \f(CW\*(C`where\*(C'\fR. This function
+accepts a single subroutine reference. That subroutine will be called
+with the value to be checked in \f(CW$_\fR (1). It is expected to return a
+true or false value indicating whether the value is valid for the
+type.
+.PP
+We can now use the \f(CW\*(C`USState\*(C'\fR type just like Moose's builtin types:
+.PP
+.Vb 1
+\& has 'state' => ( is => 'rw', isa => 'USState' );
+.Ve
+.PP
+When the \f(CW\*(C`state\*(C'\fR attribute is set, the value is checked against the
+\&\f(CW\*(C`USState\*(C'\fR constraint. If the value is not valid, an exception will be
+thrown.
+.PP
+The next \f(CW\*(C`subtype\*(C'\fR, \f(CW\*(C`USZipCode\*(C'\fR, uses
+Regexp::Common. Regexp::Common includes a regex for validating
+\&\s-1US\s0 zip codes. We use this constraint for the \f(CW\*(C`zip_code\*(C'\fR attribute.
+.PP
+.Vb 5
+\& subtype 'USZipCode'
+\& => as Value
+\& => where {
+\& /^$RE{zip}{US}{\-extended => 'allow'}$/;
+\& };
+.Ve
+.PP
+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.
+.PP
+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 \f(CW\*(C`MyApp.Type.USState\*(C'\fR.
+.PP
+These two subtypes allow us to define a simple \f(CW\*(C`Address\*(C'\fR class.
+.PP
+Then we define our \f(CW\*(C`Company\*(C'\fR 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 \f(CW\*(C`Company\*(C'\fR class's
+\&\f(CW\*(C`address\*(C'\fR attribute:
+.PP
+.Vb 1
+\& has 'address' => ( is => 'rw', isa => 'Address' );
+.Ve
+.PP
+A company also needs a name:
+.PP
+.Vb 1
+\& has 'name' => ( is => 'rw', isa => 'Str', required => 1 );
+.Ve
+.PP
+This introduces a new attribute option, \f(CW\*(C`required\*(C'\fR. 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
+\&\f(CW\*(C`required\*(C'\fR attribute can still be false or \f(CW\*(C`undef\*(C'\fR, if its type
+constraint allows that.
+.PP
+The next attribute, \f(CW\*(C`employees\*(C'\fR, uses a \fIparameterized\fR type
+constraint:
+.PP
+.Vb 1
+\& has 'employees' => ( is => 'rw', isa => 'ArrayRef[Employee]' );
+.Ve
+.PP
+This constraint says that \f(CW\*(C`employees\*(C'\fR must be an array reference
+where each element of the array is an \f(CW\*(C`Employee\*(C'\fR object. It's worth
+noting that an \fIempty\fR array reference also satisfies this
+constraint.
+.PP
+Parameterizable type constraints (or \*(L"container types\*(R"), such as
+\&\f(CW\*(C`ArrayRef[`a]\*(C'\fR, can be made more specific with a type parameter. In
+fact, we can arbitrarily nest these types, producing something like
+\&\f(CW\*(C`HashRef[ArrayRef[Int]]\*(C'\fR. However, you can also just use the type by
+itself, so \f(CW\*(C`ArrayRef\*(C'\fR is legal. (2)
+.PP
+If you jump down to the definition of the \f(CW\*(C`Employee\*(C'\fR class, you will
+see that it has an \f(CW\*(C`employer\*(C'\fR attribute.
+.PP
+When we set the \f(CW\*(C`employees\*(C'\fR for a \f(CW\*(C`Company\*(C'\fR we want to make sure
+that each of these employee objects refers back to the right
+\&\f(CW\*(C`Company\*(C'\fR in its \f(CW\*(C`employer\*(C'\fR attribute.
+.PP
+To do that, we need to hook into object construction. Moose lets us do
+this by writing a \f(CW\*(C`BUILD\*(C'\fR method in our class. When your class
+defined a \f(CW\*(C`BUILD\*(C'\fR method, it will be called immediately after an
+object construction, but before the object is returned to the caller
+(3).
+.PP
+The \f(CW\*(C`Company\*(C'\fR class uses the \f(CW\*(C`BUILD\*(C'\fR method to ensure that each
+employee of a company has the proper \f(CW\*(C`Company\*(C'\fR object in its
+\&\f(CW\*(C`employer\*(C'\fR attribute:
+.PP
+.Vb 8
+\& sub BUILD {
+\& my ( $self, $params ) = @_;
+\& if ( $self\->employees ) {
+\& foreach my $employee ( @{ $self\->employees } ) {
+\& $employee\->employer($self);
+\& }
+\& }
+\& }
+.Ve
+.PP
+The \f(CW\*(C`BUILD\*(C'\fR method is executed after type constraints are checked, so
+it is safe to assume that \f(CW\*(C`$self\->employees\*(C'\fR will return an array
+reference, and that the elements of that array will be \f(CW\*(C`Employee\*(C'\fR
+objects.
+.PP
+We also want to make sure that whenever the \f(CW\*(C`employees\*(C'\fR attribute for
+a \f(CW\*(C`Company\*(C'\fR is changed, we also update the \f(CW\*(C`employer\*(C'\fR for each
+employee.
+.PP
+To do this we can use an \f(CW\*(C`after\*(C'\fR modifier:
+.PP
+.Vb 8
+\& after 'employees' => sub {
+\& my ( $self, $employees ) = @_;
+\& if ($employees) {
+\& foreach my $employee ( @{$employees} ) {
+\& $employee\->employer($self);
+\& }
+\& }
+\& };
+.Ve
+.PP
+Again, as with the \f(CW\*(C`BUILD\*(C'\fR method, we know that the type constraint
+check has already happened, so we can just check for definedness on the
+\&\f(CW$employees\fR argument.
+.PP
+The \fBPerson\fR class does have demonstrate anything new. It has several
+\&\f(CW\*(C`required\*(C'\fR attributes. It also has a \f(CW\*(C`predicate\*(C'\fR method, which we
+first used in recipe 3.
+.PP
+The only new feature in the \f(CW\*(C`Employee\*(C'\fR class is the \f(CW\*(C`override\*(C'\fR
+method modifier:
+.PP
+.Vb 4
+\& override 'full_name' => sub {
+\& my $self = shift;
+\& super() . ', ' . $self\->title;
+\& };
+.Ve
+.PP
+This is just a sugary alternative to Perl's built in \f(CW\*(C`SUPER::\*(C'\fR
+feature. However, there is one difference. You cannot pass any
+arguments to \f(CW\*(C`super\*(C'\fR. Instead, Moose simply passes the same
+parameters that were passed to the method.
+.PP
+A more detailed example of usage can be found in
+\&\fIt/000_recipes/moose_cookbook_basics_recipe4.t\fR.
+.SH "CONCLUSION"
+.IX Header "CONCLUSION"
+This recipe was intentionally longer and more complex. It illustrates
+how Moose classes can be used together with type constraints, as well
+as the density of information that you can get out of a small amount
+of typing when using Moose.
+.PP
+This recipe also introduced the \f(CW\*(C`subtype\*(C'\fR function, the \f(CW\*(C`required\*(C'\fR
+attribute, and the \f(CW\*(C`override\*(C'\fR method modifier.
+.PP
+We will revisit type constraints in future recipes, and cover type
+coercion as well.
+.SH "FOOTNOTES"
+.IX Header "FOOTNOTES"
+.IP "(1)" 4
+.IX Item "(1)"
+The value being checked is also passed as the first argument to
+the \f(CW\*(C`where\*(C'\fR block, so it can be accessed as \f(CW$_[0]\fR.
+.IP "(2)" 4
+.IX Item "(2)"
+Note that \f(CW\*(C`ArrayRef[]\*(C'\fR will not work. Moose will not parse this as a
+container type, and instead you will have a new type named
+\&\*(L"ArrayRef[]\*(R", which doesn't make any sense.
+.IP "(3)" 4
+.IX Item "(3)"
+The \f(CW\*(C`BUILD\*(C'\fR method is actually called by \f(CW\*(C`Moose::Object\->BUILDALL\*(C'\fR, which is called by \f(CW\*(C`Moose::Object\->new\*(C'\fR. The \f(CW\*(C`BUILDALL\*(C'\fR
+method climbs the object inheritance graph and calls any \f(CW\*(C`BUILD\*(C'\fR
+methods it finds in the correct order.
+.SH "AUTHORS"
+.IX Header "AUTHORS"
+Stevan Little <stevan@iinteractive.com>
+.PP
+Dave Rolsky <autarch@urth.org>
+.SH "COPYRIGHT AND LICENSE"
+.IX Header "COPYRIGHT AND LICENSE"
+Copyright 2006\-2009 by Infinity Interactive, Inc.
+.PP
+<http://www.iinteractive.com>
+.PP
+This library is free software; you can redistribute it and/or modify
+it under the same terms as Perl itself.
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