Moose (Perl)

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Moose is an extension of the object system of the Perl programming language. Its stated purpose[1] is to bring modern object-oriented programming language features to Perl 5, and to make object-oriented Perl programming more consistent and less tedious.

## Features

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Moose is built on `Class::MOP`, a metaobject protocol (MOP). Using the MOP, Moose provides complete type introspection for all Moose-using classes.

### Classes

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Moose allows a programmer to create classes:

* A class has zero or more attributes.
* A class has zero or more methods.
* A class has zero or more superclasses (a.k.a. parent classes). A class inherits from its superclass(es). Moose supports multiple inheritance.
* A class has zero or more method modifiers. These modifiers can apply to its own methods, methods that are inherited from its ancestors or methods that are provided by roles.
* A class does zero or more roles (also known as traits in other programming languages).
* A class has a constructor and a destructor.
* A class has a metaclass.

### Attributes

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An attribute is a property of the class that defines it.

* An attribute always has a name, and it may have a number of other defining characteristics.
* An attribute's characteristics may include a read/write flag, a type, accessor method names, delegations, a default value and lazy initialization.

### Roles

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Roles in Moose are based on traits. They perform a similar task as mixins, but are composed horizontally rather than inherited. They are also somewhat like interfaces, but unlike some implementations of interfaces they can provide a default implementation. Roles can be applied to individual instances as well as Classes.

* 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.

## Extensions

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There are a number of Moose extension modules on CPAN. As of September 2012[update] there are 855 modules in 266 distributions in the MooseX namespace.[2] Most of them can be optionally installed with the Task::Moose module.[3]

## Examples

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This is an example of a class `Point` and its subclass `Point3D`:

package Point;
use Moose;
use Carp;

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

sub clear {
my $self = shift;
$self->x(0);
$self->y(0);

sub set_to {
@_ == 3 or croak "Bad number of arguments";
my $self = shift;
my ($x, $y) = @_;
$self->x($x);
$self->y($y);

package Point3D;
use Moose;
use Carp;

extends 'Point';

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

after 'clear' => sub {
my $self = shift;
$self->z(0);

sub set_to {
@_ == 4 or croak "Bad number of arguments";
my $self = shift;
my ($x, $y, $z) = @_;
$self->x($x);
$self->y($y);
$self->z($z);

There is a new `set_to()` method in the `Point3D` class so the method of the same name defined in the `Point` class is not invoked in the case of `Point3D` instances. The `clear()` method on the other hand is not replaced but extended in the subclass, so both methods are run in the correct order.

This is the same using the `MooseX::Declare` extension:

use MooseX::Declare;

class Point {
has 'x' => (isa => 'Num', is => 'rw');
has 'y' => (isa => 'Num', is => 'rw');

method clear {
$self->x(0);
$self->y(0);

method set_to (Num $x, Num $y) {
$self->x($x);
$self->y($y);

class Point3D extends Point {
has 'z' => (isa => 'Num', is => 'rw');

after clear {
$self->z(0);

method set_to (Num $x, Num $y, Num $z) {
$self->x($x);
$self->y($y);
$self->z($z);

## See also

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* Raku (programming language) § Object-oriented programming, the inspiration for Moose
* Joose (framework), a JavaScript framework inspired by Moose
* Catalyst (software), a web application framework using Moose

## References

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1. ^ "Moose - A postmodern object system for Perl". Retrieved 2017-03-06.
2. ^ Moose extensions on CPAN
3. ^ Task::Moose

## External links

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* Moose Homepage
* Moose Documentation

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