Rename Basics::Recipe7 to Basics::Immutable

[gitmo/Moose.git] / lib / Moose / Cookbook / Basics / Recipe8.pod

package Moose::Cookbook::Basics::Recipe8;

# ABSTRACT: Builder methods and lazy_build

__END__


=pod

=head1 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 );
  }

=head1 DESCRIPTION

If you've already read
L<Moose::Cookbook::Basics::BinaryTree_AttributeFeatures>, then this example
should look very familiar. In fact, all we've done here is replace the
attribute's C<default> parameter with a C<builder>.

In this particular case, the C<default> and C<builder> options act in
exactly the same way. When the C<left> or C<right> attribute is read,
Moose calls the builder method to initialize the attribute.

Note that Moose calls the builder method I<on the object which has the
attribute>. Here's an example:

  my $tree = BinaryTree->new();

  my $left = $tree->left();

When C<< $tree->left() >> is called, Moose calls C<<
$tree->_build_child_tree() >> in order to populate the C<left>
attribute. If we had passed C<left> to the original constructor, the
builder would not be called.

There are some differences between C<default> and C<builder>. Notably,
a builder is subclassable, and can be composed from a role. See
L<Moose::Manual::Attributes> for more details.

=head2 The lazy_build shortcut

The C<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 C<predicate> and C<clearer>, making them
both start with an underscore. The C<builder> method I<always> starts
with an underscore.

You can read more about C<lazy_build> in L<Moose::Meta::Attribute>

=head1 CONCLUSION

The C<builder> option is a more OO-friendly version of the C<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.

=cut