Stevan Little > Moose-0.54 > Moose::Cookbook::Recipe9


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Moose::Cookbook::Recipe9 - 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 );


If you've already read Moose::Cookbook::Recipe3, then this example should look awfully familiar. In fact, all we've done here is replace the attribute default with a builder method.

In this particular case, the default and builder options act in exactly the same way. When the left or right attribute get method is called, Moose will call 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 in code:

  my $tree = BinaryTree->new();

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

At this point, Moose will call $tree->_build_child_tree() in order to populate the left attribute. If we had passed left to the original constructor, the builer would not be called.


There are some differences between default and builder. Because builder is called by name, it goes through Perl's normal inheritance system. This means that builder methods are both inheritable and overrideable.

For example, we might make a BinaryTree subclass:

  package TrinaryTree;
  use Moose;

  extends 'BinaryTree';

  has 'middle' => (
      is        => 'rw',
      isa       => 'BinaryTree',
      predicate => 'has_middle',
      lazy      => 1,
      builder   => '_build_child_tree',

This doesn't quite work though. If you look closely at the _build_child_tree method defined in BinaryTree, you'll notice that it hard-codes a class name. Naughty us!

Also, as a bonus, we'll pass @_ through, so subclasses can override the method to pass additional options to the constructor.

Good object-oriented code should allow itself to be subclassed gracefully. Let's tweak _build_child_tree:

  sub _build_child_tree {
      my $self = shift;

      return (ref $self)->new( parent => $self, @_ );

Now _build_child_tree can be gracefully inherited and overridden.


There's more to builders than just subclassing, though. The fact that builders are called by name also makes them suitable for use in a role.

  package HasAnimal;
  use Moose::Role;

  requires '_build_animal';

  has 'animal' => (
      is      => 'ro',
      isa     => 'Animal',
      lazy    => 1,
      builder => '_build_animal',

This role provides an animal attribute, but requires that the consumer of the role provide a builder method it.

  package CatLover;
  use Moose;

  with 'HasAnimal';

  sub _build_animal {
      return Cat->new();

The lazy_build shortcut

The lazy_build attribute parameter can be used as sugar to specify a whole bunch of options at once.

  has 'animal' => (
      is         => 'ro',
      isa        => 'Animal',
      lazy_build => 1,

This is a shorthand for this:

  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, since you will want this to be private the vast majority of the time.

Note that the builder method name is created by simply taking "_build_" and appending the attribute name. This means that attributes with a leading underscore like _animal end up with a builder named _build__animal.


The builder option is a more OO-friendly version of the default functionality. It also has the property of separating out the code into a separate well-defined method. This alone makes it valuable. It is quite ugly to jam a long default code reference into your attribute definition.

Here are some good rules for determining when to use builder vs default.

If the default value is a simple scalar that only needs to be calculated once (or a constant), use default.

If the default value is an empty reference that needs to be wrapped in a coderef like sub { [] }, use default.

Otherwise, use builder.

This ensures that your classes are easily subclassable, and also helps keep crufty code out of your attribute definition blocks.


Dave Rolsky <>


Copyright 2006-2008 by Infinity Interactive, Inc.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

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