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Author image Shawn M Moore
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Mouse - Moose minus the antlers


    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 = shift;

    package Point3D;
    use Mouse;

    extends 'Point';

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

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


Moose is wonderful.

Unfortunately, it's a little slow. Though significant progress has been made over the years, the compile time penalty is a non-starter for some applications.

Mouse aims to alleviate this by providing a subset of Moose's functionality, faster. In particular, "has" in Moose is missing only a few expert-level features.

We're also going as light on dependencies as possible. Most functions we use from Scalar::Util are copied into this dist. Scalar::Util is required if you'd like weak references; there's simply no way to do it from pure Perl. Class::Method::Modifiers is required if you want support for "before", "after", and "around".


Compatibility with Moose has been the utmost concern. Fewer than 1% of the tests fail when run against Moose instead of Mouse. Mouse code coverage is also over 96%. Even the error messages are taken from Moose. The Mouse code just runs the 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, nothingmuch has written Squirrel (part of this distribution) which will act as Mouse unless Moose is loaded, in which case it will act as Moose.

Mouse also has the blessings of Moose's author, stevan.



We're working on fixing this one! stevan has suggested an implementation strategy. Mouse currently ignores methods, so that needs to be fixed next. Roles that consist entirely of attributes may be usable in this very version.

Complex types

User-defined type constraints and parameterized types may be implemented. Type coercions probably not (patches welcome).

Bootstrapped meta world

Very handy for extensions to the MOP. Not pressing, but would be nice to have.

Modification of attribute metaclass

When you declare an attribute with "has", you get the inlined accessors installed immediately. Modifying the attribute metaclass, even if possible, does nothing.

Lots more..



meta -> Mouse::Meta::Class

Returns this class' metaclass instance.

extends superclasses

Sets this class' superclasses.

before (method|methods) => Code

Installs a "before" method modifier. See "before" in Moose or "before" in Class::Method::Modifiers.

Use of this feature requires Class::Method::Modifiers!

after (method|methods) => Code

Installs an "after" method modifier. See "after" in Moose or "after" in Class::Method::Modifiers.

Use of this feature requires Class::Method::Modifiers!

around (method|methods) => Code

Installs an "around" method modifier. See "around" in Moose or "around" in Class::Method::Modifiers.

Use of this feature requires Class::Method::Modifiers!

has (name|names) => parameters

Adds an attribute (or if passed an arrayref of names, multiple attributes) to this class. Options:

is => ro|rw

If specified, inlines a read-only/read-write accessor with the same name as the attribute.

isa => TypeConstraint

Provides basic type checking in the constructor and accessor. Basic types such as Int, ArrayRef, Defined are supported. Any unknown type is taken to be a class check (e.g. isa => 'DateTime' would accept only DateTime objects).

required => 0|1

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 attribue 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 => 0|1

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

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 regular expression and coderef, are not yet supported.

weak_ref => 0|1

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, the new value, and the attribute instance.

Mouse 0.05 supported more complex triggers, but this behavior is now removed.

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 => 0|1

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 => 0|1

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

confess error -> BOOM

"confess" in Carp for your convenience.

blessed 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.


load_class Class::Name

This will load a given Class::Name (or die if it's not loadable). This function can be used in place of tricks like eval "use $module" or using require.

is_class_loaded Class::Name -> Bool

Returns whether this class is actually loaded or not. It uses a heuristic which involves checking for the existence of $VERSION, @ISA, and any locally-defined method.


Shawn M Moore, <sartak at gmail.com>

Yuval Kogman, <nothingmuch at woobling.org>



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


No known bugs.

Please report any bugs through RT: email bug-mouse at rt.cpan.org, or browse http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Mouse.


Copyright 2008 Shawn M Moore.

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