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Moose - A postmodern object system for Perl 5


  package Point;
  use Moose; # automatically turns on strict and warnings

  has 'x' => (is => 'rw', isa => 'Int');
  has 'y' => (is => 'rw', isa => 'Int');

  sub clear {
      my $self = shift;

  package Point3D;
  use Moose;

  extends 'Point';

  has 'z' => (is => 'rw', isa => 'Int');

  after 'clear' => sub {
      my $self = shift;


Moose is an extension of the Perl 5 object system.

The main goal of Moose is to make Perl 5 Object Oriented programming easier, more consistent and less tedious. With Moose you can to think more about what you want to do and less about the mechanics of OOP.

Additionally, Moose is built on top of Class::MOP, which is a metaclass system for Perl 5. This means that Moose not only makes building normal Perl 5 objects better, but it provides the power of metaclass programming as well.

New to Moose?

If you're new to Moose, the best place to start is the Moose::Manual docs, followed by the Moose::Cookbook. The intro will show you what Moose is, and how it makes Perl 5 OO better.

The cookbook recipes on Moose basics will get you up to speed with many of Moose's features quickly. Once you have an idea of what Moose can do, you can use the API documentation to get more detail on features which interest you.

Moose Extensions

The MooseX:: namespace is the official place to find Moose extensions. These extensions can be found on the CPAN. The easiest way to find them is to search for them (, or to examine Task::Moose which aims to keep an up-to-date, easily installable list of Moose extensions.


Much of the Moose documentation has been translated into other languages.


Japanese docs can be found at The source POD files can be found in GitHub:


Moose makes every attempt to provide as much convenience as possible during class construction/definition, but still stay out of your way if you want it to. Here are a few items to note when building classes with Moose.

When you use Moose, Moose will set the class's parent class to Moose::Object, unless the class using Moose already has a parent class. In addition, specifying a parent with extends will change the parent class.

Moose will also manage all attributes (including inherited ones) that are defined with has. And (assuming you call new, which is inherited from Moose::Object) this includes properly initializing all instance slots, setting defaults where appropriate, and performing any type constraint checking or coercion.


Moose provides a number of methods to all your classes, mostly through the inheritance of Moose::Object. There is however, one exception.


This is a method which provides access to the current class's metaclass.


Moose will export a number of functions into the class's namespace which may then be used to set up the class. These functions all work directly on the current class.

extends (@superclasses)

This function will set the superclass(es) for the current class.

This approach is recommended instead of use base, because use base actually pushes onto the class's @ISA, whereas extends will replace it. This is important to ensure that classes which do not have superclasses still properly inherit from Moose::Object.

Each superclass can be followed by a hash reference with options. Currently, only -version is recognized:

    extends 'My::Parent'      => { -version => 0.01 },
            'My::OtherParent' => { -version => 0.03 };

An exception will be thrown if the version requirements are not satisfied.

with (@roles)

This will apply a given set of @roles to the local class.

Like with extends, each specified role can be followed by a hash reference with a -version option:

    with 'My::Role'      => { -version => 0.32 },
         'My::Otherrole' => { -version => 0.23 };

The specified version requirements must be satisfied, otherwise an exception will be thrown.

If your role takes options or arguments, they can be passed along in the hash reference as well.

has $name|@$names => %options

This will install an attribute of a given $name into the current class. If the first parameter is an array reference, it will create an attribute for every $name in the list. The %options are the same as those provided by Class::MOP::Attribute, in addition to the list below which are provided by Moose (Moose::Meta::Attribute to be more specific):

is => 'rw'|'ro'

The is option accepts either rw (for read/write) or ro (for read only). These will create either a read/write accessor or a read-only accessor respectively, using the same name as the $name of the attribute.

If you need more control over how your accessors are named, you can use the reader, writer and accessor options inherited from Class::MOP::Attribute, however if you use those, you won't need the is option.

isa => $type_name

The isa option uses Moose's type constraint facilities to set up runtime type checking for this attribute. Moose will perform the checks during class construction, and within any accessors. The $type_name argument must be a string. The string may be either a class name or a type defined using Moose's type definition features. (Refer to Moose::Util::TypeConstraints for information on how to define a new type, and how to retrieve type meta-data).

coerce => (1|0)

This will attempt to use coercion with the supplied type constraint to change the value passed into any accessors or constructors. You must supply a type constraint, and that type constraint must define a coercion. See Moose::Cookbook::Basics::Recipe5 for an example.

does => $role_name

This will accept the name of a role which the value stored in this attribute is expected to have consumed.

required => (1|0)

This marks the attribute as being required. This means a value must be supplied during class construction, or the attribute must be lazy and have either a default or a builder. Note that c<required> does not say anything about the attribute's value, which can be undef.

weak_ref => (1|0)

This will tell the class to store the value of this attribute as a weakened reference. If an attribute is a weakened reference, it cannot also be coerced.

lazy => (1|0)

This will tell the class to not create this slot until absolutely necessary. If an attribute is marked as lazy it must have a default supplied.

auto_deref => (1|0)

This tells the accessor to automatically dereference the value of this attribute when called in list context. The accessor will still return a reference when called in scalar context. If this behavior isn't desirable, "elements" in Moose::Meta::Attribute::Native::Trait::Array or "elements" in Moose::Meta::Attribute::Native::Trait::Hash may be a better choice. The auto_deref option is only legal if your isa option is either ArrayRef or HashRef.

trigger => $code

The trigger option is a CODE reference which will be called after the value of the attribute is set. The CODE ref is passed the instance itself, the updated value, and the original value if the attribute was already set.

You can have a trigger on a read-only attribute.

NOTE: Triggers will only fire when you assign to the attribute, either in the constructor, or using the writer. Default and built values will not cause the trigger to be fired.


The handles option provides Moose classes with automated delegation features. This is a pretty complex and powerful option. It accepts many different option formats, each with its own benefits and drawbacks.

NOTE: The class being delegated to does not need to be a Moose based class, which is why this feature is especially useful when wrapping non-Moose classes.

All handles option formats share the following traits:

You cannot override a locally defined method with a delegated method; an exception will be thrown if you try. That is to say, if you define foo in your class, you cannot override it with a delegated foo. This is almost never something you would want to do, and if it is, you should do it by hand and not use Moose.

You cannot override any of the methods found in Moose::Object, or the BUILD and DEMOLISH methods. These will not throw an exception, but will silently move on to the next method in the list. My reasoning for this is that you would almost never want to do this, since it usually breaks your class. As with overriding locally defined methods, if you do want to do this, you should do it manually, not with Moose.

You do not need to have a reader (or accessor) for the attribute in order to delegate to it. Moose will create a means of accessing the value for you, however this will be several times less efficient then if you had given the attribute a reader (or accessor) to use.

Below is the documentation for each option format:


This is the most common usage for handles. You basically pass a list of method names to be delegated, and Moose will install a delegation method for each one.


This is the second most common usage for handles. Instead of a list of method names, you pass a HASH ref where each key is the method name you want installed locally, and its value is the name of the original method in the class being delegated to.

This can be very useful for recursive classes like trees. Here is a quick example (soon to be expanded into a Moose::Cookbook recipe):

  package Tree;
  use Moose;

  has 'node' => (is => 'rw', isa => 'Any');

  has 'children' => (
      is      => 'ro',
      isa     => 'ArrayRef',
      default => sub { [] }

  has 'parent' => (
      is          => 'rw',
      isa         => 'Tree',
      weak_ref => 1,
      handles     => {
          parent_node => 'node',
          siblings    => 'children',

In this example, the Tree package gets parent_node and siblings methods, which delegate to the node and children methods (respectively) of the Tree instance stored in the parent slot.

You may also use an array reference to curry arguments to the original method.

  has 'thing' => (
      handles => { set_foo => [ set => 'foo' ] },

  # $self->set_foo(...) calls $self->thing->set('foo', ...)

The first element of the array reference is the original method name, and the rest is a list of curried arguments.


The regexp option works very similar to the ARRAY option, except that it builds the list of methods for you. It starts by collecting all possible methods of the class being delegated to, then filters that list using the regexp supplied here.

NOTE: An isa option is required when using the regexp option format. This is so that we can determine (at compile time) the method list from the class. Without an isa this is just not possible.


With the role option, you specify the name of a role or a role type whose "interface" then becomes the list of methods to handle. The "interface" can be defined as; the methods of the role and any required methods of the role. It should be noted that this does not include any method modifiers or generated attribute methods (which is consistent with role composition).


With the duck type option, you pass a duck type object whose "interface" then becomes the list of methods to handle. The "interface" can be defined as; the list of methods passed to duck_type to create a duck type object. For more information on duck_type please check Moose::Util::TypeConstraints.


This is the option to use when you really want to do something funky. You should only use it if you really know what you are doing, as it involves manual metaclass twiddling.

This takes a code reference, which should expect two arguments. The first is the attribute meta-object this handles is attached to. The second is the metaclass of the class being delegated to. It expects you to return a hash (not a HASH ref) of the methods you want mapped.

metaclass => $metaclass_name

This tells the class to use a custom attribute metaclass for this particular attribute. Custom attribute metaclasses are useful for extending the capabilities of the has keyword: they are the simplest way to extend the MOP, but they are still a fairly advanced topic and too much to cover here, see Moose::Cookbook::Meta::Recipe1 for more information.

See "Metaclass and Trait Name Resolution" for details on how a metaclass name is resolved to a class name.

traits => [ @role_names ]

This tells Moose to take the list of @role_names and apply them to the attribute meta-object. This is very similar to the metaclass option, but allows you to use more than one extension at a time.

See "Metaclass and Trait Name Resolution" for details on how a trait name is resolved to a role name.

Also see Moose::Cookbook::Meta::Recipe3 for a metaclass trait example.

builder => Str

The value of this key is the name of the method that will be called to obtain the value used to initialize the attribute. See the builder option docs in Class::MOP::Attribute and/or Moose::Cookbook::Basics::Recipe8 for more information.

default => SCALAR | CODE

The value of this key is the default value which will initialize the attribute.

NOTE: If the value is a simple scalar (string or number), then it can be just passed as is. However, if you wish to initialize it with a HASH or ARRAY ref, then you need to wrap that inside a CODE reference. See the default option docs in Class::MOP::Attribute for more information.

clearer => Str

Creates a method allowing you to clear the value, see the clearer option docs in Class::MOP::Attribute for more information.

predicate => Str

Creates a method to perform a basic test to see if a value has been set in the attribute, see the predicate option docs in Class::MOP::Attribute for more information.

lazy_build => (0|1)

Automatically define lazy => 1 as well as builder => "_build_$attr", clearer => "clear_$attr', predicate => 'has_$attr' unless they are already defined.

initializer => Str

This may be a method name (referring to a method on the class with this attribute) or a CODE ref. The initializer is used to set the attribute value on an instance when the attribute is set during instance initialization (but not when the value is being assigned to). See the initializer option docs in Class::MOP::Attribute for more information.

documentation => $string

An arbitrary string that can be retrieved later by calling $attr->documentation.

has +$name => %options

This is variation on the normal attribute creator has which allows you to clone and extend an attribute from a superclass or from a role. Here is an example of the superclass usage:

  package Foo;
  use Moose;

  has 'message' => (
      is      => 'rw',
      isa     => 'Str',
      default => 'Hello, I am a Foo'

  package My::Foo;
  use Moose;

  extends 'Foo';

  has '+message' => (default => 'Hello I am My::Foo');

What is happening here is that My::Foo is cloning the message attribute from its parent class Foo, retaining the is => 'rw' and isa => 'Str' characteristics, but changing the value in default.

Here is another example, but within the context of a role:

  package Foo::Role;
  use Moose::Role;

  has 'message' => (
      is      => 'rw',
      isa     => 'Str',
      default => 'Hello, I am a Foo'

  package My::Foo;
  use Moose;

  with 'Foo::Role';

  has '+message' => (default => 'Hello I am My::Foo');

In this case, we are basically taking the attribute which the role supplied and altering it within the bounds of this feature.

Note that you can only extend an attribute from either a superclass or a role, you cannot extend an attribute in a role that composes over an attribute from another role.

Aside from where the attributes come from (one from superclass, the other from a role), this feature works exactly the same. This feature is restricted somewhat, so as to try and force at least some sanity into it. You are only allowed to change the following attributes:


Change the default value of an attribute.


Change whether the attribute attempts to coerce a value passed to it.


Change if the attribute is required to have a value.


Change the documentation string associated with the attribute.


Change if the attribute lazily initializes the slot.


You are allowed to change the type without restriction.

It is recommended that you use this freedom with caution. We used to only allow for extension only if the type was a subtype of the parent's type, but we felt that was too restrictive and is better left as a policy decision.


You are allowed to add a new handles definition, but you are not allowed to change one.


You are allowed to add a new builder definition, but you are not allowed to change one.


You are allowed to add a new metaclass definition, but you are not allowed to change one.


You are allowed to add additional traits to the traits definition. These traits will be composed into the attribute, but preexisting traits are not overridden, or removed.

before $name|@names|\@names|qr/.../ => sub { ... }
after $name|@names|\@names|qr/.../ => sub { ... }
around $name|@names|\@names|qr/.../ => sub { ... }

These three items are syntactic sugar for the before, after, and around method modifier features that Class::MOP provides. More information on these may be found in Moose::Manual::MethodModifiers and the Class::MOP::Class documentation.


The keyword super is a no-op when called outside of an override method. In the context of an override method, it will call the next most appropriate superclass method with the same arguments as the original method.

override ($name, &sub)

An override method is a way of explicitly saying "I am overriding this method from my superclass". You can call super within this method, and it will work as expected. The same thing can be accomplished with a normal method call and the SUPER:: pseudo-package; it is really your choice.


The keyword inner, much like super, is a no-op outside of the context of an augment method. You can think of inner as being the inverse of super; the details of how inner and augment work is best described in the Moose::Cookbook::Basics::Recipe6.

augment ($name, &sub)

An augment method, is a way of explicitly saying "I am augmenting this method from my superclass". Once again, the details of how inner and augment work is best described in the Moose::Cookbook::Basics::Recipe6.


This is the Carp::confess function, and exported here because I use it all the time.


This is the Scalar::Util::blessed function, it is exported here because I use it all the time. It is highly recommended that this is used instead of ref anywhere you need to test for an object's class name.


When you use Moose, you can specify which metaclass to use:

    use Moose -metaclass => 'My::Meta::Class';

You can also specify traits which will be applied to your metaclass:

    use Moose -traits => 'My::Trait';

This is very similar to the attribute traits feature. When you do this, your class's meta object will have the specified traits applied to it. See "Metaclass and Trait Name Resolution" for more details.

Metaclass and Trait Name Resolution

By default, when given a trait name, Moose simply tries to load a class of the same name. If such a class does not exist, it then looks for for a class matching Moose::Meta::$type::Custom::Trait::$trait_name. The $type variable here will be one of Attribute or Class, depending on what the trait is being applied to.

If a class with this long name exists, Moose checks to see if it has the method register_implementation. This method is expected to return the real class name of the trait. If there is no register_implementation method, it will fall back to using Moose::Meta::$type::Custom::Trait::$trait as the trait name.

The lookup method for metaclasses is the same, except that it looks for a class matching Moose::Meta::$type::Custom::$metaclass_name.

If all this is confusing, take a look at Moose::Cookbook::Meta::Recipe3, which demonstrates how to create an attribute trait.



Moose offers a way to remove the keywords it exports, through the unimport method. You simply have to say no Moose at the bottom of your code for this to work. Here is an example:

    package Person;
    use Moose;

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

    sub full_name {
        my $self = shift;
        $self->first_name . ' ' . $self->last_name

    no Moose; # keywords are removed from the Person package


To learn more about extending Moose, we recommend checking out the "Extending" recipes in the Moose::Cookbook, starting with Moose::Cookbook::Extending::Recipe1, which provides an overview of all the different ways you might extend Moose.

Moose->init_meta(for_class => $class, base_class => $baseclass, metaclass => $metaclass)

The init_meta method sets up the metaclass object for the class specified by for_class. This method injects a a meta accessor into the class so you can get at this object. It also sets the class's superclass to base_class, with Moose::Object as the default.

init_meta returns the metaclass object for $class.

You can specify an alternate metaclass with the metaclass option.

For more detail on this topic, see Moose::Cookbook::Extending::Recipe2.

This method used to be documented as a function which accepted positional parameters. This calling style will still work for backwards compatibility, but is deprecated.


Moose's import method supports the Sub::Exporter form of {into => $pkg} and {into_level => 1}.

NOTE: Doing this is more or less deprecated. Use Moose::Exporter instead, which lets you stack multiple modules sanely. It handles getting the exported functions into the right place for you.


An alias for confess, used by internally by Moose.

The MooseX:: namespace

Generally if you're writing an extension for Moose itself you'll want to put your extension in the MooseX:: namespace. This namespace is specifically for extensions that make Moose better or different in some fundamental way. It is traditionally not for a package that just happens to use Moose. This namespace follows from the examples of the LWPx:: and DBIx:: namespaces that perform the same function for LWP and DBI respectively.


Metaclass compatibility is a thorny subject. You should start by reading the "About Metaclass compatibility" section in the Class::MOP docs.

Moose will attempt to resolve a few cases of metaclass incompatibility when you set the superclasses for a class, in addition to the cases that Class::MOP handles.

Moose tries to determine if the metaclasses only "differ by roles". This means that the parent and child's metaclass share a common ancestor in their respective hierarchies, and that the subclasses under the common ancestor are only different because of role applications. This case is actually fairly common when you mix and match various MooseX::* modules, many of which apply roles to the metaclass.

If the parent and child do differ by roles, Moose replaces the metaclass in the child with a newly created metaclass. This metaclass is a subclass of the parent's metaclass, does all of the roles that the child's metaclass did before being replaced. Effectively, this means the new metaclass does all of the roles done by both the parent's and child's original metaclasses.

Ultimately, this is all transparent to you except in the case of an unresolvable conflict.


  • It should be noted that super and inner cannot be used in the same method. However, they may be combined within the same class hierarchy; see t/014_override_augment_inner_super.t for an example.

    The reason for this is that super is only valid within a method with the override modifier, and inner will never be valid within an override method. In fact, augment will skip over any override methods when searching for its appropriate inner.

    This might seem like a restriction, but I am of the opinion that keeping these two features separate (yet interoperable) actually makes them easy to use, since their behavior is then easier to predict. Time will tell whether I am right or not (UPDATE: so far so good).


We offer both a mailing list and a very active IRC channel.

The mailing list is You must be subscribed to send a message. To subscribe, send an empty message to

You can also visit us at #moose on irc:// This channel is quite active, and questions at all levels (on Moose-related topics ;) are welcome.


I blame Sam Vilain for introducing me to the insanity that is meta-models.
I blame Audrey Tang for then encouraging my meta-model habit in #perl6.
Without Yuval "nothingmuch" Kogman this module would not be possible, and it certainly wouldn't have this name ;P
The basis of the TypeContraints module was Rob Kinyon's idea originally, I just ran with it.
Thanks to mst & chansen and the whole #moose posse for all the early ideas/feature-requests/encouragement/bug-finding.
Thanks to David "Theory" Wheeler for meta-discussions and spelling fixes.


This is the official web home of Moose, it contains links to our public git repository as well as links to a number of talks and articles on Moose and Moose related technologies.

The Moose is flying, a tutorial by Randal Schwartz

Part 1 -

Part 2 -

Several Moose extension modules in the MooseX:: namespace.

See for extensions.

Moose stats on -


The Art of the MetaObject Protocol

I mention this in the Class::MOP docs too, this book was critical in the development of both modules and is highly recommended.


This paper (suggested by lbr on #moose) was what lead to the implementation of the super/override and inner/augment features. If you really want to understand them, I suggest you read this.


All complex software has bugs lurking in it, and this module is no exception.

Please report any bugs to, or through the web interface at

You can also discuss feature requests or possible bugs on the Moose mailing list ( or on IRC at irc://


We are very strict about what features we add to the Moose core, especially the user-visible features. Instead we have made sure that the underlying meta-system of Moose is as extensible as possible so that you can add your own features easily.

That said, occasionally there is a feature needed in the meta-system to support your planned extension, in which case you should either email the mailing list ( or join us on IRC at irc:// to discuss. The Moose::Manual::Contributing has more detail about how and when you can contribute.


Moose is an open project, there are at this point dozens of people who have contributed, and can contribute. If you have added anything to the Moose project you have a commit bit on this file and can add your name to the list.


However there are only a few people with the rights to release a new version of Moose. The Moose Cabal are the people to go to with questions regarding the wider purview of Moose, and help out maintaining not just the code but the community as well.

Stevan (stevan) Little <>

Jesse (doy) Luehrs <doy at tozt dot net>

Yuval (nothingmuch) Kogman

Shawn (sartak) Moore <>

Hans Dieter (confound) Pearcey <>

Chris (perigrin) Prather

Florian Ragwitz <>

Dave (autarch) Rolsky <>



Adam (Alias) Kennedy

Anders (Debolaz) Nor Berle

Nathan (kolibrie) Gray

Christian (chansen) Hansen

Eric (ewilhelm) Wilhelm

Guillermo (groditi) Roditi

Jess (castaway) Robinson

Matt (mst) Trout

Robert (phaylon) Sedlacek

Robert (rlb3) Boone

Scott (konobi) McWhirter

Shlomi (rindolf) Fish

Wallace (wreis) Reis

Jonathan (jrockway) Rockway

Piotr (dexter) Roszatycki

Sam (mugwump) Vilain

Cory (gphat) Watson

Dylan Hardison (doc fixes)

... and many other #moose folks


Copyright 2006-2010 by Infinity Interactive, Inc.

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