Read-write vs. read-only

In fact, you could even omit "is", but that gives you an attribute that has no accessor. This can be useful with other attribute options, such as "handles". However, if your attribute generates no accessors, Moose will issue a warning, because that usually means the programmer forgot to say the attribute is read-only or read-write. If you really mean to have no accessors, you can silence this warning by setting "is" to "bare".

Accessor methods

By default, the accessor method has the same name as the attribute. If you declared your attribute as "ro" then your accessor will be read-only. If you declared it as "rw", you get a read-write accessor. Simple.

Given our "Person" example above, we now have a single "first_name" accessor that can read or write a "Person" object's "first_name" attribute's value.

If you want, you can also explicitly specify the method names to be used for reading and writing an attribute's value. This is particularly handy when you'd like an attribute to be publicly readable, but only privately settable. For example:

  has 'weight' => (
      is     => 'ro',
      writer => '_set_weight',
  );

This might be useful if weight is calculated based on other methods. For example, every time the "eat" method is called, we might adjust weight. This lets us hide the implementation details of weight changes, but still provide the weight value to users of the class.

Some people might prefer to have distinct methods for reading and writing. In Perl Best Practices, Damian Conway recommends that reader methods start with "get_" and writer methods start with "set_".

We can do exactly that by providing names for both the "reader" and "writer" methods:

  has 'weight' => (
      is     => 'rw',
      reader => 'get_weight',
      writer => 'set_weight',
  );

If you're thinking that doing this over and over would be insanely tedious, you're right! Fortunately, Moose provides a powerful extension system that lets you override the default naming conventions. See Moose::Manual::MooseX for more details.

Predicate and clearer methods

A predicate method tells you whether or not a given attribute is currently set. Note that an attribute can be explicitly set to "undef" or some other false value, but the predicate will return true.

The clearer method unsets the attribute. This is not the same as setting the value to "undef", but you can only distinguish between them if you define a predicate method!

Here's some code to illustrate the relationship between an accessor, predicate, and clearer method.

  package Person;

  use Moose;

  has 'ssn' => (
      is        => 'rw',
      clearer   => 'clear_ssn',
      predicate => 'has_ssn',
  );

  ...

  my $person = Person->new();
  $person->has_ssn; # false

  $person->ssn(undef);
  $person->ssn; # returns undef
  $person->has_ssn; # true

  $person->clear_ssn;
  $person->ssn; # returns undef
  $person->has_ssn; # false

  $person->ssn('123-45-6789');
  $person->ssn; # returns '123-45-6789'
  $person->has_ssn; # true

  my $person2 = Person->new( ssn => '111-22-3333');
  $person2->has_ssn; # true

By default, Moose does not make a predicate or clearer for you. You must explicitly provide names for them, and then Moose will create the methods for you.

Required or not?

  has 'name' => (
      is       => 'ro',
      required => 1,
  );

There are a couple caveats worth mentioning in regards to what "required" actually means.

Basically, all it says is that this attribute ("name") must be provided to the constructor or it must have either a default or a builder. It does not say anything about its value, so it could be "undef".

If you define a clearer method on a required attribute, the clearer will work, so even a required attribute can be unset after object construction.

This means that if you do make an attribute required, providing a clearer doesn't make much sense. In some cases, it might be handy to have a private "clearer" and "predicate" for a required attribute.

Default and builder methods

In the simplest form, you simply provide a non-reference scalar value for the "default" option:

  has 'size' => (
      is        => 'ro',
      default   => 'medium',
      predicate => 'has_size',
  );

If the size attribute is not provided to the constructor, then it ends up being set to "medium":

  my $person = Person->new();
  $person->size; # medium
  $person->has_size; # true

You can also provide a subroutine reference for "default". This reference will be called as a method on the object.

  has 'size' => (
      is => 'ro',
      default =>
          sub { ( 'small', 'medium', 'large' )[ int( rand 3 ) ] },
      predicate => 'has_size',
  );

This is a trivial example, but it illustrates the point that the subroutine will be called for every new object created.

When you provide a "default" subroutine reference, it is called as a method on the object, with no additional parameters:

  has 'size' => (
      is      => 'ro',
      default => sub {
          my $self = shift;

          return $self->height > 200 ? 'large' : 'average';
      },
  );

When the "default" is called during object construction, it may be called before other attributes have been set. If your default is dependent on other parts of the object's state, you can make the attribute "lazy". Laziness is covered in the next section.

If you want to use a reference of any sort as the default value, you must return it from a subroutine.

  has 'mapping' => (
      is      => 'ro',
      default => sub { {} },
  );

This is necessary because otherwise Perl would instantiate the reference exactly once, and it would be shared by all objects:

  has 'mapping' => (
      is      => 'ro',
      default => {}, # wrong!
  );

Moose will throw an error if you pass a bare non-subroutine reference as the default.

If Moose allowed this then the default mapping attribute could easily end up shared across many objects. Instead, wrap it in a subroutine reference as we saw above.

This is a bit awkward, but it's just the way Perl works.

As an alternative to using a subroutine reference, you can supply a "builder" method for your attribute:

  has 'size' => (
      is        => 'ro',
      builder   => '_build_size',
      predicate => 'has_size',
  );

  sub _build_size {
      return ( 'small', 'medium', 'large' )[ int( rand 3 ) ];
  }

This has several advantages. First, it moves a chunk of code to its own named method, which improves readability and code organization. Second, because this is a named method, it can be subclassed or provided by a role.

We strongly recommend that you use a "builder" instead of a "default" for anything beyond the most trivial default.

A "builder", just like a "default", is called as a method on the object with no additional parameters.

Builders allow subclassing

Because the "builder" is called by name, it goes through Perl's method resolution. This means that builder methods are both inheritable and overridable.

If we subclass our "Person" class, we can override "_build_size":

  package Lilliputian;

  use Moose;
  extends 'Person';

  sub _build_size { return 'small' }

Builders work well with roles

Because builders are called by name, they work well with roles. For example, a role could provide an attribute but require that the consuming class provide the "builder":

  package HasSize;
  use Moose::Role;

  requires '_build_size';

  has 'size' => (
      is      => 'ro',
      lazy    => 1,
      builder => '_build_size',
  );

  package Lilliputian;
  use Moose;

  with 'HasSize';

  sub _build_size { return 'small' }

Roles are covered in Moose::Manual::Roles.

Laziness

  has 'size' => (
      is      => 'ro',
      lazy    => 1,
      builder => '_build_size',
  );

When "lazy" is true, the default is not generated until the reader method is called, rather than at object construction time. There are several reasons you might choose to do this.

First, if the default value for this attribute depends on some other attributes, then the attribute must be "lazy". During object construction, defaults are not generated in a predictable order, so you cannot count on some other attribute being populated when generating a default.

Second, there's often no reason to calculate a default before it's needed. Making an attribute "lazy" lets you defer the cost until the attribute is needed. If the attribute is never needed, you save some CPU time.

We recommend that you make any attribute with a builder or non-trivial default "lazy" as a matter of course.

Lazy defaults and $_

Please note that a lazy default or builder can be called anywhere, even inside a "map" or "grep". This means that if your default sub or builder changes $_, something weird could happen. You can prevent this by adding "local $_" inside your default or builder.

Constructor parameters ("init_arg")

You can do either of these things with the "init_arg" option:

  has 'bigness' => (
      is       => 'ro',
      init_arg => 'size',
  );

Now we have an attribute named "bigness", but we pass "size" to the constructor.

Even more useful is the ability to disable setting an attribute via the constructor. This is particularly handy for private attributes:

  has '_genetic_code' => (
      is       => 'ro',
      lazy     => 1,
      builder  => '_build_genetic_code',
      init_arg => undef,
  );

By setting the "init_arg" to "undef", we make it impossible to set this attribute when creating a new object.

Weak references

  has 'parent' => (
      is       => 'rw',
      weak_ref => 1,
  );

  $node->parent($parent_node);

This is very useful when you're building objects that may contain circular references.

When the object in a weak reference goes out of scope, the attribute's value will become "undef" "behind the scenes". This is done by the Perl interpreter directly, so Moose does not see this change. This means that triggers don't fire, coercions aren't applied, etc.

The attribute is not cleared, so a predicate method for that attribute will still return true. Similarly, when the attribute is next accessed, a default value will not be generated.

Triggers

  has 'size' => (
      is      => 'rw',
      trigger => \&_size_set,
  );

  sub _size_set {
      my ( $self, $size, $old_size ) = @_;

      my $msg = $self->name;

      if ( @_ > 2 ) {
          $msg .= " - old size was $old_size";
      }

      $msg .= " - size is now $size";
      warn $msg;
  }

The trigger is called after an attribute's value is set. It is called as a method on the object, and receives the new and old values as its arguments. If the attribute had not previously been set at all, then only the new value is passed. This lets you distinguish between the case where the attribute had no value versus when the old value was "undef".

This differs from an "after" method modifier in two ways. First, a trigger is only called when the attribute is set, as opposed to whenever the accessor method is called (for reading or writing). Second, it is also called when an attribute's value is passed to the constructor.

However, triggers are not called when an attribute is populated from a "default" or "builder".

Attribute types

  has 'first_name' => (
      is  => 'ro',
      isa => 'Str',
  );

This says that the "first_name" attribute must be a string.

Moose also provides a shortcut for specifying that an attribute only accepts objects that do a certain role:

  has 'weapon' => (
      is   => 'rw',
      does => 'MyApp::Weapon',
  );

See the Moose::Manual::Types documentation for a complete discussion of Moose's type system.

Delegation

  has 'hair_color' => (
      is      => 'ro',
      isa     => 'Graphics::Color::RGB',
      handles => { hair_color_hex => 'as_hex_string' },
  );

This adds a new method, "hair_color_hex". When someone calls "hair_color_hex", internally, the object just calls "$self->hair_color->as_hex_string".

See Moose::Manual::Delegation for documentation on how to set up delegation methods.

Attribute traits and metaclasses

You can apply one or more traits to an attribute:

  use MooseX::MetaDescription;

  has 'size' => (
      is          => 'ro',
      traits      => ['MooseX::MetaDescription::Meta::Trait'],
      description => {
          html_widget  => 'text_input',
          serialize_as => 'element',
      },
  );

The advantage of traits is that you can mix more than one of them together easily (in fact, a trait is just a role under the hood).

There are a number of MooseX modules on CPAN which provide useful attribute metaclasses and traits. See Moose::Manual::MooseX for some examples. You can also write your own metaclasses and traits. See the "Meta" and "Extending" recipes in Moose::Cookbook for examples.

Native Delegations

For example, we can pretend that an array reference has methods like "push()", "shift()", "map()", "count()", and more.

  has 'options' => (
      traits  => ['Array'],
      is      => 'ro',
      isa     => 'ArrayRef[Str]',
      default => sub { [] },
      handles => {
          all_options    => 'elements',
          add_option     => 'push',
          map_options    => 'map',
          option_count   => 'count',
          sorted_options => 'sort',
      },
  );

See Moose::Manual::Delegation for more details.

Attribute Inheritance and Method Modifiers

As a consequence, any method modifiers defined on the attribute's accessors in an ancestor class will effectively be ignored, because the new accessors live in the child class and do not see the modifiers from the parent class.

The "documentation" option

  has 'first_name' => (
      is            => 'rw',
      documentation => q{The person's first (personal) name},
  );

Moose does absolutely nothing with this information other than store it.

The "auto_deref" option

  my %map = $object->mapping;

This option only works if your attribute is explicitly typed as an "ArrayRef" or "HashRef". When the reader is called in scalar context, the reference itself is returned.

However, we recommend that you use Moose::Meta::Attribute::Native traits for these types of attributes, which gives you much more control over how they are accessed and manipulated. See also Moose::Manual::BestPractices#Use_Moose::Meta::Attribute::Native_traits_instead_of_auto_deref.

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