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Matt S Trout > Moo > Moo



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Module Version: 1.003001   Source   Latest Release: Moo-2.003004


Moo - Minimalist Object Orientation (with Moose compatibility)


 package Cat::Food;

 use Moo;

 sub feed_lion {
   my $self = shift;
   my $amount = shift || 1;

   $self->pounds( $self->pounds - $amount );

 has taste => (
   is => 'ro',

 has brand => (
   is  => 'ro',
   isa => sub {
     die "Only SWEET-TREATZ supported!" unless $_[0] eq 'SWEET-TREATZ'

 has pounds => (
   is  => 'rw',
   isa => sub { die "$_[0] is too much cat food!" unless $_[0] < 15 },


And elsewhere:

 my $full = Cat::Food->new(
    taste  => 'DELICIOUS.',
    brand  => 'SWEET-TREATZ',
    pounds => 10,


 say $full->pounds;


This module is an extremely light-weight subset of Moose optimised for rapid startup and "pay only for what you use".

It also avoids depending on any XS modules to allow simple deployments. The name Moo is based on the idea that it provides almost -- but not quite -- two thirds of Moose.

Unlike Mouse this module does not aim at full compatibility with Moose's surface syntax, preferring instead of provide full interoperability via the metaclass inflation capabilities described in "MOO AND MOOSE".

For a full list of the minor differences between Moose and Moo's surface syntax, see "INCOMPATIBILITIES WITH MOOSE".


If you want a full object system with a rich Metaprotocol, Moose is already wonderful.

However, sometimes you're writing a command line script or a CGI script where fast startup is essential, or code designed to be deployed as a single file via App::FatPacker, or you're writing a CPAN module and you want it to be usable by people with those constraints.

I've tried several times to use Mouse but it's 3x the size of Moo and takes longer to load than most of my Moo based CGI scripts take to run.

If you don't want Moose, you don't want "less metaprotocol" like Mouse, you want "as little as possible" -- which means "no metaprotocol", which is what Moo provides.

Better still, if you install and load Moose, we set up metaclasses for your Moo classes and Moo::Role roles, so you can use them in Moose code without ever noticing that some of your codebase is using Moo.

Hence, Moo exists as its name -- Minimal Object Orientation -- with a pledge to make it smooth to upgrade to Moose when you need more than minimal features.


If Moo detects Moose being loaded, it will automatically register metaclasses for your Moo and Moo::Role packages, so you should be able to use them in Moose code without anybody ever noticing you aren't using Moose everywhere.

Moo will also create Moose type constraints for classes and roles, so that isa => 'MyClass' and isa => 'MyRole' work the same as for Moose classes and roles.

Extending a Moose class or consuming a Moose::Role will also work.

So will extending a Mouse class or consuming a Mouse::Role - but note that we don't provide Mouse metaclasses or metaroles so the other way around doesn't work. This feature exists for Any::Moose users porting to Moo; enabling Mouse users to use Moo classes is not a priority for us.

This means that there is no need for anything like Any::Moose for Moo code - Moo and Moose code should simply interoperate without problem. To handle Mouse code, you'll likely need an empty Moo role or class consuming or extending the Mouse stuff since it doesn't register true Moose metaclasses like Moo does.

If you want types to be upgraded to the Moose types, use MooX::Types::MooseLike and install the MooseX::Types library to match the MooX::Types::MooseLike library you're using - Moo will load the MooseX::Types library and use that type for the newly created metaclass.

If you need to disable the metaclass creation, add:

  no Moo::sification;

to your code before Moose is loaded, but bear in mind that this switch is currently global and turns the mechanism off entirely so don't put this in library code.


If a new enough version of Class::XSAccessor is available, it will be used to generate simple accessors, readers, and writers for a speed boost. Simple accessors are those without lazy defaults, type checks/coercions, or triggers. Readers and writers generated by Class::XSAccessor will behave slightly differently: they will reject attempts to call them with the incorrect number of parameters.


Any::Moose will load Mouse normally, and Moose in a program using Moose - which theoretically allows you to get the startup time of Mouse without disadvantaging Moose users.

Sadly, this doesn't entirely work, since the selection is load order dependent - Moo's metaclass inflation system explained above in "MOO AND MOOSE" is significantly more reliable.

So if you want to write a CPAN module that loads fast or has only pure perl dependencies but is also fully usable by Moose users, you should be using Moo.

For a full explanation, see the article which explains the differing strategies in more detail and provides a direct example of where Moo succeeds and Any::Moose fails.



 Foo::Bar->new( attr1 => 3 );


 Foo::Bar->new({ attr1 => 3 });


   my ( $class, @args ) = @_;

   unshift @args, "attr1" if @args % 2 == 1;

   return { @args };

 Foo::Bar->new( 3 );

The default implementation of this method accepts a hash or hash reference of named parameters. If it receives a single argument that isn't a hash reference it throws an error.

You can override this method in your class to handle other types of options passed to the constructor.

This method should always return a hash reference of named options.


If you are inheriting from a non-Moo class, the arguments passed to the parent class constructor can be manipulated by defining a FOREIGNBUILDARGS method. It will receive the same arguments as BUILDARGS, and should return a list of arguments to pass to the parent class constructor.


Define a BUILD method on your class and the constructor will automatically call the BUILD method from parent down to child after the object has been instantiated. Typically this is used for object validation or possibly logging.


If you have a DEMOLISH method anywhere in your inheritance hierarchy, a DESTROY method is created on first object construction which will call $instance->DEMOLISH($in_global_destruction) for each DEMOLISH method from child upwards to parents.

Note that the DESTROY method is created on first construction of an object of your class in order to not add overhead to classes without DEMOLISH methods; this may prove slightly surprising if you try and define your own.


 if ($foo->does('Some::Role1')) {

Returns true if the object composes in the passed role.



 extends 'Parent::Class';

Declares base class. Multiple superclasses can be passed for multiple inheritance (but please use roles instead).

Calling extends more than once will REPLACE your superclasses, not add to them like 'use base' would.


 with 'Some::Role1';


 with 'Some::Role1', 'Some::Role2';

Composes one or more Moo::Role (or Role::Tiny) roles into the current class. An error will be raised if these roles have conflicting methods.


 has attr => (
   is => 'ro',

Declares an attribute for the class.

 package Foo;
 use Moo;
 has 'attr' => (
   is => 'ro'

 package Bar;
 use Moo;
 extends 'Foo';
 has '+attr' => (
   default => sub { "blah" },

Using the + notation, it's possible to override an attribute.

The options for has are as follows:


 before foo => sub { ... };

See "before method(s) => sub { ... }" in Class::Method::Modifiers for full documentation.


 around foo => sub { ... };

See "around method(s) => sub { ... }" in Class::Method::Modifiers for full documentation.


 after foo => sub { ... };

See "after method(s) => sub { ... }" in Class::Method::Modifiers for full documentation.


"quote_sub" in Sub::Quote allows us to create coderefs that are "inlineable," giving us a handy, XS-free speed boost. Any option that is Sub::Quote aware can take advantage of this.

To do this, you can write

  use Moo;
  use Sub::Quote;

  has foo => (
    is => 'ro',
    isa => quote_sub(q{ die "Not <3" unless $_[0] < 3 })

which will be inlined as

  do {
    local @_ = ($_[0]->{foo});
    die "Not <3" unless $_[0] < 3;

or to avoid localizing @_,

  has foo => (
    is => 'ro',
    isa => quote_sub(q{ my ($val) = @_; die "Not <3" unless $val < 3 })

which will be inlined as

  do {
    my ($val) = ($_[0]->{foo});
    die "Not <3" unless $val < 3;

See Sub::Quote for more information, including how to pass lexical captures that will also be compiled into the subroutine.


There is no built-in type system. isa is verified with a coderef; if you need complex types, just make a library of coderefs, or better yet, functions that return quoted subs. MooX::Types::MooseLike provides a similar API to MooseX::Types::Moose so that you can write

  has days_to_live => (is => 'ro', isa => Int);

and have it work with both; it is hoped that providing only subrefs as an API will encourage the use of other type systems as well, since it's probably the weakest part of Moose design-wise.

initializer is not supported in core since the author considers it to be a bad idea and Moose best practices recommend avoiding it. Meanwhile trigger or coerce are more likely to be able to fulfill your needs.

There is no meta object. If you need this level of complexity you wanted Moose - Moo succeeds at being small because it explicitly does not provide a metaprotocol. However, if you load Moose, then


will return an appropriate metaclass pre-populated by Moo.

No support for super, override, inner, or augment - the author considers augment to be a bad idea, and override can be translated:

  override foo => sub {

  around foo => sub {
    my ($orig, $self) = (shift, shift);

The dump method is not provided by default. The author suggests loading Devel::Dwarn into main:: (via perl -MDevel::Dwarn ... for example) and using $obj->$::Dwarn() instead.

"default" only supports coderefs and plain scalars, because passing a hash or array reference as a default is almost always incorrect since the value is then shared between all objects using that default.

lazy_build is not supported; you are instead encouraged to use the is => 'lazy' option supported by Moo and MooseX::AttributeShortcuts.

auto_deref is not supported since the author considers it a bad idea and it has been considered best practice to avoid it for some time.

documentation will show up in a Moose metaclass created from your class but is otherwise ignored. Then again, Moose ignores it as well, so this is arguably not an incompatibility.

Since coerce does not require isa to be defined but Moose does require it, the metaclass inflation for coerce alone is a trifle insane and if you attempt to subtype the result will almost certainly break.

Handling of warnings: when you use Moo we enable FATAL warnings. The nearest similar invocation for Moose would be:

  use Moose;
  use warnings FATAL => "all";

Additionally, Moo supports a set of attribute option shortcuts intended to reduce common boilerplate. The set of shortcuts is the same as in the Moose module MooseX::AttributeShortcuts as of its version 0.009+. So if you:

    package MyClass;
    use Moo;

The nearest Moose invocation would be:

    package MyClass;

    use Moose;
    use warnings FATAL => "all";
    use MooseX::AttributeShortcuts;

or, if you're inheriting from a non-Moose class,

    package MyClass;

    use Moose;
    use MooseX::NonMoose;
    use warnings FATAL => "all";
    use MooseX::AttributeShortcuts;

Finally, Moose requires you to call


at the end of your class to get an inlined (i.e. not horribly slow) constructor. Moo does it automatically the first time ->new is called on your class. (make_immutable is a no-op in Moo to ease migration.)

An extension MooX::late exists to ease translating Moose packages to Moo by providing a more Moose-like interface.


Users' IRC: #moose on

(click for instant chatroom login)

Development and contribution IRC: #web-simple on

(click for instant chatroom login)


Git repository: git://

Git web access:


mst - Matt S. Trout (cpan:MSTROUT) <>


dg - David Leadbeater (cpan:DGL) <>

frew - Arthur Axel "fREW" Schmidt (cpan:FREW) <>

hobbs - Andrew Rodland (cpan:ARODLAND) <>

jnap - John Napiorkowski (cpan:JJNAPIORK) <>

ribasushi - Peter Rabbitson (cpan:RIBASUSHI) <>

chip - Chip Salzenberg (cpan:CHIPS) <>

ajgb - Alex J. G. Burzyński (cpan:AJGB) <>

doy - Jesse Luehrs (cpan:DOY) <doy at tozt dot net>

perigrin - Chris Prather (cpan:PERIGRIN) <>

Mithaldu - Christian Walde (cpan:MITHALDU) <>

ilmari - Dagfinn Ilmari Mannsåker (cpan:ILMARI) <>

tobyink - Toby Inkster (cpan:TOBYINK) <>

haarg - Graham Knop (cpan:HAARG) <>

mattp - Matt Phillips (cpan:MATTP) <>


Copyright (c) 2010-2011 the Moo "AUTHOR" and "CONTRIBUTORS" as listed above.


This library is free software and may be distributed under the same terms as perl itself. See

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