
=pod

=head1 NAME

Moose::Cookbook::Basics::Recipe9 - Operator overloading, subtypes, and coercion

=head1 SYNOPSIS

  package Human;

  use Moose;
  use Moose::Util::TypeConstraints;

  subtype 'Gender'
      => as 'Str'
      => where { $_ =~ m{^[mf]$}s };

  has 'gender' => ( is => 'ro', isa => 'Gender', 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->gender() eq $two->gender() );

      my ( $mother, $father )
          = ( $one->gender eq 'f' ? ( $one, $two ) : ( $two, $one ) );

      my $gender = 'f';
      $gender = 'm' if ( rand() >= 0.5 );

      return Human->new(
          gender => $gender,
          mother => $mother,
          father => $father,
      );
  }

=head1 DESCRIPTION

This Moose cookbook recipe shows how operator overloading, coercion,
and sub types can be used to mimic the human reproductive system
(well, the selection of genes at least).

=head1 INTRODUCTION

Our C<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 genders. 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.

=head2 What is Operator Overloading?

Overloading is I<not> a Moose-specific feature. It's a general OO
concept that is implemented in Perl with the C<overload>
pragma. Overloading lets objects do something sane when used with
Perl's built in operators, like addition (C<+>) or when used as a
string.

In this example we overload addition so we can write code like
C<$child = $mother + $father>.

=head1 GENES

There are many genes which affect eye color, but there are two which
are most important, I<gey> and I<bey2>. We will start by making a
class for each gene.

=head2 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 C<color> attribute.

=head2 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 C<Humane::Gene::bey2> class, except
that the I<gey> gene allows for different colors.

=head1 EYE COLOR

We could just give add four attributes (two of each gene) to the
C<Human> class, but this is a bit messy. Instead, we'll abstract the
genes into a container class, C<Human::EyeColor>. Then a C<Human> can
have a single C<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 supply an array reference of names as the first
argument to C<has>.

We also need a method to calculate the actual eye color that results
from a set of genes. The I<bey2> brown gene is dominant over both blue
and green. The I<gey> green gene 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 C<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 to C<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 C<_overload_add()>
method will be passed two C<Human::EyeColor> objects. These are the
left and right side operands for the C<+> operator. This method
returns a new C<Human::EyeColor> object.

=head1 ADDING EYE COLOR TO C<Human>s

Our original C<Human> class requires just a few changes to incorporate
our new C<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 C<_overload_add()> in the C<Human> class to
account for eye color:

  return Human->new(
      gender    => $gender,
      eye_color => ( $one->eye_color() + $two->eye_color() ),
      mother    => $mother,
      father    => $father,
  );

=head1 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 L<overload> pragma.

To see all the code we created together, take a look at
F<t/000_recipes/basics/010_genes.t>.

=head1 NEXT STEPS

Had this been a real project we'd probably want:

=over 4

=item Better Randomization with Crypt::Random

=item Characteristic Base Class

=item Mutating Genes

=item More Characteristics

=item Artificial Life

=back

=head1 AUTHORS

Aran Clary Deltac <bluefeet@cpan.org>

Dave Rolsky E<lt>autarch@urth.orgE<gt>

=head1 LICENSE

This work is licensed under a Creative Commons Attribution 3.0 Unported License.

License details are at: L<http://creativecommons.org/licenses/by/3.0/>

=cut

