This is an expanded version of my "Refactoring with Roles" talk. People don't ask me how to use roles. They're easy to use. They ask me why to use roles. That's pretty easy to, so long as you understand what's wrong with inheritance. This talk covers that. A paper for this talk is at http://www.slideshare.net/Ovid/inheritance-versus-roles-1799996

1. OO Systems and Roles
Curtis “Ovid” Poe
Senior Software Engineer
Copyright 2009 by Curtis “Ovid” Poe.
This presentation is free and you may redistribute it and/or
modify it under the terms of the GNU Free Documentation
License.
Template Copyright 2009 by BBC.
Future Media & Technology  BBC MMIX

2. Not a Tutorial
• #ovidfail
• “How” is easy
• “Why” is not
Future Media & Technology  BBC MMIX

3. The Problem Domain
• 5,613 Brands
• 6,755 Series
• 386,943 Episodes
• 394,540 Versions
• 1,106,246 Broadcasts
• 1,701,309 On Demands
July 2, 2009
Future Media & Technology  BBC MMIX

4. A Brief History of Pain
•Simula 67
–Classes
–Polymorphism
–Encapsulation
–Inheritance
Future Media & Technology  BBC MMIX

5. Multiple Inheritance
•C++
•Eiffel
•Perl
•CLOS
Future Media & Technology  BBC MMIX

6. Single Inheritance
•C#
•Java
•BETA
•Ruby
Future Media & Technology  BBC MMIX

7. Handling Inheritance
•Liskov
•Strict equivalence
•Interfaces
•Mixins
•C3
Future Media & Technology  BBC MMIX

8. Four Decades of Pain
•Code smell
–In the language!
Future Media & Technology  BBC MMIX

9. B:: Object Hierarchy
Future Media & Technology  BBC MMIX

10. A Closer Look
Future Media & Technology  BBC MMIX

11. A Closer Look
• Multiple Inheritance
Future Media & Technology  BBC MMIX

12. B::PVIV Pseudo-Code
• B::PVIV Internals
bless => {
pv => 'three',
iv => 3,
} => 'B::PVIV';
Future Media & Technology  BBC MMIX

13. Printing Numbers?
• Perl
my $number = 3;
$number += 2;
print "I have $number apples";
• Java
int number = 3;
number += 2;
System.out.println(
"I have " + number + " apples"
);
Future Media & Technology  BBC MMIX

14. Class Details
• More pseudo-code
package B::PVIV;
use parent qw( B::PV B::IV );
sub B::PV::as_string { $_[0]->{pv} }
sub B::IV::as_string { $_[0]->{iv} }
• Printing:
print $pviv->as_string; # Str
print $pviv->B::IV::as_string; # Int
Future Media & Technology  BBC MMIX

15. Real World Pain
Future Media & Technology  BBC MMIX

16. Real World Pain
Future Media & Technology  BBC MMIX

17. Real World Pain
Future Media & Technology  BBC MMIX

18. Real World Pain
Future Media & Technology  BBC MMIX

19. Systems Grow
Future Media & Technology  BBC MMIX

20. The Problem
• Responsibility
–Wants larger classes
• Reuse
–Wants smaller classes
Future Media & Technology  BBC MMIX

21. Solution
Decouple!
Future Media & Technology  BBC MMIX

22. Solutions
• Interfaces
–Reimplementing
Future Media & Technology  BBC MMIX

23. Solutions
• Delegation
–Scaffolding
–Communication
Future Media & Technology  BBC MMIX

24. Solutions
• Mixins
–Ordering
Future Media & Technology  BBC MMIX

25. PracticalJoke
• Needs:
–explode()
–fuse()
Future Media & Technology  BBC MMIX

26. Shared Behavior
Method Description
✓ Bomb::fuse() Deterministic
Spouse::fuse() Non-deterministic
Bomb::explode() Lethal
✓ Spouse::explode() Wish it was lethal
Future Media & Technology  BBC MMIX

27. Ruby Mixins
module Bomb
def explode
puts "Bomb explode"
end
def fuse
puts "Bomb fuse"
end
end
module Spouse
def explode
puts "Spouse explode"
end
def fuse
puts "Spouse fuse"
end
end
Future Media & Technology  BBC MMIX

28. Ruby Mixins
class PracticalJoke
include Spouse
include Bomb
end
joke = PracticalJoke.new()
joke.fuse
joke.explode
Prints out:
Future Media & Technology  BBC MMIX

29. Ruby Mixins
class PracticalJoke
include Spouse
include Bomb
end
joke = PracticalJoke.new()
joke.fuse
joke.explode
Prints out:
Bomb fuse
Bomb explode
Future Media & Technology  BBC MMIX

30. Moose Roles
{
package Bomb;
use Moose::Role;
sub fuse { say "Bomb explode" }
sub explode { say "Bomb fuse" }
}
{
package Spouse;
use Moose::Role;
sub fuse { say "Spouse explode" }
sub explode { say "Spouse fuse" }
}
Future Media & Technology  BBC MMIX

31. Moose Roles
{
package PracticalJoke;
use Moose;
with qw(Bomb Spouse);
}
my $joke = PracticalJoke->new();
$joke->fuse();
$joke->explode();
Prints out:
Future Media & Technology  BBC MMIX

32. Moose Roles
{
package PracticalJoke;
use Moose;
with qw(Bomb Spouse);
}
my $joke = PracticalJoke->new();
$joke->fuse();
$joke->explode();
Prints out:
Due to a method name conflict in roles 'Bomb' and
'Spouse', the method 'fuse' must be implemented
or excluded by 'PracticalJoke'
Future Media & Technology  BBC MMIX

33. Moose Roles
{
package PracticalJoke;
use Moose;
with 'Bomb' => { excludes => 'explode' },
'Spouse' => { excludes => 'fuse' };
}
my $joke = PracticalJoke->new();
$joke->fuse();
$joke->explode();
Prints out:
Spouse fuse
Bomb explode
Future Media & Technology  BBC MMIX

34. Moose Roles
package PracticalJoke;
use Moose;
with 'Bomb' => { excludes => 'explode' },
'Spouse' => { excludes => 'fuse',
alias =>
{ fuse => 'random_fuse' }
};
And in your actual code:
$joke->fuse(14); # timed fuse
# or
$joke->random_fuse(); # who knows?
Future Media & Technology  BBC MMIX

35. Role Example
package Does::Serialize::YAML
use Moose::Role;
use YAML::Syck;
requires 'as_hash';
sub serialize {
my $self = shift;
return Dump($self->as_hash);
}
1;
And in your actual code:
package My::Object;
use Moose;
with 'Does::Serialize::YAML';
Future Media & Technology  BBC MMIX

36. Back To Work
Future Media & Technology  BBC MMIX

37. Work + Roles
Future Media & Technology  BBC MMIX

38. Work + Roles
package Country;
use Moose;
extends "My::ResultSource";
with qw(DoesStatic DoesAuditing);
Future Media & Technology  BBC MMIX

39. Old BBC ResultSet Classes
Future Media & Technology  BBC MMIX

40. New ResultSet Classes
Future Media & Technology  BBC MMIX

41. Work + Roles
package BBC::Programme::Episode;
use Moose;
extends 'BBC::ResultSet';
with qw(
DoesSearch::Broadcasts
DoesSearch::Tags
DoesSearch::Titles
DoesSearch::Promotions
DoesIdentifier::Universal
);
Future Media & Technology  BBC MMIX

42. Conclusion
• Increases comprehension
• Increases safety
• Decreases complexity
Future Media & Technology  BBC MMIX