oracular (3) Class::Meta.3pm.gz
NAME
Class::Meta - Class automation, introspection, and data validation
SYNOPSIS
Generate a class:
package MyApp::Thingy;
use strict;
use Class::Meta;
BEGIN {
# Create a Class::Meta object for this class.
my $cm = Class::Meta->new(
key => 'thingy',
default_type => 'string',
);
# Add a constructor.
$cm->add_constructor(
name => 'new',
create => 1,
);
# Add a couple of attributes with generated methods.
$cm->add_attribute(
name => 'uuid',
authz => 'READ',
required => 1,
default => sub { Data::UUID->new->create_str },
);
$cm->add_attribute(
name => 'name',
is => 'string',
default => undef,
);
$cm->add_attribute(
name => 'age',
is => 'integer',
default => undef,
);
# Add a custom method.
$cm->add_method(
name => 'chk_pass',
view => 'PUBLIC',
code => sub { ... },
);
$cm->build;
}
sub chck_pass { ... }
Or use Class::Meta::Express for a more pleasant declarative syntax (highly recommended!):
package MyApp::Thingy;
use strict;
use Class::Meta::Express;
class {
meta thingy => ( default_type => 'string' );
ctor 'new';
has uuid => (
authz => 'READ',
required => 1,
default => sub { Data::UUID->new->create_str },
);
has name => ( required => 1 );
has age => ( is => 'integer' );
method chk_pass => sub { ... }
};
Now isn't that nicer? Then use the class:
use MyApp::Thingy;
my $thingy = MyApp::Thingy->new( id => 19 );
print "ID: ", $thingy->id, $/;
$thingy->name('Larry');
print "Name: ", $thingy->name, $/;
$thingy->age(42);
print "Age: ", $thingy->age, $/;
Or make use of the introspection API:
use MyApp::Thingy;
my $class = MyApp::Thingy->my_class;
my $thingy;
print "Examining object of class ", $class->package, $/;
print "\nConstructors:\n";
for my $ctor ($class->constructors) {
print " o ", $ctor->name, $/;
$thingy = $ctor->call($class->package);
}
print "\nAttributes:\n";
for my $attr ($class->attributes) {
print " o ", $attr->name, " => ", $attr->get($thingy), $/;
if ($attr->authz >= Class::Meta::SET && $attr->type eq 'string') {
$attr->get($thingy, 'hey there!');
print " Changed to: ", $attr->get($thingy), $/;
}
}
print "\nMethods:\n";
for my $meth ($class->methods) {
print " o ", $meth->name, $/;
$meth->call($thingy);
}
DESCRIPTION
Class::Meta provides an interface for automating the creation of Perl classes with attribute data type
validation. It differs from other such modules in that it includes an introspection API that can be used
as a unified interface for all Class::Meta-generated classes. In this sense, it is an implementation of
the "Facade" design pattern.
USAGE
Before we get to the introspection API, let's take a look at how to create classes with Class::Meta.
Unlike many class automation modules for Perl, the classes that Class::Meta builds do not inherit from
Class::Meta. This frees you from any dependencies on the interfaces that such a base class might compel.
For example, you can create whatever constructors you like, and name them whatever you like.
First of all, you really want to be using Class::Meta::Express to declare your Class::Meta classes. It
provides a much more pleasant class declaration experience than Class::Meta itself does. But since its
functions support many of the same arguments as the declaration methods described here, it's worth it to
skim the notes here, as well. Or if you're just a masochist and want to use the Class::Meta interface
itself, well, read on!
I recommend that you create your Class::Meta classes in a "BEGIN" block. Although this is not strictly
necessary, it helps ensure that the classes you're building are completely constructed and ready to go by
the time compilation has completed. Creating classes with Class::Meta is easy, using the Class::Meta
object oriented interface. Here is an example of a very simple class:
package MyApp::Dog;
use strict;
use Class::Meta;
use Class::Meta::Types::Perl;
BEGIN {
# Create a Class::Meta object for this class.
my $cm = Class::Meta->new( key => 'dog' );
# Add a constructor.
$cm->add_constructor(
name => 'new',
create => 1,
);
# Add an attribute.
$cm->add_attribute(
name => 'tail',
type => 'scalar',
);
# Add a custom method.
$cm->add_method( name => 'wag' );
$cm->build;
}
sub wag {
my $self = shift;
print "Wagging ", $self->tail;
}
This simple example shows of the construction of all three types of objects supported by Class::Meta:
constructors, attributes, and methods. Here's how it does it:
• First we load Class::Meta and Class::Meta::Types::Perl. The latter module creates data types that can
be used for attributes, including a "scalar" data type.
• Second, we create a Class::Meta object. It's okay to create it within the "BEGIN" block, as it won't
be needed beyond that. All Class::Meta classes have a "key" that uniquely identifies them across an
application. If none is provided, the class name will be used, instead.
• Next, we create a Class::Meta::Constructor object to describe a constructor method for the class. The
"create" parameter to the "add_constructor()" method tells Class::Meta to create the constructor
named ""new()"".
• Then we call "add_attribute()" to create a single attribute, "tail". This is a simple scalar
attribute, meaning that any scalar value can be stored in it. Class::Meta will create a
Class::Meta::Attribute object that describes this attribute, and will also shortly create accessor
methods for the attribute.
• The "add_method()" method constructs a Class::Meta::Method object to describe any methods written for
the class. In this case, we've told Class::Meta that there will be a "wag()" method.
• And finally, we tell Class::Meta to build the class. This is the point at which all constructors and
accessor methods will be created in the class. In this case, these include the "new()" constructor
and a "tail()" accessor for the "tail" attribute. And finally, Class::Meta will install another
method, "my_class()". This method will return a Class::Meta::Class object that describes the class,
and provides the complete introspection API.
Thus, the class the above code creates has this interface:
sub my_class;
sub new;
sub tail;
sub wag;
Data Types
By default, Class::Meta loads no data types. If you attempt to create an attribute without creating or
loading the appropriate data type, you will get an error.
But I didn't want to leave you out in the cold, so I created a whole bunch of data types to get you
started. Any of these will automatically be loaded by Class::Meta if it is used to create an attribute.
They can also be loaded simply by "use"ing the appropriate module. The modules are:
Class::Meta::Types::Perl
Typical Perl data types.
scalar
Any scalar value.
scalarref
A scalar reference.
array
arrayref
An array reference.
hash
hashref
A hash reference.
code
coderef
closure
A code reference.
Class::Meta::Types::String
string
Attributes of this type must contain a string value. Essentially, this means anything other than
a reference.
Class::Meta::Types::Boolean
boolean
bool
Attributes of this type store a boolean value. Implementation-wise, this means either a 1 or a 0.
Class::Meta::Types::Numeric
These data types are validated by the functions provided by Data::Types.
whole
A whole number.
integer
An integer.
decimal
A decimal number.
real
A real number.
float
a floating point number.
Other data types may be added in the future. See the individual data type modules for more information.
Accessors
Class::Meta supports the creation of three different types of attribute accessors: typical Perl single-
method accessors, "affordance" accessors, and "semi-affordance" accessors. The single accessors are named
for their attributes, and typically tend to look like this:
sub tail {
my $self = shift;
return $self->{tail} unless @_;
return $self->{tail} = shift;
}
Although this can be an oversimplification if the data type has associated validation checks.
Affordance accessors provide at up to two accessors for every attribute: One to set the value and one to
retrieve the value. They tend to look like this:
sub get_tail { shift->{tail} }
sub set_tail { shift->{tail} = shift }
These accessors offer a bit less overhead than the traditional Perl accessors, in that they don't have to
check whether they're called to get or set a value. They also have the benefit of creating a
psychological barrier to misuse. Since traditional Perl accessors can be created as read-only or write-
only accessors, one can't tell just by looking at them which is the case. The affordance accessors make
this point moot, as they make clear what their purpose is.
Semi-affordance accessors are similar to affordance accessors in that they provide at least two accessors
for every attribute. However, the accessor that fetches the value is named for the attribute. Thus, they
tend to look like this:
sub tail { shift->{tail} }
sub set_tail { shift->{tail} = shift }
To get Class::Meta's data types to create affordance accessors, simply pass the string "affordance" to
them when you load them:
use Class::Meta::Types::Perl 'affordance';
Likewise, to get them to create semi-affordance accessors, pass the string "semi-affordance":
use Class::Meta::Types::Perl 'semi-affordance';
The boolean data type is the only one that uses a slightly different approach to the creation of
affordance accessors: It creates three of them. Assuming you're creating a boolean attribute named
"alive", it will create these accessors:
sub is_alive { shift->{alive} }
sub set_alive_on { shift->{alive} = 1 }
sub set_alive_off { shift->{alive} = 0 }
Incidentally, I stole the term "affordance" from Damian Conway's "Object Oriented Perl," pp 83-84, where
he borrows it from Donald Norman.
See Class::Meta::Type for details on creating new data types.
Introspection API
Class::Meta provides four classes the make up the introspection API for Class::Meta-generated classes.
Those classes are:
Class::Meta::Class
Describes the class. Each Class::Meta-generated class has a single constructor object that can be
retrieved by calling a class' "my_class()" class method. Using the Class::Meta::Class object, you can get
access to all of the other objects that describe the class. The relevant methods are:
constructors
Provides access to all of the Class::Meta::Constructor objects that describe the class' constructors,
and provide indirect access to those constructors.
attributes
Provides access to all of the Class::Meta::Attribute objects that describe the class' attributes, and
provide methods for indirectly getting and setting their values.
methods
Provides access to all of the Class::Meta::Method objects that describe the class' methods, and
provide indirect execution of those constructors.
Class::Meta::Constructor
Describes a class constructor. Typically a class will have only a single constructor, but there could be
more, and client code doesn't necessarily know its name. Class::Meta::Constructor objects resolve these
issues by describing all of the constructors in a class. The most useful methods are:
name
Returns the name of the constructor, such as "new".
call
Calls the constructor on an object, passing in the arguments passed to "call()" itself.
Class::Meta::Attribute
Describes a class attribute, including its name and data type. Attribute objects are perhaps the most
useful Class::Meta objects, in that they can provide a great deal of information about the structure of a
class. The most interesting methods are:
name
Returns the name of the attribute.
type
Returns the name of the attribute's data type.
required
Returns true if the attribute is required to have a value.
once
Returns true if the attribute value can be set to a defined value only once.
set Sets the value of an attribute on an object.
get Returns the value of an attribute on an object.
Class::Meta::Method
Describes a method of a class, including its name and context (class vs. instance). The relevant methods
are:
name
The method name.
context
The context of the method indicated by a value corresponding to either Class::Meta::OBJECT or
Class::Meta::CLASS.
call
Calls the method, passing in the arguments passed to "call()" itself.
Consult the documentation of the individual classes for a complete description of their interfaces.
INTERFACE
Class Methods
default_error_handler
Class::Meta->default_error_handler($code);
my $default_error_handler = Class::Meta->default_error_handler;
Sets the default error handler for Class::Meta classes. If no "error_handler" attribute is passed to new,
then this error handler will be associated with the new class. The default default error handler uses
"Carp::croak()" to handle errors.
Note that if other modules are using Class::Meta that they will use your default error handler unless you
reset the default error handler to its original value before loading them.
handle_error
Class::Meta->handle_error($err);
Uses the code reference returned by "default_error_handler()" to handle an error. Used internally
Class::Meta classes when no Class::Meta::Class object is available. Probably not useful outside of
Class::Meta unless you're creating your own accessor generation class. Use the "handle_error()" instance
method in Class::Meta::Class, instead.
for_key
my $class = Class::Meta->for_key($key);
Returns the Class::Meta::Class object for a class by its key name. This can be useful in circumstances
where the key has been used to track a class, and you need to get a handle on that class. With the class
package name, you can of course simply call "$pkg->my_class"; this method is the solution for getting the
class object for a class key.
keys
my @keys = Class::Meta->keys;
Returns the keys for all Class::Meta::Class objects. The order of keys is not guaranteed. In scalar
context, this method returns an array reference containing the keys.
clear
Class::Meta->clear;
Class::Meta->clear($key);
Called without arguments, "clear" will remove all Class::Meta::Class objects from memory. Called with an
argument, "clear" attempts to remove only that key from memory. Calling it with a non-existent key is a
no-op.
In general, you probably won't want to use this method, except perhaps in tests, when you might need to
do funky things with your classes.
Constructors
new
my $cm = Class::Meta->new( key => $key );
Constructs and returns a new Class::Meta object that can then be used to define and build the complete
interface of a class. Many of the supported parameters values will default to values specified for the
most immediate Class::Meta-built parent class, if any. The supported parameters are:
package
The package that defines the class. Defaults to the package of the code that calls "new()".
key A key name that uniquely identifies a class within an application. Defaults to the value of the
"package" parameter if not specified.
name
The human name to use for the class. Defaults to the value of "key" with underscores replaced with
spaces and each word capitalized by the "ucfirst" operator. So "foo" will become "Foo" and
"contact_type" will become "Contact Type".
abstract
A boolean indicating whether the class being defined is an abstract class. An abstract class, also
known as a "virtual" class, is not intended to be used directly. No objects of an abstract class
should every be created. Instead, classes that inherit from an abstract class must be implemented.
default_type
A data type to use for attributes added to the class with no explicit data type. See "Data Types" for
some possible values for this parameter. Inheritable from parent class.
trust
An array reference of key names or packages that are trusted by the class.
trust => ['Foo::Bar', 'Foo::Bat'],
Trusted packages and the classes that inherit from them can retrieve trusted attributes and methods
of the class. Trusted packages need not be Class::Meta classes. Trusted classes do not include the
declaring class by default, so if you want the class that declares an attribute to be able to use
trusted attribute accessors, be sure to include it in the list of trusted packages:
trust => [__PACKAGE__, 'Foo::Bar', 'Foo::Bat'],
If you need to trust a single class, you may pass in the key name or package of that class rather
than an array reference:
trust => 'Foo::Bar',
class_class
The name of a class that inherits from Class::Meta::Class to be used to create all of the class
objects for the class. Defaults to Class::Meta::Class. Inheritable from parent class.
constructor_class
The name of a class that inherits from Class::Meta::Constructor to be used to create all of the
constructor objects for the class. Defaults to Class::Meta::Constructor. Inheritable from parent
class.
attribute_class
The name of a class that inherits from Class::Meta::Attribute to be used to create all of the
attribute objects for the class. Defaults to Class::Meta::Attribute. Inheritable from parent class.
method_class
The name of a class that inherits from Class::Meta::Method to be used to create all of the method
objects for the class. Defaults to Class::Meta::Method. Inheritable from parent class.
error_handler
A code reference that will be used to handle errors thrown by the methods created for the new class.
Defaults to the value returned by "Class::Meta->default_error_handler". Inheritable from parent
class.
add_constructor
$cm->add_constructor(
name => 'construct',
create => 1,
);
Creates and returns a Class::Meta::Constructor object that describes a constructor for the class. The
supported parameters are:
name
The name of the constructor. The name must consist of only alphanumeric characters or "_". Required.
create
When true, Class::Meta::Constructor will automatically create and install a constructor named for the
"name" parameter. Defaults to true unless "code" is passed. In general you won't need to specify this
parameter unless you've written your own constructor in the package, in which case you'll want to
specify "create => 0".
label
A label for the constructor. Generally used for displaying its name in a user interface. Optional.
desc
A description of the constructor. Possibly useful for displaying help text in a user interface.
Optional.
code
You can implicitly define the constructor in your class by passing a code reference via the "code"
parameter. Once "build()" is called, Kinetic::Meta::Constructor will install the constructor into the
package for which the Class::Meta object was defined, and with the name specified via the "name"
parameter. Note that if the constructor view is PRIVATE or PROTECTED, the constructor will be wrapped
in extra code to constrain the view. Optional.
view
The visibility of the constructor. The possible values are defined by the following constants:
Class::Meta::PUBLIC
Can be used by any client.
Class::Meta::PRIVATE
Can only be used by the declaring class.
Class::Meta::TRUSTED
Can only be used by the classes specified by the "trust" parameter to "new()".
Class::Meta::PROTECTED
Can only be used by the declaring class or by classes that inherit from it.
Defaults to Class::Meta::PUBLIC if not defined. You can also use strings aliases to the above
constants, although the constant values will actually be stored in the Class::Meta::Constructor
object, rather than the string. The supported strings are "PUBLIC", "PRIVATE", "TRUSTED", and
"PROTECTED".
caller
A code reference that calls the constructor. Defaults to a code reference that calls a method with
the name provided by the "name" attribute on the class being defined.
If Class::Meta creates the constructor, it will be a simple parameter-list constructor, wherein attribute
values can be passed as a list of attribute-name/value pairs, e.g.:
my $thingy = MyApp::Thingy->new(
name => 'Larry',
age => 32,
);
Required attributes must have a value passed to the constructor, with one exception: You can pass an
optional subroutine reference as the last argument to the constructor. After all parameter values and
default values have been set on the object, but before any exceptions are thrown for undefined required
attributes, the constructor will execute this subroutine reference, passing in the object being
constructed as the sole argument. So, for example, if "name" is required but, for some reason, could not
be set before constructing the object, you could set it like so:
my $thingy = MyApp::Thingy->new(
age => 32,
sub {
my $thingy = shift;
# age and attributes with default values are already set.
my $name = calculate_name( $thingy );
$thingy->name($name);
},
);
This allows developers to have a scope-limited context in which to work before required constraints are
enforced.
add_attribute
$cm->add_attribute(
name => 'tail',
type => 'scalar',
);
Creates and returns a Class::Meta::Attribute object that describes an attribute of the class. The
supported parameters are:
name
The name of the attribute. The name must consist of only alphanumeric characters or "_". Required.
type
is The data type of the attribute. See "Data Types" for some possible values for this parameter. If the
type name corresponds to a data type in a package in the Class::Meta::Types name space, that package
will automatically be loaded and configured with Perl-style accessors, so that the data type can
simply be used. If both "type" and "is" are passed, "is" will be used. Required unless the class was
declared with a "default_type".
required
A boolean value indicating whether the attribute is required to have a value. Defaults to false.
once
A boolean value indicating whether the attribute can be set to a defined value only once. Defaults to
false.
label
A label for the attribute. Generally used for displaying its name in a user interface. Optional.
desc
A description of the attribute. Possibly useful for displaying help text in a user interface.
Optional.
view
The visibility of the attribute. See the description of the "view" parameter to "add_constructor" for
a description of its value.
authz
The authorization of the attribute. This value indicates whether it is read-only, write-only,
read/write, or inaccessible. The possible values are defined by the following constants:
Class::Meta::READ
Class::Meta::WRITE
Class::Meta::RDWR
Class::Meta::NONE
Defaults to Class::Meta::RDWR if not defined. You can also use strings aliases to the above
constants, although the constant values will actually be stored in the Class::Meta::Attribute object,
rather than the string. The supported strings are "READ", "WRITE", "RDWR", and "NONE".
create
Indicates what type of accessor or accessors are to be created for the attribute.
Class::Meta::GET
Create read-only accessor(s).
Class::Meta::SET
Create write-only accessor(s).
Class::Meta::GETSET
Create read/write accessor(s).
Class::Meta::NONE
Create no accessors.
You can also use strings aliases to the above constants, although the constant values will actually
be stored in the Class::Meta::Attribute object, rather than the string. The supported strings are
"GET", "SET", "GETSET", and "NONE".
If not unspecified, the value of the "create" parameter will correspond to the value of the "authz"
parameter like so:
authz create
------------------
READ => GET
WRITE => SET
RDWR => GETSET
NONE => NONE
The "create" parameter differs from the "authz" parameter in case you've taken it upon yourself to
create some accessors, and therefore don't need Class::Meta to do so. For example, if you were using
standard Perl-style accessors, and needed to do something a little different by coding your own
accessor, you'd specify it like this:
$cm->add_attribute(
name => $name,
type => $type,
authz => Class::Meta::RDWR,
create => Class::Meta::NONE
);
Just be sure that your custom accessor compiles before you call "$cm->build" so that
Class::Meta::Attribute can get a handle on it for its "get()" and/or "set()" methods.
context
The context of the attribute. This indicates whether it's a class attribute or an object attribute.
The possible values are defined by the following constants:
Class::Meta::CLASS
Class::Meta::OBJECT
You can also use strings aliases to the above constants, although the constant values will actually
be stored in the Class::Meta::Attribute object, rather than the string. The supported strings are
"CLASS", and "OBJECT".
default
The default value for the attribute, if any. This may be either a literal value or a code reference
that will be executed to generate a default value.
override
If an attribute being added to a class has the same name as an attribute in a parent class,
Class::Meta will normally throw an exception. However, in some cases you might want to override an
attribute in a parent class to change its properties. In such a case, pass a true value to the
"override" parameter to override the attribute and avoid the exception.
add_method
$cm->add_method( name => 'wag' );
Creates and returns a Class::Meta::Method object that describes a method of the class. The supported
parameters are:
name
The name of the method. The name must consist of only alphanumeric characters or "_".
label
A label for the method. Generally used for displaying its name in a user interface. Optional.
desc
A description of the method. Possibly useful for displaying help text in a user interface. Optional.
view
The visibility of the method. See the description of the "view" parameter to "add_constructor" for a
description of its value. Class::Meta only enforces the "view" if the "code" parameter is used to
define the method body. Otherwise, it's up to the class implementation itself to do the job.
code
You can implicitly define the method in your class by passing a code reference via the "code"
parameter. Once "build()" is called, Kinetic::Meta::Method will install the method into the package
for which the Class::Meta object was defined, and with the name specified via the "name" parameter.
If the "view" is anything other than PUBLIC, it will be enforced.
context
The context of the method. This indicates whether it's a class method or an object method. See the
description of the "context" parameter to "add_attribute" for a description of its value.
caller
A code reference that calls the method. This code reference will be be used by the "call()" method of
Class::Meta::Method to execute the method on behalf of an object. Defaults to a code reference that
calls a method with the name provided by the "name" attribute on the class being defined.
args
A description of the arguments to the method. This can be anything you like, but I recommend
something like a string for a single argument, an array reference for a list of arguments, or a hash
reference for parameter arguments.
returns
A string describing the return value or values of the method.
Instance Methods
class
my $class = $cm->class;
Returns the instance of the Class::Meta::Class object that will be used to provide the introspection API
for the class being generated.
build
$cm->build;
Builds the class defined by the Class::Meta object, including the "my_class()" class method, and all
requisite constructors and accessors.
JUSTIFICATION
One might argue that there are already too many class automation and parameter validation modules on
CPAN. And one would be right. They range from simple accessor generators, such as Class::Accessor, to
simple parameter validators, such as Params::Validate, to more comprehensive systems, such as
Class::Contract and Class::Tangram. But, naturally, none of them could do exactly what I needed.
What I needed was an implementation of the "Facade" design pattern. Okay, this isn't a facade like the
"Gang of Four" meant it, but it is in the respect that it creates classes with a common API so that
objects of these classes can all be used identically, calling the same methods on each. This is done via
the implementation of an introspection API. So the process of creating classes with Class::Meta not only
creates attributes and accessors, but also creates objects that describe those classes. Using these
descriptive objects, client applications can determine what to do with objects of Class::Meta-generated
classes. This is particularly useful for user interface code.
TO DO
• Add support for an "accessor" parameter to "add_attribute()" that will be used for the accessor
instead of generating one.
• Make class attribute accessors behave as they do in Class::Data::Inheritable.
• Modify class attribute accessors so that they are thread safe. This will involve sharing the
attributes across threads, and locking them before changing their values. If they've also been made
to behave as they do in Class::Data::Inheritable, we'll have to figure out a way to make it so that
newly generated accessors for subclasses are shared between threads, too. This may not be easy.
SUPPORT
This module is stored in an open GitHub repository <http://github.com/theory/class-meta/>. Feel free to fork and contribute! Please file bug reports via GitHub Issues <http://github.com/theory/class-meta/issues/> or by sending mail to bug-Class-Meta@rt.cpan.org <mailto:bug-Class-Meta@rt.cpan.org>.
AUTHOR
David E. Wheeler <david@justatheory.com>
SEE ALSO
First of all, use Class::Meta::Express instead of Class::Meta to declare your classes. I hope I've made
that clear enough by now.
Other classes of interest within the Class::Meta distribution include:
Class::Meta::Class
Class::Meta::Constructor
Class::Meta::Attribute
Class::Meta::Method
Class::Meta::Type
Class::Meta::Types::Perl
Class::Meta::Types::String
Class::Meta::Types::Boolean
Class::Meta::Types::Numeric
For comparative purposes, you might also want to check out these fine modules:
Class::Accessor
Accessor and constructor automation.
Params::Validate
Parameter validation.
Class::Contract
Design by contract.
Class::Tangram
Accessor automation and data validation for Tangram applications.
Class::Maker
An ambitious yet under-documented module that also manages accessor and constructor generation, data
validation, and provides a reflection API. It also supports serialization.
Class::MOP
Stevan Little's application of Perl 6 meta classes to Perl 5.
Moose
"It's the new camel." Another extension of the Perl 5 object system, built on Class::MOP.
COPYRIGHT AND LICENSE
Copyright (c) 2002-2011, David E. Wheeler. Some Rights Reserved.
This module is free software; you can redistribute it and/or modify it under the same terms as Perl
itself.