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 => $father,
);
}
While this example works as-is, we can take it a lot further by adding genes into the mix. We'll add the two genes that control eye color, and use overloading to combine the genes from the parent to model the biology.
In this example we overload addition so we can write code like "$child = $mother + $father".
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.
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.
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.
use List::SomeUtils 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,
);
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_genome_overloadingsubtypesandcoercion.t.
License details are at: <http://creativecommons.org/licenses/by/3.0/>