Moose::Intro - What is Moose, and how do I use it?


Moose is a complete object system for Perl 5. If you've used a modern object-oriented language (which Perl 5 definitely isn't), you know they provide keywords for attribute declaration, object construction, and inheritance. These keywords are part of the language, and you don't care how they are implemented.

Moose aims to do the same thing for Perl 5 OO. We can't actually create new keywords, but we do offer "sugar" that looks a lot like them. More importantly, with Moose, you declaritively define your class, without needing to know about blessed hashrefs, accessor methods, and so on.

Moose lets you focus on the logical structure of your classes, so you can focus on "what" rather than "how". With Moose, a class definition should read like a list of very concise English sentences.

Moose is built in top of Class::MOP, a meta-object protocol (aka MOP). Using the MOP, Moose provides complete introspection for all Moose-using classes. This means you can ask classes about their attributes, parents, children, methods, etc., all using a well-defined API. The MOP abstracts away tedious digging about in the Perl symbol table, looking at @ISA vars, and all the other crufty Perl tricks we know and love (?).

Moose is based in large part on the Perl 6 object system, as well as drawing on the best ideas from CLOS, Smalltalk, and many other languages.


Moose makes Perl 5 OO both simpler and more powerful. It encapsulates all the tricks of Perl 5 power users in high-level declarative APIs which are easy to use, and don't require any special knowledge of how Perl works under the hood.

Moose makes Perl 5 OO fun, accessible, and powerful. And if you want to dig about in the guts, Moose lets you do that too, by using and extending its powerful introspection API.


  package Person;

  use Moose;

  has 'first_name' => (
      is  => 'rw',
      isa => 'Str',

  has 'last_name' => (
      is  => 'rw',
      isa => 'Str',

This is a complete and usable class definition!

  package User;

  use DateTime;
  use Moose;

  extends 'Person';

  has 'password' => (
      is  => 'rw',
      isa => 'Str',

  has 'last_login' => (
      is      => 'rw',
      isa     => 'DateTime',
      handles => { 'date_of_last_login' => 'date' },

  sub login {
      my $self = shift;
      my $pw   = shift;

      return 0 if $pw ne $self->password;

      $self->last_login( DateTime->now() );

      return 1;

We'll leave the line-by-line explanation of this code to other documentation, but you can see how Moose reduces common OO idioms to simple declarative constructs.

Where's the Constructor?

One point of confusion that might come up with Moose is how it handles object construction. You should not define a new() method for your classes!

Moose will provide one for you. It will accept a hash or hash reference of named parameters matching your attributes. This is just another way in which Moose keeps your from worrying how classes are implemented. Simply define a class and you're ready to start creating objects!


In the past, you may not have thought too much about the difference between packages and classes, attributes and methods, constructors vs methods, etc. Part of what the MOP provides is well-defined introspection features for each of those things, and in turn Moose provides distinct sugar for each of them. Moose also introduces concepts that are uncommon (or entirely new) like roles, method modifiers, and declarative delegation.

Knowing what these concepts mean in Moose-speak, and how they used to be done in old school Perl 5 OO is a good way to start learning to use Moose.


When you say "use Moose" in a package, you are defining your package as a class. At its simplest, a class will consist simply of attributes and/or methods. It can also include roles, method modifiers, and more.

A class has zero or more attributes.

A class has zero or more methods.

A class may have one or more superclasses (aka parent classes). A class inherits from its superclass(es).

A class may have method modifiers. These modifiers can apply to its own methods or methods that are inherited from its ancestors.

A class may do one or more roles.

A class has a constructor and a destructor. These are provided for you "for free" by Moose.

The constructor accepts named parameters corresponding to the class's attributes and uses them to initialize an object instances.

A class has a metaclass, which in turn has meta-attributes, meta-methods, and meta-roles. This metaclass describes the class.

A class is usually analogous to a category of nouns, like "People" or "Users".

  package Person;

  use Moose;
  # now it's a Moose class!


An attribute is a property of the class that defines it. It always has a name, and it may have a number of other defining characteristics.

These characteristics may include a read/write flag, a type, accessor method names, delegations, a default value, and more.

Attributes are not methods, but defining them causes various accessor methods to be created. At a minimum, a normal attribute will always have a reader accessor method. Many attributes have things like a writer method, clearer method, and predicate method ("has it been set?").

An attribute may also define delegations, which will create additional methods based on the delegation specification.

By default, Moose stores attributes in the object instance, which is a hashref, but this is invisible to the author of a Moose-base class! It is best to think of Moose attributes as "properties" of the opaque object instance. These properties are accessed through well-defined accessor methods.

An attribute is usually analagous to specific feature of something in the class's category. For example, People have first and last names. Users have passwords and last login datetimes.

  has 'first_name' => (
      is  => 'rw',
      isa => 'Str',


A method is very straightforward. Any subroutine you define in your class is a method.

Methods correspond to verbs, and are what your objects can do. For example, a User can login.

  sub login { ... }


A role is something that a class does. For example, a Machine class might do the Breakable role, and a so could a Bone class. A role is used to define some concept that cuts across multiple unrelated classes, like "breakability", or "has a color".

A role has zero or more attributes.

A role has zero or more methods.

A role has zero or more method modifiers.

A role has zero or more required methods.

A required method is not implemented by the role. Instead, a required method says "to use this Role you must implement this method".

Roles are composed into classes (or other roles). When a role is composed into a class, its attributes and methods are "flattened" into the class. Roles do not show up in the inheritance hierarchy. When a role is composed, it's attributes and methods appear as if they were defined in the consuming class.

Role are somewhat like mixins or interfaces in other OO languages.

  package Breakable;

  use Moose::Role;

  has is_broken => (
      is  => 'rw',
      isa => 'Bool',

  requires 'break';

  before 'break' => {
      my $self = shift;


Method Modifiers

A method modifier is a way of defining an action to be taken when a named method is called. Think of it as a hook on the named method. For example, you could say "before calling login(), call this modifier first". Modifiers come in different flavors like "before", "after", "around", and "augment", and you can apply more than one modifier to a single method.

Method modifiers are often used as an alternative to overriding a method in a parent class. They are also used in roles as a way of modifying methods in the consuming class.

Under the hood, a method modifier is just a plain old Perl subroutine that gets called before or after (or around, etc.) some named method.

  before 'login' => sub {
      my $self = shift;
      my $pw   = shift;

      warn "Called login() with $pw\n";


Moose also comes with a (miniature) type system. This allows you to define types for attributes. Moose has a set of built-in types based on what Perl provides, such as "Str", "Num", "Bool", "HashRef", etc.

In addition, every class name in your application can also be used as a type name. We saw an example using "DateTime" earlier.

Finally, you can define your own types, either as subtypes or entirely new types, with their own constraints. For example, you could define a type "PosInt", a subtype of "Int" which only allows positive numbers.


Moose attributes provide declarative syntax for defining delegations. A delegation is a method which delegates the real work to some attribute of the class.

You saw this in the User example, where we defined a delegation for the date_of_last_login() method. Under the hood, this simple calls date() on the User object's last_login attribute.


A constructor creates an object instance for the class. In old school Perl, this was usually done by defining a method called new() which in turn called bless on a reference.

With Moose, this new() method is created for you, and it simply does the right thing. You should never need to define your own constructor!

Sometimes you want to do something whenever an object is created. In those cases, you can provide a BUILD() method in your class. Moose will call this for you after creating a new object.


This is a special method called when an object instance goes out of scope. You can specialize what your class does in this method if you need to, but you usually don't.

With old school Perl 5, this is the DESTROY() method, but with Moose it is the DEMOLISH() method.

Object Instance

An object instance is a specific noun in the class's "category". For example, one specific Person or User. An instance is created by the class's constructor.

An instance has values for its attributes. For example, a specific person has a first and last name,

In old school Perl 5, this is often a blessed hash reference. With Moose, you should never need to know what your object instance actually is. (ok, it's usually a blessed hashref with Moose too)

Moose VS Old School Summary

  • Class

    A package with no introspection other than mucking about in the symbol table.

    With Moose, you get well-defined declaration and introspection.

  • Attributes

    Hand-written accessor methods, symbol table hackery, or a helper module like Class::Accessor.

    With Moose, these are declaritively defined, and distinct from methods.

  • Method

    These are pretty much the same in Moose as in old school Perl.

  • Roles

    Class::Trait or Class::Role, or maybe

    With Moose, they're part of the core feature set, and are introspectable like everything else.

  • Method Modifiers

    Could only be done through serious symbol table wizardry, and you probably never saw this before (at least in Perl 5).

  • Type

    Hand-written parameter checking in your new() method and accessors.

    With Moose, you define types declaratively, and then use them by name in your attributes.

  • Delegation

    Class::Delegation or Class::Delegator, but probably even more hand-written code.

    With Moose, this is also declarative.

  • Constructor

    A new() method which calls bless on a reference.

    Comes for free when you define a class with Moose.

  • Destructor

    A DESTROY() method.

    With Moose, this is called DEMOLISH().

  • Object Instance

    A blessed reference, usually a hash reference.

    With Moose, this is an opaque thing which has a bunch of attributes and methods, as defined by its class.

  • Immutabilization

    Moose comes with a feature called "immutabilization". When you make your class immutable, it means you're done adding methods, attributes, roles, etc. This lets Moose optimize your class with a bunch of extremely dirty in-place code generation tricks that speed up things like object construction and so on.


A metaclass is a class that describes classes. With Moose, every class you define gets a meta() method. It returns a Moose::Meta::Class object, which has an introspection API that can tell you about the class it represents.

  my $meta = User->meta();

  for my $attribute ( $meta->compute_all_applicable_attributes ) {
      print $attribute->name(), "\n";

      if ( $attribute->has_type_constraint ) {
          print "  type: ", $attribute->type_constraint->name, "\n";

  for my $method ( $meta->compute_all_applicable_methods ) {
      print $method->name, "\n";

Almost every concept we defined earlier has a meta class, so we have Moose::Meta::Class, Moose::Meta::Attribute, Moose::Meta::Method, Moose::Meta::Role, Moose::Meta::TypeConstraint, Moose::Meta::Instance, and so on.


One of the great things about Moose, is that if you dig down and find that it does something the "wrong way", you can change it by extending a metaclass. For example, you can have arrayref based objects, you can make your constructors strict (no unknown params allowed!), you can define a naming scheme for attribute accessors, you can make a class a Singleton, and much, much more.

Many of these extensions require surprisingly small amounts of code, and once you've done it once, you'll never have to hand-code "your way of doing things" again. Instead you ll just load your favorite extensions.

  package MyWay::User;

  use Moose;
  use MooseX::StrictConstructor
  use MooseX::MyWay;

  has ...;


If you're still still asking yourself "Why do I need this?", then this section is for you.

Another object system!?!?

Yes, I know there has been an explosion recently of new ways to build objects in Perl 5, most of them based on inside-out objects and other such things. Moose is different because it is not a new object system for Perl 5, but instead an extension of the existing object system.

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 also provides the power of metaclass programming.

Is this for real? Or is this just an experiment?

Moose is based on the prototypes and experiments Stevan did for the Perl 6 meta-model. However, Moose is NOT an experiment or prototype; it is for real.

Is this ready for use in production?


Moose has been used successfully in production environments by several people and companies. There are Moose applications which have been in production with little or no issue now for well over two years. We consider it highly stable and we are commited to keeping it stable.

Of course, in the end, you need to make this call yourself. If you have any questions or concerns, please feel free to email Stevan, the list, or just stop by and ask away.

Is Moose just Perl 6 in Perl 5?

No. While Moose is very much inspired by Perl 6, it is not itself Perl 6. Instead, it is an OO system for Perl 5. Stevan built Moose because he was tired of writing the same old boring Perl 5 OO code, and drooling over Perl 6 OO. So instead of switching to Ruby, he wrote Moose :)

Wait, post modern, I thought it was just modern?

Stevan read Larry Wall's talk from the 1999 Linux World entitled "Perl, the first postmodern computer language" in which he talks about how he picked the features for Perl because he thought they were cool and he threw out the ones that he thought sucked. This got him thinking about how we have done the same thing in Moose. For Moose, we have "borrowed" features from Perl 6, CLOS (LISP), Smalltalk, Java, BETA, OCaml, Ruby and more, and the bits we didn't like (cause they sucked) we tossed aside. So for this reason (and a few others) Stevan has re-dubbed Moose a postmodern object system.

Nuff Said.


So you're sold on Moose. Time to learn how to really use it.

We recommend that you start with the Moose::Cookbook. If you work your way through all the recipes under the basics section, you should have a pretty good sense of how Moose works, and all of its basic OO features.

After that, check out the Role recipes. If you're really curious, go on and read the Meta and Extending recipes, but those are mostly there for people who want to be Moose wizards and change how Moose works.

If you want to see how Moose would translate directly old school Perl 5 OO code, check out the Moose::Unsweetened.


Dave Rolsky <> and Stevan Little <>


Copyright 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.