6 Moose::Cookbook::Recipe3 - A lazy B<BinaryTree> example
13 has 'node' => (is => 'rw', isa => 'Any');
18 predicate => 'has_parent',
25 predicate => 'has_left',
27 default => sub { BinaryTree->new(parent => $_[0]) },
33 predicate => 'has_right',
35 default => sub { BinaryTree->new(parent => $_[0]) },
38 before 'right', 'left' => sub {
39 my ($self, $tree) = @_;
40 $tree->parent($self) if defined $tree;
45 In this recipe we take a closer look at attributes, and see how
46 some of their more advanced features can be used to create fairly
49 The class in this recipe is a classic binary tree, each node in the
50 tree is represented by an instance of the B<BinaryTree> class. Each
51 instance has a C<node> slot to hold an arbitrary value, a C<right>
52 slot to hold the right node, a C<left> slot to hold the left node,
53 and finally a C<parent> slot to hold a reference back up the tree.
55 Now, let's start with the code, our first attribute is the C<node>
56 slot, defined as such:
58 has 'node' => (is => 'rw', isa => 'Any');
60 If you recall from the previous recipes, this slot will have a
61 read/write accessor generated for it, and has a type constraint on it.
62 The new item here is the type constraint of C<Any>. In the type
63 constraint hierarchy in L<Moose::Utils::TypeConstraints>, the C<Any>
64 constraint is the "root" of the hierarchy. It means exactly what it
65 says, it allows anything to pass. Now, you could just as easily have
66 left out the C<isa>, left the C<node> slot unconstrained and gotten the
67 same behavior. But here, we are really including the type constraint
68 for the benefit of other programmers, not the computer. It makes
69 clear my intent that the C<node> can be of any type, and that the
70 class is a polymorphic container. Next, lets move onto the C<parent>
76 predicate => 'has_parent',
80 As you already know from reading the previous recipes, this code
81 tells you that C<parent> gets a read/write accessor and is constrained
82 to only accept instances of B<BinaryTree>. You will of course remember
83 from the second recipe that the C<BinaryTree> type constraint is
84 automatically created for us by Moose.
86 The next attribute option is new though, the C<predicate> option.
87 This option creates a method, which can be used to check to see if
88 a given slot (in this case C<parent>) has a defined value in it. In
89 this case it will create a method called C<has_parent>. Quite simple,
90 and also quite handy too.
92 This brings us to our last attribute, and also a new one. Since the
93 C<parent> is a circular reference (the tree in C<parent> should
94 already have a reference in either it's C<left> or C<right> nodes),
95 we want to make sure that it is also a weakened reference to avoid
96 memory leaks. The C<weak_ref> attribute option will do just that,
97 C<weak_ref> simply takes a boolean value (C<1> or C<0>) and it will
98 then add the extra capability to the accessor function to weaken
99 the reference of any value stored in the C<parent> slot (1).
101 Now, onto the C<left> and C<right> attributes. They are essentially
102 the same things, only with different names, so I will just describe
108 predicate => 'has_left',
110 default => sub { BinaryTree->new(parent => $_[0]) },
113 You already know what the C<is>, C<isa> and C<predicate> options
114 do, but now we have two more new options. These two options are
115 actually linked together, in fact, you cannot use the C<lazy>
116 option unless you have set the C<default> option. The class
117 creation will fail with an exception (2).
119 Before I go into detail about how C<lazy> works, let me first
120 explain how C<default> works, and in particular why it is wrapped
123 In the second recipe the B<BankAccount>'s C<balance> slot had a
124 default value of C<0>. Since Perl will copy strings and numbers
125 by value, this was all we had to say. But for any other item
126 (ARRAY ref, HASH ref, object instance, etc) you would need to
127 wrap this into a CODE reference, so this:
129 has 'foo' => (is => 'rw', default => []);
131 is actually illegal in Moose. Instead, what you really want is
134 has 'foo' => (is => 'rw', default => sub { [] });
136 This assures that each instance of this class will get it's own
137 ARRAY ref in the C<foo> slot.
139 One other feature of the sub ref version of the C<default> option
140 is that when the subroutine is executed (to get back the expected
141 default value), we also pass in the instance where the slot will
142 be stored. This added feature can come in quite handy at times, as
143 is illustrated above, with this code:
145 default => sub { BinaryTree->new(parent => $_[0]) },
147 The default value being generated is a new C<BinaryTree> instance
148 for the C<left> (or C<right>) slot. Here we set up the parental
149 relationship by passing the current instance to the constructor.
151 Now, before we go on to the C<lazy> option, I want you to think
152 for a moment. When an instance of this class is created, and the
153 slots are being initialized, the "normal" behavior would be for
154 the C<left> and C<right> slots to be populated with a new instance
155 of B<BinaryTree>. In creating that instance of the C<left> or
156 C<right> slots, we would need to create new instances to populate
157 the C<left> and C<right> slots of I<those> instances. This would
158 continue in an I<infinitely recursive spiral of death> until you had
159 exhausted all available memory on your machine.
161 This is, of course, not good :)
163 Which brings us to the C<lazy> attribute option. The C<lazy> option
164 does just what it says. It lazily initializes the slot within the
165 instance. This means that it waits till the I<absolute> last possible
166 moment to populate the slot. This means that if you, the user, write
167 to the slot, everything happens as normal and what you pass in is stored.
168 However, if you I<read> the slot, then at that I<exact> moment (and no
169 sooner), the slot will be populated with the value of the C<default>
172 This option is what allows the B<BinaryTree> class to instantiate
173 objects without fear of the I<infinitely recursive spiral of death>
176 So, we have described a quite complex set of behaviors here, and not
177 one method had to be written. But wait, we can't get away that
178 easily. The autogenerated C<right> and C<left> accessors are not
179 completely correct. They will not install the parental relationships
180 that we need. We could write our own accessors, but that would require
181 us to implement all those features we got automatically (the type
182 constraint, the lazy initialization, etc). So instead we use the
183 method modifiers again.
185 before 'right', 'left' => sub {
186 my ($self, $tree) = @_;
187 $tree->parent($self) if defined $tree;
190 This is a C<before> modifier, just like we saw in the second recipe,
191 but with two slight differences. First, we are applying this to more
192 than one method at a time. Since both the C<left> and C<right> methods
193 need the same feature, it makes sense. The second difference is that
194 we are not wrapping an inherited method anymore, but instead a method
195 of our own local class. Wrapping local methods is no different, the
196 only requirement is that the wrappee be created before the wrapper
197 (after all, you cannot wrap something which doesn't exist right?).
199 Now, as with all the other recipes, you can go about using
200 B<BinaryTree> like any other Perl 5 class. A more detailed example of
201 usage can be found in F<t/003_recipe.t>.
205 This recipe introduced you to some of the more advanced behavioral
206 possibilities of Moose's attribute mechanism. I hope that it has
207 opened your mind to the powerful possibilities of Moose. In the next
208 recipe we explore how we can create custom subtypes and take
209 advantage of the plethora of useful modules out on CPAN with Moose.
217 Weak references are tricky things, and should be used sparingly
218 and appropriately (such as in the case of circular refs). If you
219 are not careful, you will have slot values disappear "mysteriously"
220 because perls reference counting garbage collector has gone and
221 removed the item you are weak-referencing.
223 In short, don't use them unless you know what you are doing :)
227 You I<can> use the C<default> option without the C<lazy> option if
228 you like, as we showed in the second recipe.
234 Stevan Little E<lt>stevan@iinteractive.comE<gt>
236 =head1 COPYRIGHT AND LICENSE
238 Copyright 2006, 2007 by Infinity Interactive, Inc.
240 L<http://www.iinteractive.com>
242 This library is free software; you can redistribute it and/or modify
243 it under the same terms as Perl itself.