| Moose::Cookbook::Basics::Recipe3 - A lazy B<BinaryTree> example |
Moose::Cookbook::Basics::Recipe3 - A lazy BinaryTree example
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] ) }, trigger => \&_set_parent_for_child );
has 'right' => ( is => 'rw', isa => 'BinaryTree', predicate => 'has_right', lazy => 1, default => sub { BinaryTree->new( parent => $_[0] ) }, trigger => \&_set_parent_for_child );
sub _set_parent_for_child { my ( $self, $child ) = @_;
confess "You cannot insert a tree which already has a parent"
if $child->has_parent;
$child->parent($self);
}
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 predicate, lazy,
and trigger.
The example class is a classic binary tree. Each node in the tree is
itself an instance of BinaryTree. It has a node, which holds
some arbitrary value. It has right and left attributes, which
refer to its child trees, and a parent.
Let's take a look at the node attribute:
has 'node' => ( is => 'rw', isa => 'Any' );
Moose generates a read-write accessor for this attribute. The type
constraint is Any, which means literally means it can contain
anything.
We could have left out the isa option, but in this case, we are
including it for the benefit of other programmers, not the computer.
Next, let's move on to the parent attribute:
has 'parent' => ( is => 'rw', isa => 'BinaryTree', predicate => 'has_parent', weak_ref => 1, );
Again, we have a read-write accessor. This time, the isa option
says that this attribute must always be an instance of
BinaryTree. In the second recipe, we saw that every time we create
a Moose-based class, we also get a corresponding class type
constraint.
The predicate 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 has_parent.
This brings us to our last attribute option, weak_ref. Since
parent is a circular reference (the tree in parent should
already have a reference to this one, in its left or right
attribute), we want to make sure that we weaken the reference to avoid
memory leaks. If weak_ref is true, it alters the accessor function
so that the reference is weakened when it is set.
Finally, we have the the left and right attributes. They are
essentially identical except for their names, so we'll just look at
left:
has 'left' => ( is => 'rw', isa => 'BinaryTree', predicate => 'has_left', lazy => 1, default => sub { BinaryTree->new( parent => $_[0] ) }, trigger => \&_set_parent_for_child );
There are three new options here, lazy, default, and
trigger. The lazy and default options options are linked. In
fact, you cannot have a lazy attribute unless it has a default
(or a builder, 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)
In the second recipe the BankAccount's balance attribute had a
default value of 0. Given a non-reference, Perl copies the
value. 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.
As a workaround, we use an anonymous subroutine to generate a new reference every time the default is called.
has 'foo' => ( is => 'rw', default => sub { [] } );
In fact, using a non-subroutine reference as a default is illegal in Moose.
# will fail has 'foo' => ( is => 'rw', default => [] );
This will blow up, so don't do it.
You'll notice that we use $_[0] in our default sub. When the
default subroutine is executed, it is called as a method on the
object.
In our case, we're making a new BinaryTree object in our default,
with the current tree as the parent.
Normally, when an object is instantiated, any defaults are evaluated
immediately. With our BinaryTree class, this would be a big
problem! We'd create the first object, which would immediately try to
populate its left and right attributes, which would create a new
BinaryTree, which would populate its left and right
slots. Kaboom!
By making our left and right attributes lazy, we avoid this
problem. If the attribute has a value when it is read, the default is
never executed at all.
We still have one last bit of behavior to add. The autogenerated
right and left accessors are not quite correct. When one of
these is set, we want to make sure that we update the parent of the
left or right attribute's tree.
We could write our own accessors, but then why use Moose at all?
Instead, we use a trigger. A trigger 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 default or builder.
sub _set_parent_for_child { my ( $self, $child ) = @_;
confess "You cannot insert a tree which already has a parent"
if $child->has_parent;
$child->parent($self);
}
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.
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 parent attribute.
As with all the other recipes, BinaryTree can be used just like any other Perl 5 class. A more detailed example of its usage can be found in t/000_recipes/moose_cookbook_basics_recipe3.t.
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.
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 "mysteriously" because Perl's 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 :)
You can use the default option without the lazy option if you
like, as we showed in the second recipe.
Also, you can use builder instead of default. See
the Moose::Cookbook::Basics::Recipe9 manpage for details.
Stevan Little <stevan@iinteractive.com>
Dave Rolsky <autarch@urth.org>
Copyright 2006-2009 by Infinity Interactive, Inc.
This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
| Moose::Cookbook::Basics::Recipe3 - A lazy B<BinaryTree> example |