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Moose::Cookbook::BMoose::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)