Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild - Builder methods and lazy_build

       •   Stevan Little <stevan@cpan.org>

       •   Dave Rolsky <autarch@urth.org>

       •   Jesse Luehrs <doy@cpan.org>

       •   Shawn M Moore <sartak@cpan.org>

       •   יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>

       •   Karen Etheridge <ether@cpan.org>

       •   Florian Ragwitz <rafl@debian.org>

       •   Hans Dieter Pearcey <hdp@cpan.org>

       •   Chris Prather <chris@prather.org>

       •   Matt S Trout <mstrout@cpan.org>

       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.

       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 v5.40.0                                       2024-10-20              Moose::Cookboo...derAndLazyBuild(3pm)

       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 ontheobjectwhichhastheattribute. 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.

   Thelazy_buildshortcut
       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

       Moose::Cookbook::Basics::BinaryTree_BuilderAndLazyBuild - Builder methods and lazy_build

         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 );
         }

       version 2.2207