Too few projects demand good API design as a critical goal. A clean and extensible API will pay for itself many times over in fostering a community of plugins. We certainly cannot anticipate the ways in which our users will bend our modules, but designing an extensible system alleviates the pain. There are many lessons to be learned from Moose, HTTP::Engine and IM::Engine, Dist::Zilla, KiokuDB, Fey, and TAEB. The most important lesson is to decouple the core functionality from the "fluff" such as sugar and middleware. This forces you to have a solid API that ordinary users can extend. This also lets users write their own sugar and middleware. In a tightly-coupled system, there is little hope for extensibility. In this talk, you will learn how to make very productive use of Moose's roles to form the foundation of a pluggable system. Roles provide excellent means of code reuse and safe composition. I will also demonstrate how to use Sub::Exporter to construct a more useful and flexible sugar layer.

1. API Design
Shawn M Moore
Best Practical Solutions
http://sartak.org

2. Google API

3. CC-BY-SA Yuval Kogman,
2006

4. CC-BY-SA Yuval Kogman,
2006

5. CC-BY-SA Hans Dieter
Pearcey, 2009

6. Moose
Path::Dispatcher
HTTP::Engine
Dist::Zilla
IM::Engine
API

7. CC-BY-SA-NC Will Spaetzel,
2005

8. THE SECRET

9. THE
WRITE
TESTS

10. WRITE TESTS
WRITEWRITE
TESTSTESTS
WRITEWRITE
TESTSTESTS

11. DO THIS
WRITE WRITE RITE WRITE WRITE
W
WRITE
WRITE
WRI
WRITE
WRI TE
WRITE TE
WRITE WRIT E WR ITE

12. Test!
use base 'Class::Accessor::Fast';
__PACKAGE__->mk_ro_accessors('birthday');
use Moose;
has birthday => (is => 'ro');
API

13. Test!

14. Test!

15. Test!

16. Test!

17. Moose
package Point;
use Moose;
has x => (
is => 'ro',
isa => 'Num',
);
Moose

18. Metaobject Protocol
Class::MOP
Moose Class::MOP

19. Metaobject Protocol
has cache => (
is => 'ro',
);

20. Metaobject Protocol
has cache => (
is => 'ro',
);
Moose::Meta::Attribute
has Moose::Meta::Attribute

21. Metaobject Protocol
has cache => (
is => 'ro',
);
Moose::Meta::Method::Accessor
cache

22. Metaobject Protocol
class PersistentAttr
extends Moose::Meta::Attribute {
…
}
has cache => (
metaclass => 'PersistentAttr',
is => 'ro',
);
Moose

23. Metaobject Protocol
role PersistentAttr {
…
}
has cache => (
traits => ['PersistentAttr'],
is => 'ro',
);

24. Moose
X
MooseX MOP

25. Sugar Layer
Moose

26. Sugar Layer
my $class = get_class();

27. Sugar Layer
my $class = get_class();
$class->has(
birthday => (
is => 'ro',
)
);
has

28. Sugar Layer
my $class = get_class();
no strict 'refs';
*{$class.'::has'}->(
birthday => (
is => 'ro',
)
);
has

29. Sugar Layer
my $class = get_class();
no strict 'refs';
*{$class.'::has'}->(
birthday => (
is => 'ro',
)
);

30. Sugar Layer
Class::MOP::Class
->initialize($class)
->add_attribute(
$name, %options);
has add_attribute

31. Sugar Layer
my $class = get_class();
$class->meta->add_attribute(
birthday => (
is => 'ro',
)
);

32. Sugar Layer
use MooseX::Declare;
class Point3D extends Point {
has z => (…);
after clear {
$self->z(0);
}
}

33. Path::Dispatcher
use Path::Dispatcher::Declarative -base;
on ['wield', qr/^w+$/] => sub {
wield_weapon($2);
}
under display => sub {
on inventory => sub { show_inventory };
on score => sub { show_score };
};
YourDispatcher->run('display score');
Jifty::Dispatcher Prophet

34. Path::Dispatcher::Declar
use Sub::Exporter -setup => {
exports => [
on => &build_on,
under => &build_under,
…,
],
};
Sub::Exporter

35. Path::Dispatcher::Declar
use Sub::Exporter -setup => {
exports => [
on => &build_on,
under => &build_under,
…,
],
};

36. Path::Dispatcher::Builder
Robert Krimen
"grink"
Robert Krimen

37. Path::Dispatcher::Builder
return {
on => sub { $builder->on(@_) },
under => sub { $builder->under(@_) },
…,
};

38. grink++
OO

39. HTTP::Engine
HTTP::Engine->new(
interface => {
module => 'ServerSimple',
args => { … },
request_handler => sub {
…
},
},
)->run;
HTTP::Engine

40. HTTP::Engine
HTTP::Engine->new(
interface => {
module => 'ModPerl',
args => { … },
request_handler => sub {
…
},
},
)->run;
mod_perl

41. HTTP::Engine
HTTP::Engine->new(
interface => {
module => 'FCGI',
args => { … },
request_handler => sub {
…
},
},
)->run;
FastCGI

42. HTTP::Engine
request_handler => sub {
my $request = shift;
return $response;
}
HTTP::Engine IO

43. HTTP::Engine
request_handler => sub {
my $request = shift;
return $response;
}
HTTP::Engine OK

44. HTTP::Engine
request_handler => sub {
my $request = shift;
return $response;
}

45. HTTP::Engine++
1

46. CC-BY-SA Hans Dieter
Pearcey, 2009

47. Dist::Zilla
AllFiles
ExtraTests
InstallDirs
License
MakeMaker
Manifest
ManifestSkip
MetaYAML
PkgVersion
PodTests
PodVersion
PruneCruft
Readme
UploadToCPAN

48. Dist::Zilla
AllFiles
ExtraTests
InstallDirs
License $_->gather_files
MakeMaker for
Manifest $self->plugins_with(
ManifestSkip
-FileGatherer
MetaYAML
PkgVersion );
PodTests
PodVersion
PruneCruft
Readme
UploadToCPAN
Dist::Zilla

49. Dist::Zilla
AllFiles
ExtraTests
InstallDirs
License $_->gather_files
MakeMaker for
Manifest $self->plugins_with(
ManifestSkip
-FileGatherer
MetaYAML
PkgVersion );
PodTests
PodVersion
PruneCruft
Readme
UploadToCPAN
plugins_with

50. Dist::Zilla
AllFiles
ExtraTests
InstallDirs
License $_->gather_files
MakeMaker for
Manifest $self->plugins_with(
ManifestSkip
-FileGatherer
MetaYAML
PkgVersion );
PodTests
PodVersion
PruneCruft
Readme
UploadToCPAN

51. Dist::Zilla
AllFiles
ExtraTests
InstallDirs
License $_->gather_files
MakeMaker for
Manifest $self->plugins_with(
ManifestSkip
-FileGatherer
MetaYAML
PkgVersion );
PodTests
PodVersion
PruneCruft
Readme
UploadToCPAN
gather_files

52. Dist::Zilla
AllFiles
ExtraTests
InstallDirs
License $_->munge_files
MakeMaker for
Manifest $self->plugins_with(
ManifestSkip
-FileMunger
MetaYAML
PkgVersion );
PodTests
PodVersion
PruneCruft
Readme
UploadToCPAN
Dist::Zilla

53. Request Tracker
User/Prefs.html
$m->callback(
CallbackName => 'FormEnd',
UserObj => $UserObj,
…,
);
RT

54. Request Tracker
User/Prefs.html
$m->callback(
CallbackName => 'FormEnd',
UserObj => $UserObj,
…,
);

55. Request Tracker
User/Prefs.html
$m->callback(
CallbackName => 'FormEnd',
UserObj => $UserObj,
…,
);
FormEnd

56. Request Tracker
User/Prefs.html
$m->callback(
CallbackName => 'FormEnd',
UserObj => $UserObj,
…,
);

57. Request Tracker
User/Prefs.html
$m->callback(
CallbackName => 'FormEnd',
UserObj => $UserObj,
…,
);

58. Request Tracker
commit 4c05a6835eef112701ac58dfd1b133e220059d4f
Author: Jesse Vincent <jesse@bestpractical.com>
Date: Fri Dec 27 18:50:06 2002 -0500
Attempting mason callouts
Ticket/Update.html
<& /Elements/Callback,
Name => 'BeforeTextarea',
%ARGS
&>
7

59. Dist::Zilla
Choice
ModuleBuild or MakeMaker
MetaYAML or MetaJSON

60. Dist::Zilla
Extensibility
Dist::Zilla::Plugin::CriticTests
Dist::Zilla::Plugin::Repository
Dist::Zilla::Plugin::PerlTidy

61. Dist::Zilla
Extensibility
Dist::Zilla::Plugin::CriticTests
InlineFiles
Dist::Zilla::Plugin::Repository
MetaProvider
Dist::Zilla::Plugin::PerlTidy
FileMunger

62. Dist::Zilla
Extensibility
Dist::Zilla::Plugin::BPS::Secret

63. CC-BY-SA Hans Dieter
Pearcey, 2009

64. IM::Engine
incoming_callback => sub {
my $incoming = shift;
my $message = $incoming->plaintext;
$message =~ tr[a-zA-Z][n-za-mN-ZA-M];
return $incoming->reply($message);
}
IM::Engine

65. IM::Engine
$self->plugin_collect(
role => 'ExtendsObject::User',
method => 'traits',
);
plugin_collect

66. IM::Engine
$self->plugin_collect(
role => 'ExtendsObject::User',
method => 'traits',
);

67. IM::Engine
$self->plugin_collect(
role => 'ExtendsObject::User',
method => 'traits',
);
traits

68. IM::Engine
my @all_traits =
$self->plugin_collect(
role => 'ExtendsObject::User',
method => 'traits',
);

69. method plugin_collect {
my @items;
$self->each_plugin(
callback => sub {
push @items,
shift->$method
},
);
return @items;
}
plugin_collect

70. IM::Engine
method new_with_plugins {
my %args = (
$self->plugin_collect(
role => …,
method => 'ctor_args',
),
@_,
);
$self->new(%args);
}

71. IM::Engine
push @{ $args{traits} },
$self->plugin_collect(
role => …,
method => 'traits',
);
$self->new_with_traits(%args);

72. MooseX::Traits
$object = Class->new_with_traits(
traits => ['Counter'],
);
$other = Class->new;
$object->counter; # 0
$other->counter; # Can't locate...
new_with_traits MooseX::Traits

73. MooseX::Traits
$object = Class->new_with_traits(
traits => ['Counter'],
);
$other = Class->new;
$object->counter; # 0
$other->counter; # Can't locate...

74. Role = Trait
Moose Role Trait

75. Moose
Path::Dispatcher
HTTP::Engine
Dist::Zilla
IM::Engine

76. Moose
Extensibility
Separation of sugar
Dist::Zilla
IM::Engine
Moose

77. Moose
Path::Dispatcher
OO sugar layer
Dist::Zilla
IM::Engine
OO

78. Moose
Path::Dispatcher
HTTP::Engine
Omit inconsequential details
IM::Engine

79. Moose
Path::Dispatcher
HTTP::Engine
Dist::Zilla
Explicit pluggability

80. Moose
Path::Dispatcher
HTTP::Engine
Dist::Zilla
IM::Engine
Extreme pluggability
DRY
IM::Engine DRY

81. WRITEMoose
Path::Dispatcher
HTTP::Engine
TESTS!!!
Dist::Zilla
IM::Engine

82. Moose
Path::Dispatcher
HTTP::Engine
Dist::Zilla
IM::Engine

83. Thanks to my
Kenichi Ishigaki
83