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Moose::Cookbook::Basics::BinaryUser_CMoose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild(3pm)
NAME
Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild - Builder methods and lazy_build
VERSION
version 2.2200
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,
builder => '_build_child_tree',
);
has 'right' => (
is => 'rw',
isa => 'BinaryTree',
predicate => 'has_right',
lazy => 1,
builder => '_build_child_tree',
);
before 'right', 'left' => sub {
my ($self, $tree) = @_;
$tree->parent($self) if defined $tree;
};
sub _build_child_tree {
my $self = shift;
return BinaryTree->new( parent => $self );
}
DESCRIPTION
If you've already read Moose::Cookbook::Basics::BinaryTree_AttributeFeatures, then this
example should look very familiar. In fact, all we've done here is replace the attribute's
"default" parameter with a "builder".
In this particular case, the "default" and "builder" options act in exactly the same way.
When the "left" or "right" attribute is read, Moose calls the builder method to initialize
the attribute.
Note that Moose calls the builder method on the object which has the attribute. Here's an
example:
my $tree = BinaryTree->new();
my $left = $tree->left();
When "$tree->left()" is called, Moose calls "$tree->_build_child_tree()" in order to
populate the "left" attribute. If we had passed "left" to the original constructor, the
builder would not be called.
There are some differences between "default" and "builder". Notably, a builder is
subclassable, and can be composed from a role. See Moose::Manual::Attributes for more
details.
The lazy_build shortcut
The "lazy_build" attribute option can be used as sugar to specify a whole set of attribute
options at once:
has 'animal' => (
is => 'ro',
isa => 'Animal',
lazy_build => 1,
);
This is a shorthand for:
has 'animal' => (
is => 'ro',
isa => 'Animal',
required => 1,
lazy => 1,
builder => '_build_animal',
predicate => 'has_animal',
clearer => 'clear_animal',
);
If your attribute starts with an underscore, Moose is smart and will do the right thing
with the "predicate" and "clearer", making them both start with an underscore. The
"builder" method always starts with an underscore.
You can read more about "lazy_build" in Moose::Meta::Attribute
CONCLUSION
The "builder" option is a more OO-friendly version of the "default" functionality. It also
separates the default-generating code into a well-defined method. Sprinkling your
attribute definitions with anonymous subroutines can be quite ugly and hard to follow.
AUTHORS
o Stevan Little <stevan AT cpan.org>
o Dave Rolsky <autarch AT urth.org>
o Jesse Luehrs <doy AT cpan.org>
o Shawn M Moore <sartak AT cpan.org>
o ' (Yuval Kogman) <nothingmuch AT woobling.org>
o Karen Etheridge <ether AT cpan.org>
o Florian Ragwitz <rafl AT debian.org>
o Hans Dieter Pearcey <hdp AT cpan.org>
o Chris Prather <chris AT prather.org>
o Matt S Trout <mstrout AT cpan.org>
COPYRIGHT AND LICENSE
This software is copyright (c) 2006 by Infinity Interactive, Inc.
This is free software; you can redistribute it and/or modify it under the same terms as
the Perl 5 programming language system itself.
perl v5.34.0 Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild(3pm)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache
Under GNU General Public License
2024-12-21 07:51 @52.15.187.41 CrawledBy Mozilla/5.0 AppleWebKit/537.36 (KHTML, like Gecko; compatible; ClaudeBot/1.0; +claudebot@anthropic.com)