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+.\" ========================================================================
+.\"
+.IX Title "Moose::Cookbook::Basics::Recipe3 3"
+.TH Moose::Cookbook::Basics::Recipe3 3 "2009-08-11" "perl v5.8.7" "User Contributed Perl Documentation"
+.SH "NAME"
+Moose::Cookbook::Basics::Recipe3 \- A lazy \fBBinaryTree\fR example
+.SH "SYNOPSIS"
+.IX Header "SYNOPSIS"
+.Vb 2
+\& package BinaryTree;
+\& use Moose;
+.Ve
+.PP
+.Vb 1
+\& has 'node' => ( is => 'rw', isa => 'Any' );
+.Ve
+.PP
+.Vb 6
+\& has 'parent' => (
+\& is => 'rw',
+\& isa => 'BinaryTree',
+\& predicate => 'has_parent',
+\& weak_ref => 1,
+\& );
+.Ve
+.PP
+.Vb 8
+\& has 'left' => (
+\& is => 'rw',
+\& isa => 'BinaryTree',
+\& predicate => 'has_left',
+\& lazy => 1,
+\& default => sub { BinaryTree\->new( parent => $_[0] ) },
+\& trigger => \e&_set_parent_for_child
+\& );
+.Ve
+.PP
+.Vb 8
+\& has 'right' => (
+\& is => 'rw',
+\& isa => 'BinaryTree',
+\& predicate => 'has_right',
+\& lazy => 1,
+\& default => sub { BinaryTree\->new( parent => $_[0] ) },
+\& trigger => \e&_set_parent_for_child
+\& );
+.Ve
+.PP
+.Vb 2
+\& sub _set_parent_for_child {
+\& my ( $self, $child ) = @_;
+.Ve
+.PP
+.Vb 2
+\& confess "You cannot insert a tree which already has a parent"
+\& if $child\->has_parent;
+.Ve
+.PP
+.Vb 2
+\& $child\->parent($self);
+\& }
+.Ve
+.SH "DESCRIPTION"
+.IX Header "DESCRIPTION"
+This recipe shows how various advanced attribute features can be used
+to create complex and powerful behaviors. In particular, we introduce
+a number of new attribute options, including \f(CW\*(C`predicate\*(C'\fR, \f(CW\*(C`lazy\*(C'\fR,
+and \f(CW\*(C`trigger\*(C'\fR.
+.PP
+The example class is a classic binary tree. Each node in the tree is
+itself an instance of \f(CW\*(C`BinaryTree\*(C'\fR. It has a \f(CW\*(C`node\*(C'\fR, which holds
+some arbitrary value. It has \f(CW\*(C`right\*(C'\fR and \f(CW\*(C`left\*(C'\fR attributes, which
+refer to its child trees, and a \f(CW\*(C`parent\*(C'\fR.
+.PP
+Let's take a look at the \f(CW\*(C`node\*(C'\fR attribute:
+.PP
+.Vb 1
+\& has 'node' => ( is => 'rw', isa => 'Any' );
+.Ve
+.PP
+Moose generates a read-write accessor for this attribute. The type
+constraint is \f(CW\*(C`Any\*(C'\fR, which literally means it can contain anything.
+.PP
+We could have left out the \f(CW\*(C`isa\*(C'\fR option, but in this case, we are
+including it for the benefit of other programmers, not the computer.
+.PP
+Next, let's move on to the \f(CW\*(C`parent\*(C'\fR attribute:
+.PP
+.Vb 6
+\& has 'parent' => (
+\& is => 'rw',
+\& isa => 'BinaryTree',
+\& predicate => 'has_parent',
+\& weak_ref => 1,
+\& );
+.Ve
+.PP
+Again, we have a read-write accessor. This time, the \f(CW\*(C`isa\*(C'\fR option
+says that this attribute must always be an instance of
+\&\f(CW\*(C`BinaryTree\*(C'\fR. In the second recipe, we saw that every time we create
+a Moose-based class, we also get a corresponding class type
+constraint.
+.PP
+The \f(CW\*(C`predicate\*(C'\fR option is new. It creates a method which can be used
+to check whether or not a given attribute has been initialized. In
+this case, the method is named \f(CW\*(C`has_parent\*(C'\fR.
+.PP
+This brings us to our last attribute option, \f(CW\*(C`weak_ref\*(C'\fR. Since
+\&\f(CW\*(C`parent\*(C'\fR is a circular reference (the tree in \f(CW\*(C`parent\*(C'\fR should
+already have a reference to this one, in its \f(CW\*(C`left\*(C'\fR or \f(CW\*(C`right\*(C'\fR
+attribute), we want to make sure that we weaken the reference to avoid
+memory leaks. If \f(CW\*(C`weak_ref\*(C'\fR is true, it alters the accessor function
+so that the reference is weakened when it is set.
+.PP
+Finally, we have the the \f(CW\*(C`left\*(C'\fR and \f(CW\*(C`right\*(C'\fR attributes. They are
+essentially identical except for their names, so we'll just look at
+\&\f(CW\*(C`left\*(C'\fR:
+.PP
+.Vb 8
+\& has 'left' => (
+\& is => 'rw',
+\& isa => 'BinaryTree',
+\& predicate => 'has_left',
+\& lazy => 1,
+\& default => sub { BinaryTree\->new( parent => $_[0] ) },
+\& trigger => \e&_set_parent_for_child
+\& );
+.Ve
+.PP
+There are three new options here, \f(CW\*(C`lazy\*(C'\fR, \f(CW\*(C`default\*(C'\fR, and
+\&\f(CW\*(C`trigger\*(C'\fR. The \f(CW\*(C`lazy\*(C'\fR and \f(CW\*(C`default\*(C'\fR options options are linked. In
+fact, you cannot have a \f(CW\*(C`lazy\*(C'\fR attribute unless it has a \f(CW\*(C`default\*(C'\fR
+(or a \f(CW\*(C`builder\*(C'\fR, but we'll cover that later). If you try to make an
+attribute lazy without a default, class creation will fail with an
+exception. (2)
+.PP
+In the second recipe the \fBBankAccount\fR's \f(CW\*(C`balance\*(C'\fR attribute had a
+default value of \f(CW0\fR. Given a non\-reference, Perl copies the
+\&\fIvalue\fR. However, given a reference, it does not do a deep clone,
+instead simply copying the reference. If you just specified a simply
+reference for a default, Perl would create it once and it would be
+shared by all objects with that attribute.
+.PP
+As a workaround, we use an anonymous subroutine to generate a new
+reference every time the default is called.
+.PP
+.Vb 1
+\& has 'foo' => ( is => 'rw', default => sub { [] } );
+.Ve
+.PP
+In fact, using a non-subroutine reference as a default is illegal in Moose.
+.PP
+.Vb 2
+\& # will fail
+\& has 'foo' => ( is => 'rw', default => [] );
+.Ve
+.PP
+This will blow up, so don't do it.
+.PP
+You'll notice that we use \f(CW$_[0]\fR in our default sub. When the
+default subroutine is executed, it is called as a method on the
+object.
+.PP
+In our case, we're making a new \f(CW\*(C`BinaryTree\*(C'\fR object in our default,
+with the current tree as the parent.
+.PP
+Normally, when an object is instantiated, any defaults are evaluated
+immediately. With our \f(CW\*(C`BinaryTree\*(C'\fR class, this would be a big
+problem! We'd create the first object, which would immediately try to
+populate its \f(CW\*(C`left\*(C'\fR and \f(CW\*(C`right\*(C'\fR attributes, which would create a new
+\&\f(CW\*(C`BinaryTree\*(C'\fR, which would populate \fIits\fR \f(CW\*(C`left\*(C'\fR and \f(CW\*(C`right\*(C'\fR
+slots. Kaboom!
+.PP
+By making our \f(CW\*(C`left\*(C'\fR and \f(CW\*(C`right\*(C'\fR attributes \f(CW\*(C`lazy\*(C'\fR, we avoid this
+problem. If the attribute has a value when it is read, the default is
+never executed at all.
+.PP
+We still have one last bit of behavior to add. The autogenerated
+\&\f(CW\*(C`right\*(C'\fR and \f(CW\*(C`left\*(C'\fR accessors are not quite correct. When one of
+these is set, we want to make sure that we update the parent of the
+\&\f(CW\*(C`left\*(C'\fR or \f(CW\*(C`right\*(C'\fR attribute's tree.
+.PP
+We could write our own accessors, but then why use Moose at all?
+Instead, we use a \f(CW\*(C`trigger\*(C'\fR. A \f(CW\*(C`trigger\*(C'\fR accepts a subroutine
+reference, which will be called as a method whenever the attribute is
+set. This can happen both during object construction or later by
+passing a new object to the attribute's accessor method. However, it
+is not called when a value is provided by a \f(CW\*(C`default\*(C'\fR or \f(CW\*(C`builder\*(C'\fR.
+.PP
+.Vb 2
+\& sub _set_parent_for_child {
+\& my ( $self, $child ) = @_;
+.Ve
+.PP
+.Vb 2
+\& confess "You cannot insert a tree which already has a parent"
+\& if $child\->has_parent;
+.Ve
+.PP
+.Vb 2
+\& $child\->parent($self);
+\& }
+.Ve
+.PP
+This trigger does two things. First, it ensures that the new child
+node does not already have a parent. This is done for the sake of
+simplifying the example. If we wanted to be more clever, we would
+remove the child from its old parent tree and add it to the new one.
+.PP
+If the child has no parent, we will add it to the current tree, and we
+ensure that is has the correct value for its \f(CW\*(C`parent\*(C'\fR attribute.
+.PP
+As with all the other recipes, \fBBinaryTree\fR can be used just like any
+other Perl 5 class. A more detailed example of its usage can be found
+in \fIt/000_recipes/moose_cookbook_basics_recipe3.t\fR.
+.SH "CONCLUSION"
+.IX Header "CONCLUSION"
+This recipe introduced several of Moose's advanced features. We hope
+that this inspires you to think of other ways these features can be
+used to simplify your code.
+.SH "FOOTNOTES"
+.IX Header "FOOTNOTES"
+.IP "(1)" 4
+.IX Item "(1)"
+Weak references are tricky things, and should be used sparingly and
+appropriately (such as in the case of circular refs). If you are not
+careful, attribute values could disappear \*(L"mysteriously\*(R" because
+Perl's reference counting garbage collector has gone and removed the
+item you are weak\-referencing.
+.Sp
+In short, don't use them unless you know what you are doing :)
+.IP "(2)" 4
+.IX Item "(2)"
+You \fIcan\fR use the \f(CW\*(C`default\*(C'\fR option without the \f(CW\*(C`lazy\*(C'\fR option if you
+like, as we showed in the second recipe.
+.Sp
+Also, you can use \f(CW\*(C`builder\*(C'\fR instead of \f(CW\*(C`default\*(C'\fR. See
+Moose::Cookbook::Basics::Recipe8 for details.
+.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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