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Man Pages

Manual Reference Pages  -  MOUSE (3)

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Mouse - Moose minus the antlers



This document describes Mouse version v2.4.5


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

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

    sub clear {
        my($self) = @_;


    package Point3D;
    use Mouse;

    extends Point;

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

    after clear => sub {
        my($self) = @_;



Moose is a postmodern object system for Perl5. Moose is wonderful.

Unfortunately, Moose has a compile-time penalty. Though significant progress has been made over the years, the compile time penalty is a non-starter for some very specific applications. If you are writing a command-line application or CGI script where startup time is essential, you may not be able to use Moose (we recommend that you instead use persistent Perl executing environments like FastCGI for the latter, if possible).

Mouse is a Moose compatible object system, which aims to alleviate this penalty by providing a subset of Moose’s functionality.

We’re also going as light on dependencies as possible. Mouse currently has <B>no dependenciesB> except for building/testing modules. Mouse also works without XS, although it has an XS backend to make it much faster.

    Moose Compatibility

Compatibility with Moose has been the utmost concern. The sugary interface is highly compatible with Moose. Even the error messages are taken from Moose. The Mouse code just runs its test suite 4x faster.

The idea is that, if you need the extra power, you should be able to run s/Mouse/Moose/g on your codebase and have nothing break. To that end, we have written Any::Moose which will act as Mouse unless Moose is loaded, in which case it will act as Moose. Since Mouse is a little sloppier than Moose, if you run into weird errors, it would be worth running:

    ANY_MOOSE=Moose perl

to see if the bug is caused by Mouse. Moose’s diagnostics and validation are also better.

See also Mouse::Spec for compatibility and incompatibility with Moose.

    Mouse Extentions

Please don’t copy MooseX code to MouseX. If you need extensions, you really should upgrade to Moose. We don’t need two parallel sets of extensions!

If you really must write a Mouse extension, please contact the Moose mailing list or #moose on IRC beforehand.


CW$object->meta -> Mouse::Meta::Class

Returns this class’ metaclass instance.

CWextends superclasses

Sets this class’ superclasses.

CWbefore (method|methods|regexp) => CodeRef

Installs a before method modifier. See before in Moose.

CWafter (method|methods|regexp) => CodeRef

Installs an after method modifier. See after in Moose.

CWaround (method|methods|regexp) => CodeRef

Installs an around method modifier. See around in Moose.

CWhas (name|names) => parameters

Adds an attribute (or if passed an arrayref of names, multiple attributes) to this class. Options:
is => ro|rw|bare The is option accepts either rw (for read/write), ro (for read only) or bare (for nothing). 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, however if you use those, you won’t need the is option.

isa => TypeName | ClassName Provides type checking in the constructor and accessor. The following types are supported. Any unknown type is taken to be a class check (e.g. isa => DateTime would accept only DateTime objects).

    Any Item Bool Undef Defined Value Num Int Str ClassName
    Ref ScalarRef ArrayRef HashRef CodeRef RegexpRef GlobRef
    FileHandle Object

For more documentation on type constraints, see Mouse::Util::TypeConstraints.

does => RoleName This will accept the name of a role which the value stored in this attribute is expected to have consumed.
coerce => Bool This will attempt to use coercion with the supplied type constraint to change the value passed into any accessors or constructors. You <B>mustB> have supplied a type constraint in order for this to work. See Moose::Cookbook::Basics::Recipe5 for an example.
required => Bool Whether this attribute is required to have a value. If the attribute is lazy or has a builder, then providing a value for the attribute in the constructor is optional.
init_arg => Str | Undef Allows you to use a different key name in the constructor. If undef, the attribute can’t be passed to the constructor.
default => Value | CodeRef Sets the default value of the attribute. If the default is a coderef, it will be invoked to get the default value. Due to quirks of Perl, any bare reference is forbidden, you must wrap the reference in a coderef. Otherwise, all instances will share the same reference.
lazy => Bool If specified, the default is calculated on demand instead of in the constructor.
predicate => Str Lets you specify a method name for installing a predicate method, which checks that the attribute has a value. It will not invoke a lazy default or builder method.
clearer => Str Lets you specify a method name for installing a clearer method, which clears the attribute’s value from the instance. On the next read, lazy or builder will be invoked.
handles => HashRef|ArrayRef|Regexp Lets you specify methods to delegate to the attribute. ArrayRef forwards the given method names to method calls on the attribute. HashRef maps local method names to remote method names called on the attribute. Other forms of handles, such as RoleName and CodeRef, are not yet supported.
weak_ref => Bool Lets you automatically weaken any reference stored in the attribute.

Use of this feature requires Scalar::Util!

trigger => CodeRef Any time the attribute’s value is set (either through the accessor or the constructor), the trigger is called on it. The trigger receives as arguments the instance, and the new value.
builder => Str Defines a method name to be called to provide the default value of the attribute. builder => build_foo is mostly equivalent to default => sub { $_[0]->build_foo }.
auto_deref => Bool Allows you to automatically dereference ArrayRef and HashRef attributes in list context. In scalar context, the reference is returned (NOT the list length or bucket status). You must specify an appropriate type constraint to use auto_deref.
lazy_build => Bool Automatically define the following options:

    has $attr => (
        # ...
        lazy      => 1
        builder   => "_build_$attr",
        clearer   => "clear_$attr",
        predicate => "has_$attr",

CWconfess(message) -> BOOM

confess in Carp for your convenience.

CWblessed(value) -> ClassName | undef

blessed in Scalar::Util for your convenience.



Importing Mouse will default your class’ superclass list to Mouse::Object. You may use extends to replace the superclass list.


Please unimport Mouse (no Mouse) so that if someone calls one of the keywords (such as extends) it will break loudly instead breaking subtly.


We have a public git repository <>:.

    git clone git://










Shawn M Moore <sartak at>

Yuval Kogman <nothingmuch at>




Goro Fuji (gfx) <>

with plenty of code borrowed from Class::MOP and Moose


All complex software has bugs lurking in it, and this module is no exception. Please report any bugs to bug-mouse at, or through the web interface at <>


Copyright (c) 2008-2010 Infinity Interactive, Inc.

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

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