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Moose-0.29
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/ Moose::Cookbook::Recipe3

NAME

Moose::Cookbook::Recipe3 - A lazy BinaryTree example

SYNOPSIS

package BinaryTree;
use Moose;

has 'node' => (is => 'rw', isa => 'Any');

has 'parent' => (
    is        => 'rw',
    isa       => 'BinaryTree',	
    predicate => 'has_parent',
    weak_ref  => 1,
);

has 'left' => (
    is        => 'rw',	
    isa       => 'BinaryTree',		
    predicate => 'has_left',  
    lazy      => 1,
    default   => sub { BinaryTree->new(parent => $_[0]) },       
);

has 'right' => (
    is        => 'rw',	
    isa       => 'BinaryTree',		
    predicate => 'has_right',   
    lazy      => 1,       
    default   => sub { BinaryTree->new(parent => $_[0]) },       
);

before 'right', 'left' => sub {
    my ($self, $tree) = @_;
    $tree->parent($self) if defined $tree;   
};

DESCRIPTION

In this recipe we take a closer look at attributes, and see how some of their more advanced features can be used to create fairly complex behaviors.

The class in this recipe is a classic binary tree, each node in the tree is represented by an instance of the BinaryTree class. Each instance has a node slot to hold an arbitrary value, a right slot to hold the right node, a left slot to hold the left node, and finally a parent slot to hold a reference back up the tree.

Now, let's start with the code. Our first attribute is the node slot, defined as such:

has 'node' => (is => 'rw', isa => 'Any');

If you recall from the previous recipes, this slot will have a read/write accessor generated for it, and has a type constraint on it. The new item here is the type constraint of Any. Any is the "root" of the Moose::Util::TypeConstraints type hierarchy. It means exactly what it says: any value passes the constraint. Now, you could just as easily have left out the isa, leaving the node slot unconstrained and retaining this behavior. But in this case, we are really including the type constraint for the benefit of other programmers, not the computer. It makes clear my intent that the node attribute can be of any type, and that the class is a polymorphic container.

Next, let's move on to the parent slot:

has 'parent' => (
    is        => 'rw',
    isa       => 'BinaryTree',	
    predicate => 'has_parent',
    weak_ref  => 1,
);

As you already know, this code tells you that parent gets a read/write accessor and is constrained to only accept instances of BinaryTree. You will of course remember from the second recipe that the BinaryTree type constraint is automatically created for us by Moose.

The next attribute option is new, though: the predicate option. This option creates a method which can be used to check whether a given slot (in this case parent) contains a defined value. In this case it will create a method called has_parent. Quite simple, and quite handy too.

This brings us to our last attribute option, also a new one. Since parent is a circular reference (the tree in parent should already have a reference to this one, in its left or right node), we want to make sure that it is also a weakened reference to avoid memory leaks. The weak_ref attribute option will do just that, weak_ref simply takes a boolean value (1 or 0) and then alters the accessor function to weaken the reference to any value stored in the parent slot (1).

Now, onto the left and right attributes. They are essentially identical, save for different names, so I will just describe one here:

has 'left' => (
    is        => 'rw',	
    isa       => 'BinaryTree',		
    predicate => 'has_left',  
    lazy      => 1,
    default   => sub { BinaryTree->new(parent => $_[0]) },       
);

You already know what the is, isa and predicate options do, but now we have two new options. These two options are actually linked together, in fact: you cannot use the lazy option unless you have set the default option. Class creation will fail with an exception (2).

Before I go into detail about how lazy works, let me first explain how default works, and in particular why it is wrapped in a CODE ref.

In the second recipe the BankAccount's balance slot had a default value of 0. Since Perl will copy strings and numbers by value, this was all we had to say. But for any other item (ARRAY ref, HASH ref, object instance, etc) you would need to wrap it in a CODE reference, so this:

has 'foo' => (is => 'rw', default => []);

is actually illegal in Moose. Instead, what you really want is this:

has 'foo' => (is => 'rw', default => sub { [] });

This ensures that each instance of this class will get its own ARRAY ref in the foo slot.

One other feature of the CODE ref version of the default option is that when the subroutine is executed (to get the default value), we pass in the instance where the slot will be stored. This can come in quite handy at times, as illustrated above, with this code:

default => sub { BinaryTree->new(parent => $_[0]) },

The default value being generated is a new BinaryTree instance for the left (or right) slot. Here we set up the correct relationship by passing the current instance as the parent argument to the constructor.

Now, before we go on to the lazy option, I want you to think for a moment. When an instance of this class is created, and the slots are being initialized, the "normal" behavior would be for the left and right slots to be populated with a new instance of BinaryTree. In creating that instance of the left or right slots, we would need to create new instances to populate the left and right slots of those instances. This would continue in an infinitely recursive spiral of death until you had exhausted all available memory on your machine.

This is, of course, not good :)

Which brings us to the lazy attribute option. The lazy option does just what it says: it lazily initializes the slot within the instance. This means that it waits till absolutely the latest possible moment to populate the slot. So if you, the user, store a value in the slot, everything works normally, and what you pass in is stored. However, if you read the slot before storing a value in it, then at that exact moment (and no sooner), the slot will be populated with the value of the default option.

This option is what allows the BinaryTree class to instantiate objects without fear of the infinitely recursive spiral of death mentioned earlier.

So, we have described a quite complex set of behaviors here, and not one method had to be written. But wait, we aren't quite done yet; the autogenerated right and left accessors are not completely correct. They will not install the parental relationships that we need. We could write our own accessors, but that would require us to implement all those features we got automatically (type constraints, lazy initialization, and so on). Instead, we use method modifiers again:

before 'right', 'left' => sub {
    my ($self, $tree) = @_;
    $tree->parent($self) if defined $tree;   
};

This is a before modifier, just like we saw in the second recipe, but with two slight differences. First, we are applying this to more than one method at a time. Since both the left and right methods need the same feature, it makes sense. The second difference is that we are not wrapping an inherited method anymore, but instead a method of our own local class. Wrapping local methods is no different, the only requirement is that the wrappee be created before the wrapper (after all, you cannot wrap something which doesn't exist, right?).

Now, as with all the other recipes, you can go about using BinaryTree like any other Perl 5 class. A more detailed example of its usage can be found in t/003_recipe.t.

CONCLUSION

This recipe introduced you to some of the more advanced behavioral possibilities of Moose's attribute mechanism. I hope that it has opened your mind to the powerful possibilities of Moose. In the next recipe we explore how we can create custom subtypes and take advantage of the plethora of useful modules out on CPAN with Moose.

FOOTNOTES

(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, you will have slot values disappear "mysteriously" because perls reference counting garbage collector has gone and removed the item you are weak-referencing.

In short, don't use them unless you know what you are doing :)

(2)

You can use the default option without the lazy option if you like, as we showed in the second recipe.

AUTHOR

Stevan Little <stevan@iinteractive.com>

COPYRIGHT AND LICENSE

Copyright 2006, 2007 by Infinity Interactive, Inc.

http://www.iinteractive.com

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