Moose::CookboMoose::Cookbook::Basics::Genome_OverloadingSubtypesAndCoercion(3)NAMEMoose::Cookbook::Basics::Genome_OverloadingSubtypesAndCoercion-
Operator overloading, subtypes, and coercion
VERSION
version 2.1005
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/>
AUTHOR
Moose is maintained by the Moose Cabal, along with the help of many
contributors. See "CABAL" in Moose and "CONTRIBUTORS" in Moose for
details.
COPYRIGHT AND LICENSE
This software is copyright (c) 2013 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.
perl v5.18.1 Moose::Cookbook::Basics::Genome_OverloadingSubtypesAndCoercion(3)
