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.
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Not a Tutorial
• #ovidfail
• “How” is easy
• “Why” is not
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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
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A Brief History of Pain
•Simula 67
–Classes
–Polymorphism
–Encapsulation
–Inheritance
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Multiple Inheritance
•C++
•Eiffel
•Perl
•CLOS
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Single Inheritance
•C#
•Java
•BETA
•Ruby
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Handling Inheritance
•Liskov
•Strict equivalence
•Interfaces
•Mixins
•C3
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Four Decades of Pain
•Code smell
–In the language!
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B:: Object Hierarchy
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A Closer Look
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A Closer Look
• Multiple Inheritance
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B::PVIV Pseudo-Code
• B::PVIV Internals
bless => {
pv => 'three',
iv => 3,
} => 'B::PVIV';
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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"
);
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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
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Real World Pain
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Real World Pain
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Real World Pain
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Real World Pain
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Systems Grow
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The Problem
• Responsibility
–Wants larger classes
• Reuse
–Wants smaller classes
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Solution
Decouple!
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Solutions
• Interfaces
–Reimplementing
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Solutions
• Delegation
–Scaffolding
–Communication
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Solutions
• Mixins
–Ordering
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PracticalJoke
• Needs:
–explode()
–fuse()
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Shared Behavior
Method Description
✓ Bomb::fuse() Deterministic
Spouse::fuse() Non-deterministic
Bomb::explode() Lethal
✓ Spouse::explode() Wish it was lethal
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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
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Ruby Mixins
class PracticalJoke
include Spouse
include Bomb
end
joke = PracticalJoke.new()
joke.fuse
joke.explode
Prints out:
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Ruby Mixins
class PracticalJoke
include Spouse
include Bomb
end
joke = PracticalJoke.new()
joke.fuse
joke.explode
Prints out:
Bomb fuse
Bomb explode
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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" }
}
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Moose Roles
{
package PracticalJoke;
use Moose;
with qw(Bomb Spouse);
}
my $joke = PracticalJoke->new();
$joke->fuse();
$joke->explode();
Prints out:
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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'
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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
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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?
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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';
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Back To Work
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Work + Roles
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Work + Roles
package Country;
use Moose;
extends "My::ResultSource";
with qw(DoesStatic DoesAuditing);
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Old BBC ResultSet Classes
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New ResultSet Classes
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Work + Roles
package BBC::Programme::Episode;
use Moose;
extends 'BBC::ResultSet';
with qw(
DoesSearch::Broadcasts
DoesSearch::Tags
DoesSearch::Titles
DoesSearch::Promotions
DoesIdentifier::Universal
);
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Conclusion
• Increases comprehension
• Increases safety
• Decreases complexity
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