NAME¶
Moose::Cookbook::Basics::Genome_OverloadingSubtypesAndCoercion - Operator
overloading, subtypes, and coercion
VERSION¶
version 2.1213
SYNOPSIS¶
package Human;
use Moose;
use Moose::Util::TypeConstraints;
subtype 'Sex'
=> as 'Str'
=> where { $_ =~ m{^[mf]$}s };
has 'sex' => ( is => 'ro', isa => 'Sex', required => 1 );
has 'mother' => ( is => 'ro', isa => 'Human' );
has 'father' => ( is => 'ro', isa => 'Human' );
use overload '+' => \&_overload_add, fallback => 1;
sub _overload_add {
my ( $one, $two ) = @_;
die('Only male and female humans may create children')
if ( $one->sex() eq $two->sex() );
my ( $mother, $father )
= ( $one->sex eq 'f' ? ( $one, $two ) : ( $two, $one ) );
my $sex = 'f';
$sex = 'm' if ( rand() >= 0.5 );
return Human->new(
sex => $sex,
mother => $mother,
father => $father,
);
}
DESCRIPTION¶
This Moose cookbook recipe shows how operator overloading, coercion, and
subtypes can be used to mimic the human reproductive system (well, the
selection of genes at least).
INTRODUCTION¶
Our "Human" class uses operator overloading to allow us to
"add" two humans together and produce a child. Our implementation
does require that the two objects be of opposite sex. Remember, we're talking
about biological reproduction, not marriage.
While this example works as-is, we can take it a lot further by adding genes
into the mix. We'll add the two genes that control eye color, and use
overloading to combine the genes from the parent to model the biology.
What is Operator Overloading?¶
Overloading is
not a Moose-specific feature. It's a general OO concept
that is implemented in Perl with the "overload" pragma. Overloading
lets objects do something sane when used with Perl's built in operators, like
addition ("+") or when used as a string.
In this example we overload addition so we can write code like "$child =
$mother + $father".
GENES¶
There are many genes which affect eye color, but there are two which are most
important,
gey and
bey2. We will start by making a class for
each gene.
Human::Gene::bey2¶
package Human::Gene::bey2;
use Moose;
use Moose::Util::TypeConstraints;
type 'bey2_color' => where { $_ =~ m{^(?:brown|blue)$} };
has 'color' => ( is => 'ro', isa => 'bey2_color' );
This class is trivial. We have a type constraint for the allowed colors, and a
"color" attribute.
Human::Gene::gey¶
package Human::Gene::gey;
use Moose;
use Moose::Util::TypeConstraints;
type 'gey_color' => where { $_ =~ m{^(?:green|blue)$} };
has 'color' => ( is => 'ro', isa => 'gey_color' );
This is nearly identical to the "Humane::Gene::bey2" class, except
that the
gey gene allows for different colors.
EYE COLOR¶
We could just give four attributes (two of each gene) to the "Human"
class, but this is a bit messy. Instead, we'll abstract the genes into a
container class, "Human::EyeColor". Then a "Human" can
have a single "eye_color" attribute.
package Human::EyeColor;
use Moose;
use Moose::Util::TypeConstraints;
coerce 'Human::Gene::bey2'
=> from 'Str'
=> via { Human::Gene::bey2->new( color => $_ ) };
coerce 'Human::Gene::gey'
=> from 'Str'
=> via { Human::Gene::gey->new( color => $_ ) };
has [qw( bey2_1 bey2_2 )] =>
( is => 'ro', isa => 'Human::Gene::bey2', coerce => 1 );
has [qw( gey_1 gey_2 )] =>
( is => 'ro', isa => 'Human::Gene::gey', coerce => 1 );
The eye color class has two of each type of gene. We've also created a coercion
for each class that coerces a string into a new object. Note that a coercion
will fail if it attempts to coerce a string like "indigo", because
that is not a valid color for either type of gene.
As an aside, you can see that we can define several identical attributes at once
by supplying an array reference of names as the first argument to
"has".
We also need a method to calculate the actual eye color that results from a set
of genes. The
bey2 brown gene is dominant over both blue and green. The
gey green gene is dominant over blue.
sub color {
my ($self) = @_;
return 'brown'
if ( $self->bey2_1->color() eq 'brown'
or $self->bey2_2->color() eq 'brown' );
return 'green'
if ( $self->gey_1->color() eq 'green'
or $self->gey_2->color() eq 'green' );
return 'blue';
}
We'd like to be able to treat a "Human::EyeColor" object as a string,
so we define a string overloading for the class:
use overload '""' => \&color, fallback => 1;
Finally, we need to define overloading for addition. That way we can add
together two "Human::EyeColor" objects and get a new one with a new
(genetically correct) eye color.
use overload '+' => \&_overload_add, fallback => 1;
sub _overload_add {
my ( $one, $two ) = @_;
my $one_bey2 = 'bey2_' . _rand2();
my $two_bey2 = 'bey2_' . _rand2();
my $one_gey = 'gey_' . _rand2();
my $two_gey = 'gey_' . _rand2();
return Human::EyeColor->new(
bey2_1 => $one->$one_bey2->color(),
bey2_2 => $two->$two_bey2->color(),
gey_1 => $one->$one_gey->color(),
gey_2 => $two->$two_gey->color(),
);
}
sub _rand2 {
return 1 + int( rand(2) );
}
When two eye color objects are added together, the "_overload_add()"
method will be passed two "Human::EyeColor" objects. These are the
left and right side operands for the "+" operator. This method
returns a new "Human::EyeColor" object.
ADDING EYE COLOR TO "Human"s¶
Our original "Human" class requires just a few changes to incorporate
our new "Human::EyeColor" class.
use List::MoreUtils qw( zip );
coerce 'Human::EyeColor'
=> from 'ArrayRef'
=> via { my @genes = qw( bey2_1 bey2_2 gey_1 gey_2 );
return Human::EyeColor->new( zip( @genes, @{$_} ) ); };
has 'eye_color' => (
is => 'ro',
isa => 'Human::EyeColor',
coerce => 1,
required => 1,
);
We also need to modify "_overload_add()" in the "Human"
class to account for eye color:
return Human->new(
sex => $sex,
eye_color => ( $one->eye_color() + $two->eye_color() ),
mother => $mother,
father => $father,
);
CONCLUSION¶
The three techniques we used, overloading, subtypes, and coercion, combine to
provide a powerful interface.
If you'd like to learn more about overloading, please read the documentation for
the overload pragma.
To see all the code we created together, take a look at
t/recipes/basics_recipe9.t.
NEXT STEPS¶
Had this been a real project we'd probably want:
- Better Randomization with Crypt::Random
- Characteristic Base Class
- Mutating Genes
- More Characteristics
- Artificial Life
LICENSE¶
This work is licensed under a Creative Commons Attribution 3.0 Unported License.
License details are at: <
http://creativecommons.org/licenses/by/3.0/>
AUTHORS¶
- •
- Stevan Little <stevan.little@iinteractive.com>
- •
- Dave Rolsky <autarch@urth.org>
- •
- Jesse Luehrs <doy@tozt.net>
- •
- Shawn M Moore <code@sartak.org>
- •
- XXXX XXX'XX (Yuval Kogman) <nothingmuch@woobling.org>
- •
- Karen Etheridge <ether@cpan.org>
- •
- Florian Ragwitz <rafl@debian.org>
- •
- Hans Dieter Pearcey <hdp@weftsoar.net>
- •
- Chris Prather <chris@prather.org>
- •
- Matt S Trout <mst@shadowcat.co.uk>
COPYRIGHT AND LICENSE¶
This software is copyright (c) 2006 by Infinity Interactive, Inc..
This is free software; you can redistribute it and/or modify it under the same
terms as the Perl 5 programming language system itself.