new

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

or

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

The constructor for the class. By default it will accept attributes either as a hashref, or a list of key value pairs. This can be customized with the "BUILDARGS" method.

does

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

Returns true if the object composes in the passed role.

DOES

  if ($foo->DOES('Some::Role1') || $foo->DOES('Some::Class1')) {
    ...
  }

Similar to "does", but will also return true for both composed roles and superclasses.

meta

  my $meta = Foo::Bar->meta;
  my @methods = $meta->get_method_list;

Returns an object that will behave as if it is a Moose metaclass object for the class. If you call anything other than "make_immutable" on it, the object will be transparently upgraded to a genuine Moose::Meta::Class instance, loading Moose in the process if required. "make_immutable" itself is a no-op, since we generate metaclasses that are already immutable, and users converting from Moose had an unfortunate tendency to accidentally load Moose by calling it.

BUILDARGS

  around BUILDARGS => sub {
    my ( $orig, $class, @args ) = @_;
    return { attr1 => $args[0] }
      if @args == 1 && !ref $args[0];
    return $class->$orig(@args);
  };
  Foo::Bar->new( 3 );

This class method is used to transform the arguments to "new" into a hash reference of attribute values.

The default implementation accepts a hash or hash reference of named parameters. If it receives a single argument that isn't a hash reference it will throw 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.

FOREIGNBUILDARGS

  sub FOREIGNBUILDARGS {
    my ( $class, $options ) = @_;
    return $options->{foo};
  }

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.

BUILD

  sub BUILD {
    my ($self, $args) = @_;
    die "foo and bar cannot be used at the same time"
      if exists $args->{foo} && exists $args->{bar};
  }

On object creation, any "BUILD" methods in the class's inheritance hierarchy will be called on the object and given the results of "BUILDARGS". They each will be called in order from the parent classes down to the child, and thus should not themselves call the parent's method. Typically this is used for object validation or possibly logging.

DEMOLISH

  sub DEMOLISH {
    my ($self, $in_global_destruction) = @_;
    ...
  }

When an object is destroyed, any "DEMOLISH" methods in the inheritance hierarchy will be called on the object. They are given boolean to inform them if global destruction is in progress, and are called from the child class upwards to the parent. This is similar to "BUILD" methods but in the opposite order.

Note that this is implemented by a "DESTROY" method, which is only created on on the first construction of an object of your class. This saves on overhead for classes that are never instantiated or those without "DEMOLISH" methods. If you try to define your own "DESTROY", this will cause undefined results.

extends

  extends 'Parent::Class';

Declares a base class. Multiple superclasses can be passed for multiple inheritance but please consider using roles instead. The class will be loaded but no errors will be triggered if the class can't be found and there are already subs in the class.

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

with

  with 'Some::Role1';

or

  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 cannot be composed because they have conflicting method definitions. The roles will be loaded using the same mechanism as "extends" uses.

has

  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.

  has [qw(attr1 attr2 attr3)] => (
    is => 'ro',
  );

Using an arrayref with multiple attribute names, it's possible to declare multiple attributes with the same options.

The options for "has" are as follows:

"is"

required, may be "ro", "lazy", "rwp" or "rw".

"ro" stands for "read-only" and generates an accessor that dies if you attempt to write to it - i.e. a getter only - by defaulting "reader" to the name of the attribute.

"lazy" generates a reader like "ro", but also sets "lazy" to 1 and "builder" to "_build_${attribute_name}" to allow on-demand generated attributes. This feature was my attempt to fix my incompetence when originally designing "lazy_build", and is also implemented by MooseX::AttributeShortcuts. There is, however, nothing to stop you using "lazy" and "builder" yourself with "rwp" or "rw" - it's just that this isn't generally a good idea so we don't provide a shortcut for it.

"rwp" stands for "read-write protected" and generates a reader like "ro", but also sets "writer" to "_set_${attribute_name}" for attributes that are designed to be written from inside of the class, but read-only from outside. This feature comes from MooseX::AttributeShortcuts.

"rw" stands for "read-write" and generates a normal getter/setter by defaulting the "accessor" to the name of the attribute specified.

"isa"

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

  use Scalar::Util qw(looks_like_number);
  ...
  isa => sub {
    die "$_[0] is not a number!" unless looks_like_number $_[0]
  },
    

Note that the return value for "isa" is discarded. Only if the sub dies does type validation fail.

Sub::Quote aware

Since Moo does not run the "isa" check before "coerce" if a coercion subroutine has been supplied, "isa" checks are not structural to your code and can, if desired, be omitted on non-debug builds (although if this results in an uncaught bug causing your program to break, the Moo authors guarantee nothing except that you get to keep both halves).

If you want Moose compatible or MooseX::Types style named types, look at Type::Tiny.

To cause your "isa" entries to be automatically mapped to named Moose::Meta::TypeConstraint objects (rather than the default behaviour of creating an anonymous type), set:

  $Moo::HandleMoose::TYPE_MAP{$isa_coderef} = sub {
    require MooseX::Types::Something;
    return MooseX::Types::Something::TypeName();
  };
    

Note that this example is purely illustrative; anything that returns a Moose::Meta::TypeConstraint object or something similar enough to it to make Moose happy is fine.

"coerce"

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

 coerce => sub {
   $_[0] % 2 ? $_[0] : $_[0] + 1
 },
    

Note that Moo will always execute your coercion: this is to permit "isa" entries to be used purely for bug trapping, whereas coercions are always structural to your code. We do, however, apply any supplied "isa" check after the coercion has run to ensure that it returned a valid value.

Sub::Quote aware

If the "isa" option is a blessed object providing a "coerce" or "coercion" method, then the "coerce" option may be set to just 1.

"handles"

Takes a string

  handles => 'RobotRole'
    

Where "RobotRole" is a 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. This includes the constructor, but not default or built values. The coderef will be invoked against the object with the new value as an argument.

If you set this to just 1, it generates a trigger which calls the "_trigger_${attr_name}" method on $self. This feature comes from MooseX::AttributeShortcuts.

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 for that attribute was supplied to the constructor. Alternatively, if the attribute is lazy, "default" executes when the attribute is first retrieved if no value has yet been provided.

If a simple scalar is provided, it will be inlined as a string. Any non-code reference (hash, array) will result in an error - for that case instead use a code reference that returns the desired value.

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.

If you set this to just 1, the predicate is automatically named "has_${attr_name}" if your attribute's name does not start with an underscore, or "_has_${attr_name_without_the_underscore}" if it does. This feature comes from MooseX::AttributeShortcuts.

"builder"

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

  $default->($self);
    

Moo will call

  $self->$builder;
    

The following features come from MooseX::AttributeShortcuts:

If you set this to just 1, the builder is automatically named "_build_${attr_name}".

If you set this to a coderef or code-convertible object, that variable will be installed under "$class::_build_${attr_name}" and the builder set to the same name.

"clearer"

Takes a method name which will clear the attribute.

If you set this to just 1, the clearer is automatically named "clear_${attr_name}" if your attribute's name does not start with an underscore, or "_clear_${attr_name_without_the_underscore}" if it does. This feature comes from MooseX::AttributeShortcuts.

NOTE: If the attribute is "lazy", it will be regenerated from "default" or "builder" the next time it is accessed. If it is not lazy, it will be "undef".

"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 object instantiation.

"reader"

The name of the method that returns 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, which cause memory leaks, are possible.

"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 is ignored.

"moosify"

Takes either a coderef or array of coderefs which is meant to transform the given attributes specifications if necessary when upgrading to a Moose role or class. You shouldn't need this by default, but is provided as a means of possible extensibility.

before

  before foo => sub { ... };

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

around

  around foo => sub { ... };

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

after

  after foo => sub { ... };

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

TYPES

There is no built-in type system. "isa" is verified with a coderef; if you need complex types, Type::Tiny can provide types, type libraries, and will work seamlessly with both Moo and Moose. Type::Tiny can be considered the successor to MooseX::Types and provides a similar API, so that you can write

  use Types::Standard qw(Int);
  has days_to_live => (is => 'ro', isa => Int);

API INCOMPATIBILITIES

"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.

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 {
    ...
    super();
    ...
  };
  around foo => sub {
    my ($orig, $self) = (shift, shift);
    ...
    $self->$orig(@_);
    ...
  };

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 strict and warnings, in a similar way to Moose. The authors recommend the use of "strictures", which enables FATAL warnings, and several extra pragmas when used in development: indirect, multidimensional, and bareword::filehandles.

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;
  use strictures 2;

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;

META OBJECT

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

  Class::MOP::class_of($moo_class_or_role)

will return an appropriate metaclass pre-populated by Moo.

IMMUTABILITY

Finally, Moose requires you to call

  __PACKAGE__->meta->make_immutable;

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.