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Author image Peter Rabbitson 🐟‍👂
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Moo - Minimalist Object Orientation (with Moose compatiblity)


 package Cat::Food;

 use Moo;
 use Sub::Quote;

 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 => quote_sub q{ die "$_[0] is too much cat food!" unless $_[0] < 15 },


and else where

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


 say $full->pounds;


This module is an extremely light-weight, high-performance Moose replacement. 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 Moose compatibility. See "INCOMPATIBILITIES" for more details.


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

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.

By Moo 1.0 I intend to have Moo's equivalent of Any::Moose built in - if Moose gets loaded, any Moo class or role will act as a Moose equivalent if treated as such.

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.



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


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


 around BUILDARGS => sub {
   my $orig = shift;
   my ( $class, @args ) = @_;

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

   return $class->$orig(@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.


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::Role2';

Composes a Role::Tiny into current class. Only one role may be composed in at a time to allow the code to remain as simple as possible.


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

Declares an attribute for the class.

The options for has are as follows:

  • is

    required, must be ro or rw. Unsurprisingly, ro generates an accessor that will not respond to arguments; to be clear: a getter only. rw will create a perlish getter/setter.

  • isa

    Takes a coderef which is meant to validate the attribute. Unlike Moose Moo does not include a basic type system, so instead of doing isa => 'Num', one should do

     isa => quote_sub q{
       die "$_[0] is not a number!" unless looks_like_number $_[0]

    Sub::Quote aware

  • coerce

    Takes a coderef which is meant to coerce the attribute. The basic idea is to do something like the following:

     coerce => quote_sub q{
       $_[0] + 1 unless $_[0] % 2

    Coerce does not require isa to be defined.

    Sub::Quote aware

  • handles

    Takes a string

      handles => 'RobotRole'

    Where RobotRole is a role (Moo::Role) that defines an interface which becomes the list of methods to handle.

    Takes a list of methods

     handles => [ qw( one two ) ]

    Takes a hashref

     handles => {
       un => 'one',
  • trigger

    Takes a coderef which will get called any time the attribute is set. Coderef will be invoked against the object with the new value as an argument.

    Note that Moose also passes the old value, if any; this feature is not yet supported.

    Sub::Quote aware

  • default

    Takes a coderef which will get called with $self as its only argument to populate an attribute if no value is supplied to the constructor - or if the attribute is lazy, when the attribute is first retrieved if no value has yet been provided.

    Note that if your default is fired during new() there is no guarantee that other attributes have been populated yet so you should not rely on their existence.

    Sub::Quote aware

  • predicate

    Takes a method name which will return true if an attribute has a value.

    A common example of this would be to call it has_$foo, implying that the object has a $foo set.

  • builder

    Takes a method name which will be called to create the attribute - functions exactly like default except that instead of calling


    Moo will call

  • clearer

    Takes a method name which will clear the attribute.

  • lazy

    Boolean. Set this if you want values for the attribute to be grabbed lazily. This is usually a good idea if you have a "builder" which requires another attribute to be set.

  • required

    Boolean. Set this if the attribute must be passed on instantiation.

  • reader

    The value of this attribute will be the name of the method to get the value of the attribute. If you like Java style methods, you might set this to get_foo

  • writer

    The value of this attribute will be the name of the method to set the value of the attribute. If you like Java style methods, you might set this to set_foo

  • weak_ref

    Boolean. Set this if you want the reference that the attribute contains to be weakened; use this when circular references are possible, which will cause leaks.

  • init_arg

    Takes the name of the key to look for at instantiation time of the object. A common use of this is to make an underscored attribute have a non-underscored initialization name. undef means that passing the value in on instantiation


 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.


You can only compose one role at a time. If your application is large or complex enough to warrant complex composition, you wanted Moose.

There is no complex 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.

initializer is not supported in core since the author considers it to be a bad idea but may be supported by an extension in future.

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.

No support for super, override, inner, or augment - override can be handled by around albeit with a little more typing, and the author considers augment to be a bad idea.

"default" only supports coderefs, because doing otherwise is usually a mistake anyway.

lazy_build is not supported per se, but of course it will work if you manually set all the options it implies.

auto_deref is not supported since the author considers it a bad idea.

documentation is not supported since it's a very poor replacement for POD.

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. So if you:

    package MyClass;
    use Moo;

The nearest Moose invocation would be:

    package MyClass;

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


mst - Matt S. Trout (cpan:MSTROUT) <mst@shadowcat.co.uk>


dg - David Leadbeater (cpan:DGL) <dgl@dgl.cx>

frew - Arthur Axel "fREW" Schmidt (cpan:FREW) <frioux@gmail.com>

hobbs - Andrew Rodland (cpan:ARODLAND) <arodland@cpan.org>

jnap - John Napiorkowski (cpan:JJNAPIORK) <jjn1056@yahoo.com>

ribasushi - Peter Rabbitson (cpan:RIBASUSHI) <ribasushi@cpan.org>

chip - Chip Salzenberg (cpan:CHIPS) <chip@pobox.com>

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

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


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.