NAME

Moose::Cookbook::Basics::Recipe9 - Builder methods and lazy_build

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 the Moose::Cookbook::Basics::Recipe3 manpage, 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 the Moose::Manual::Attributes manpage 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 the Moose::Manual::Attributes manpage

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.

AUTHOR

Dave Rolsky <autarch@urth.org>

COPYRIGHT AND LICENSE

Copyright 2006-2009 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.