Using Moose provides many immediate and obvious benefits, starting with the obviation of typing "use strict" and "use warnings" in your classes. The real power of Moose, however, rests in its extensibility. By subclassing Moose's metaclasses, you can augment and change Moose's behavior to suit your application's needs. This extensibility is powered by the meta-object protocol of Moose's foundation, Class::MOP. The motivating example for extending Moose will be the creation of a small web framework to empower a Twitter-alike. The focus will be creating meta-level roles so that metaclasses may select exactly which changes in behavior they wish to exhibit. Modules that will be used include Moose::Exporter (to define sugary keywords) and Moose::Util::MetaRole (to extend classes composably). Experience with using Moose to create regular classes is expected. Having some familiarity with roles will let you get more out of the talk. No experience with metaprogramming is required.

1. Extending Moose
for Applications
Shawn M Moore
Best Practical Solutions
1

2. Twitter!
@sartak Hey wait, what is "meta"?
2

3. (or IRC)
3

4. Extending Moose for
Applications
4

5. Domain-specific
Metaprogramming
5

6. Extending Moose for
Applications
Domain-specific
Metaprogramming
6

7. Extending Moose for
Applications
Domain-specific
Metaprogramming
7

8. Extending Moose for
Applications
Domain-specific
Metaprogramming
8

9. 9

10. Context-oriented Programming with
ContextL
Towards a Secure Programming Language:
An Access Control System for CommonLisp
Rhapsody: How CLOS simplifies System-in-
a-Package Design
10

11. Metaprogramming
11

12. Metaprogramming
$obj->can('method_name')
12

13. Metaprogramming
$obj->can('method_name')
$obj->isa('Class::Name')
13

14. Metaprogramming
$obj->can('method_name')
$obj->isa('Class::Name')
$obj->DOES('RoleName')
14

15. Metaprogramming
$obj->can('method_name')
$obj->isa('Class::Name')
$obj->DOES('RoleName')
eval "code"
15

16. Metaprogramming
my $code = setup();
$code .= important_stuff();
$code .= teardown();
eval $code;
16

17. Metaprogramming
__PACKAGE__->meta->make_immutable
17

18. Metaprogramming
__PACKAGE__->meta->make_immutable
18

19. Metaprogramming
my $meta = __PACKAGE__->meta;
$meta->make_immutable;
19

20. Metaprogramming
print $meta;
Moose::Meta::Class=HASH(0x966910)
20

21. Moose::Meta::Class
Attributes Methods
name new_object
version clone_object
attributes rebless_instance
methods subclasses
superclasses linearized_isa
roles add_attribute
attribute_metaclass has_method
method_metaclass get_all_method_names
constructor_name is_immutable
constructor_class calculate_all_roles
21

22. Metaprogramming
my $code = setup();
$code .= important_stuff();
$code .= teardown();
eval $code;
22

23. package Point;
use Moose;
has 'x' => (is => 'rw', isa => 'Int');
has 'y' => (is => 'rw', isa => 'Int');
sub clear { ... }
23

24. Point->meta
my $point = Point->meta;
24

25. Point->meta
my $point = Point->meta;
$point->name # ?
25

26. Point->meta
my $point = Point->meta;
$point->name # Point
26

27. Point->meta
my $point = Point->meta;
$point->name # Point
$point->get_attribute_list # ?
27

28. Point->meta
my $point = Point->meta;
$point->name # Point
$point->get_attribute_list # y, x
28

29. Point->meta
my $point = Point->meta;
$point->name # Point
$point->get_attribute_list # y, x
$point->has_method('clear') # ?
29

30. Point->meta
my $point = Point->meta;
$point->name # Point
$point->get_attribute_list # y, x
$point->has_method('clear') # 1
30

31. package Point3D;
use Moose;
extends 'Point';
has 'z' => (is => 'rw', isa => 'Int');
after clear => sub { ... };
31

32. Point3D->meta
my $point3d = Point3D->meta;
32

33. Point3D->meta
my $point3d = Point3D->meta;
$point3d->name # ?
33

34. Point3D->meta
my $point3d = Point3D->meta;
$point3d->name # Point3D
34

35. Point3D->meta
my $point3d = Point3D->meta;
$point3d->name # Point3D
$point3d->get_attribute_list # ?
35

36. Point3D->meta
my $point3d = Point3D->meta;
$point3d->name # Point3D
$point3d->get_attribute_list # z
36

37. Point3D->meta
my $point3d = Point3D->meta;
$point3d->name # Point3D
$point3d->get_attribute_list # z
$point3d->has_method('clear') # ?
37

38. Point3D->meta
my $point3d = Point3D->meta;
$point3d->name # Point3D
$point3d->get_attribute_list # z
$point3d->has_method('clear') # 1
38

39. package Point3D;
use Moose;
extends 'Point';
has 'z' => (is => 'rw', isa => 'Int');
after clear => sub { ... };
39

40. Finding Things
Local Global
has_attribute find_attribute_by_name
has_method find_method_by_name
get_attribute_list get_all_attributes
get_method_list get_all_methods
superclasses class_precedence_list
40

41. REST Interface
my $class = url2class($url);
my $meta = $class->meta;
for ($meta->get_all_attributes) {
my $name = $_->name;
my $tc = $_->type_constraint;
my $default = $_->default;
if ($_->is_required) { ... }
}
41

42. use Moose;
42

43. my $point = Moose::Meta::Class->create(
'Point',
);
43

44. my $point = Moose::Meta::Class->create(
'Point',
);
$point->superclasses('Moose::Object');
44

45. my $point = Moose::Meta::Class->create(
'Point',
);
$point->superclasses('Moose::Object');
$point->add_attribute(
'x',
is => 'ro',
isa => 'Int',
);
45

46. …
$point->add_attribute(
'y',
is => 'ro',
isa => 'Int',
);
46

47. …
$point->add_attribute(
'y',
is => 'ro',
isa => 'Int',
);
$point->add_method(clear => sub {
...
});
47

48. Ecosystem
Classes
Attributes
Methods
Roles
Type Constraint
Type Coercion
48

49. Ecosystem
Classes Moose::Meta::Class
Attributes
Methods
Roles
Type Constraint
Type Coercion
49

50. Ecosystem
Classes Moose::Meta::Class
Attributes Moose::Meta::Attribute
Methods Moose::Meta::Method
Roles Moose::Meta::Role
Type Constraint Moose::Meta::TypeConstraint
Type Coercion Moose::Meta::TypeCoercion
50

51. Ecosystem
my $x =
Point->meta->get_attribute('x')
$x->name #x
$x->get_read_method # x
$x->type_constraint # Int
51

52. Ecosystem
my $clear =
Point->meta->get_method('clear')
$clear->name # clear
$clear->package_name # Point
52

53. Extending
1. Extend a metaclass
2. Use it

54. Extending
Class that counts its instances

55. package HasCounter;
use Moose::Role;
has count => (
is => 'rw',
isa => 'Int',
default => 0,
);
sub increment {
my $self = shift;
$self->count(
$self->count + 1
);
}

56. package CountInstances;
use Moose::Role;
with 'HasCounter';
after new_object => sub {
my $self = shift;
$self->increment;
};

57. package Point;
use Moose -traits => [
'CountInstances',
];
has x => (
is => 'ro',
isa => 'Int',
);
...

58. Point->meta->count # 0
Point->new
Point->meta->count # 1
Point->new for 1 .. 5
Point->meta->count # 6

59. Base Meta
Point->new
Point->meta->new_object
after new_object
Point->meta->increment

60. package Line;
use Moose;
has start => (
is => 'ro',
isa => 'Point',
);
has end => (
is => 'ro',
isa => 'Point',
);

61. Base Meta
Line->new
Line->meta->new_object

62. package HasCounter;
use Moose::Role;
use MooseX::AttributeHelpers;
has count => (
traits => ['Counter'],
provides => {
inc => 'increment',
},
);

63. package FieldType;
use Moose::Role;
use Moose::Util::TypeConstraints;
has render_as => (
is => 'ro',
isa => (enum [
'text',
'textarea',
'password',
...,
]),
);

64. package User;
use Moose;
has name => (
traits => ['FieldType'],
is => 'rw',
isa => 'Str',
render_as => 'text',
);

65. ...
has password => (
traits => ['FieldType'],
is => 'rw',
isa => 'Str',
render_as => 'password',
);

66. ...
has biography => (
traits => ['FieldType'],
is => 'rw',
isa => 'Str',
render_as => 'textarea',
);

67. traits => ['FieldType'],
render_as => 'text',
traits => ['FieldType'],
render_as => 'password',
traits => ['FieldType'],
render_as => 'textarea',

68. traits => ['FieldType'],
render_as => 'text',
traits => ['FieldType'],
render_as => 'password',
traits => ['FieldType'],
render_as => 'textarea',

69. Moose::Exporter
Moose::Util::MetaRole

70. Moose::Exporter
Moose::Util::MetaRole

71. Moose::Exporter
Moose::Util::MetaRole

72. package MyWeb::OO;
use Moose ();
use Moose::Exporter;
use Moose::Util::MetaRole;
use FieldType;
Moose::Exporter->setup_import_methods(
also => 'Moose',
);
sub init_meta {
...
}

73. my $class = shift;
my %options = @_;
Moose->init_meta(%options);
Moose::Util::MetaRole::apply_metaclass_roles(
for_class => $options{for_class},
attribute_metaclass_roles => ['FieldType'],
);
return $options{for_class}->meta;

74. package User;
use MyWeb::OO;
has name => (
is => 'rw',
isa => 'Str',
field_type => 'text',
);
...

75. package User;
use MyWeb::OO;
use MyWeb::OO::Persistent;
use MyWeb::OO::RESTful;
use MyWeb::OO::IncludeInAdminUI;
use MyWeb::OO::SpamTarget;
database_name 'person'; # legacy
has email => (
is => 'rw',
isa => 'Str',
field_type => 'text',
spam_this => 1,
admin_editable => 0,
primary_key => 1,
);

76. Immutability

77. sub _initialize_body {
my $self = shift;
my $source = 'sub {';
$source .= "n" . 'my $class = shift;';
$source .= "n" . 'return $class->Moose::Object::new(@_)';
$source .= "n if $class ne '" . $self->associated_metaclass->name
. "';n";
$source .= $self->_generate_params('$params', '$class');
$source .= $self->_generate_instance('$instance', '$class');
$source .= $self->_generate_slot_initializers;
$source .= $self->_generate_triggers();
$source .= ";n" . $self->_generate_BUILDALL();
$source .= ";nreturn $instance";
$source .= ";n" . '}';
...

78. KiokuDB
Fey::ORM

79. KiokuDB
Fey::ORM
ContextL (CLOS)

80. ROLES!

82. Thank you!