The London Perl and Raku Workshop takes place on 26th Oct 2024. If your company depends on Perl, please consider sponsoring and/or attending.


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


version 2.2207


  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 );


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


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.


  • Stevan Little <>

  • Dave Rolsky <>

  • Jesse Luehrs <>

  • Shawn M Moore <>

  • יובל קוג'מן (Yuval Kogman) <>

  • Karen Etheridge <>

  • Florian Ragwitz <>

  • Hans Dieter Pearcey <>

  • Chris Prather <>

  • Matt S Trout <>


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.