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Edit File: Comparable_CodeReuse.pod

package Moose::Cookbook::Roles::Comparable_CodeReuse;

# ABSTRACT: Using roles for code reuse

__END__

=pod

=head1 NAME

Moose::Cookbook::Roles::Comparable_CodeReuse - Using roles for code reuse

=head1 VERSION

version 2.1005

=head1 SYNOPSIS

package Eq;
  use Moose::Role;

requires 'equal_to';

sub not_equal_to {
      my ( $self, $other ) = @_;
      not $self->equal_to($other);
  }

package Comparable;
  use Moose::Role;

with 'Eq';

requires 'compare';

sub equal_to {
      my ( $self, $other ) = @_;
      $self->compare($other) == 0;
  }

sub greater_than {
      my ( $self, $other ) = @_;
      $self->compare($other) == 1;
  }

sub less_than {
      my ( $self, $other ) = @_;
      $self->compare($other) == -1;
  }

sub greater_than_or_equal_to {
      my ( $self, $other ) = @_;
      $self->greater_than($other) || $self->equal_to($other);
  }

sub less_than_or_equal_to {
      my ( $self, $other ) = @_;
      $self->less_than($other) || $self->equal_to($other);
  }

package Printable;
  use Moose::Role;

requires 'to_string';

package US::Currency;
  use Moose;

with 'Comparable', 'Printable';

has 'amount' => ( is => 'rw', isa => 'Num', default => 0 );

sub compare {
      my ( $self, $other ) = @_;
      $self->amount <=> $other->amount;
  }

sub to_string {
      my $self = shift;
      sprintf '$%0.2f USD' => $self->amount;
  }

=head1 DESCRIPTION

Roles have two primary purposes: as interfaces, and as a means of code
reuse. This recipe demonstrates the latter, with roles that define
comparison and display code for objects.

Let's start with C<Eq>. First, note that we've replaced C<use Moose>
with C<use Moose::Role>. We also have a new sugar function, C<requires>:

requires 'equal_to';

This says that any class which consumes this role must provide an
C<equal_to> method. It can provide this method directly, or by
consuming some other role.

The C<Eq> role defines its C<not_equal_to> method in terms of the
required C<equal_to> method. This lets us minimize the methods that
consuming classes must provide.

The next role, C<Comparable>, builds on the C<Eq> role. We include
C<Eq> in C<Comparable> using C<with>, another new sugar function:

with 'Eq';

The C<with> function takes a list of roles to consume. In our example,
the C<Comparable> role provides the C<equal_to> method required by
C<Eq>. However, it could opt not to, in which case a class that
consumed C<Comparable> would have to provide its own C<equal_to>. In
other words, a role can consume another role I<without> providing any
required methods.

The C<Comparable> role requires a method,  C<compare>:

requires 'compare';

The C<Comparable> role also provides a number of other methods, all of
which ultimately rely on C<compare>.

sub equal_to {
      my ( $self, $other ) = @_;
      $self->compare($other) == 0;
  }

sub greater_than {
      my ( $self, $other ) = @_;
      $self->compare($other) == 1;
  }

sub less_than {
      my ( $self, $other ) = @_;
      $self->compare($other) == -1;
  }

sub greater_than_or_equal_to {
      my ( $self, $other ) = @_;
      $self->greater_than($other) || $self->equal_to($other);
  }

sub less_than_or_equal_to {
      my ( $self, $other ) = @_;
      $self->less_than($other) || $self->equal_to($other);
  }

Finally, we define the C<Printable> role. This role exists solely to
provide an interface. It has no methods, just a list of required methods.
In this case, it just requires a C<to_string> method.

An interface role is useful because it defines both a method and a
I<name>. We know that any class which does this role has a
C<to_string> method, but we can also assume that this method has the
semantics we want. Presumably, in real code we would define those
semantics in the documentation for the C<Printable> role. (1)

Finally, we have the C<US::Currency> class which consumes both the
C<Comparable> and C<Printable> roles.

with 'Comparable', 'Printable';

It also defines a regular Moose attribute, C<amount>:

has 'amount' => ( is => 'rw', isa => 'Num', default => 0 );

Finally we see the implementation of the methods required by our
roles. We have a C<compare> method:

sub compare {
      my ( $self, $other ) = @_;
      $self->amount <=> $other->amount;
  }

By consuming the C<Comparable> role and defining this method, we gain
the following methods for free: C<equal_to>, C<greater_than>,
C<less_than>, C<greater_than_or_equal_to> and
C<less_than_or_equal_to>.

Then we have our C<to_string> method:

sub to_string {
      my $self = shift;
      sprintf '$%0.2f USD' => $self->amount;
  }

=head1 CONCLUSION

Roles can be very powerful. They are a great way of encapsulating
reusable behavior, as well as communicating (semantic and interface)
information about the methods our classes provide.

=head1 FOOTNOTES

=over 4

=item (1)

Consider two classes, C<Runner> and C<Process>, both of which define a
C<run> method. If we just require that an object implements a C<run>
method, we still aren't saying anything about what that method
I<actually does>. If we require an object that implements the
C<Executable> role, we're saying something about semantics.

=back

=begin testing

ok( US::Currency->does('Comparable'), '... US::Currency does Comparable' );
ok( US::Currency->does('Eq'),         '... US::Currency does Eq' );
ok( US::Currency->does('Printable'),  '... US::Currency does Printable' );

my $hundred = US::Currency->new( amount => 100.00 );
isa_ok( $hundred, 'US::Currency' );

ok( $hundred->DOES("US::Currency"), "UNIVERSAL::DOES for class" );
ok( $hundred->DOES("Comparable"),   "UNIVERSAL::DOES for role" );

can_ok( $hundred, 'amount' );
is( $hundred->amount, 100, '... got the right amount' );

can_ok( $hundred, 'to_string' );
is( $hundred->to_string, '$100.00 USD',
    '... got the right stringified value' );

ok( $hundred->does('Comparable'), '... US::Currency does Comparable' );
ok( $hundred->does('Eq'),         '... US::Currency does Eq' );
ok( $hundred->does('Printable'),  '... US::Currency does Printable' );

my $fifty = US::Currency->new( amount => 50.00 );
isa_ok( $fifty, 'US::Currency' );

can_ok( $fifty, 'amount' );
is( $fifty->amount, 50, '... got the right amount' );

can_ok( $fifty, 'to_string' );
is( $fifty->to_string, '$50.00 USD', '... got the right stringified value' );

ok( $hundred->greater_than($fifty),             '... 100 gt 50' );
ok( $hundred->greater_than_or_equal_to($fifty), '... 100 ge 50' );
ok( !$hundred->less_than($fifty),               '... !100 lt 50' );
ok( !$hundred->less_than_or_equal_to($fifty),   '... !100 le 50' );
ok( !$hundred->equal_to($fifty),                '... !100 eq 50' );
ok( $hundred->not_equal_to($fifty),             '... 100 ne 50' );

ok( !$fifty->greater_than($hundred),             '... !50 gt 100' );
ok( !$fifty->greater_than_or_equal_to($hundred), '... !50 ge 100' );
ok( $fifty->less_than($hundred),                 '... 50 lt 100' );
ok( $fifty->less_than_or_equal_to($hundred),     '... 50 le 100' );
ok( !$fifty->equal_to($hundred),                 '... !50 eq 100' );
ok( $fifty->not_equal_to($hundred),              '... 50 ne 100' );

ok( !$fifty->greater_than($fifty),            '... !50 gt 50' );
ok( $fifty->greater_than_or_equal_to($fifty), '... !50 ge 50' );
ok( !$fifty->less_than($fifty),               '... 50 lt 50' );
ok( $fifty->less_than_or_equal_to($fifty),    '... 50 le 50' );
ok( $fifty->equal_to($fifty),                 '... 50 eq 50' );
ok( !$fifty->not_equal_to($fifty),            '... !50 ne 50' );

## ... check some meta-stuff

# Eq

my $eq_meta = Eq->meta;
isa_ok( $eq_meta, 'Moose::Meta::Role' );

ok( $eq_meta->has_method('not_equal_to'), '... Eq has_method not_equal_to' );
ok( $eq_meta->requires_method('equal_to'),
    '... Eq requires_method not_equal_to' );

# Comparable

my $comparable_meta = Comparable->meta;
isa_ok( $comparable_meta, 'Moose::Meta::Role' );

ok( $comparable_meta->does_role('Eq'), '... Comparable does Eq' );

foreach my $method_name (
    qw(
    equal_to not_equal_to
    greater_than greater_than_or_equal_to
    less_than less_than_or_equal_to
    )
    ) {
    ok( $comparable_meta->has_method($method_name),
        '... Comparable has_method ' . $method_name );
}

ok( $comparable_meta->requires_method('compare'),
    '... Comparable requires_method compare' );

# Printable

my $printable_meta = Printable->meta;
isa_ok( $printable_meta, 'Moose::Meta::Role' );

ok( $printable_meta->requires_method('to_string'),
    '... Printable requires_method to_string' );

# US::Currency

my $currency_meta = US::Currency->meta;
isa_ok( $currency_meta, 'Moose::Meta::Class' );

ok( $currency_meta->does_role('Comparable'),
    '... US::Currency does Comparable' );
ok( $currency_meta->does_role('Eq'), '... US::Currency does Eq' );
ok( $currency_meta->does_role('Printable'),
    '... US::Currency does Printable' );

foreach my $method_name (
    qw(
    amount
    equal_to not_equal_to
    compare
    greater_than greater_than_or_equal_to
    less_than less_than_or_equal_to
    to_string
    )
    ) {
    ok( $currency_meta->has_method($method_name),
        '... US::Currency has_method ' . $method_name );
}

=end testing

=head1 AUTHOR

Moose is maintained by the Moose Cabal, along with the help of many contributors. See L<Moose/CABAL> and L<Moose/CONTRIBUTORS> for details.

=head1 COPYRIGHT AND LICENSE

This is free software; you can redistribute it and/or modify it under
the same terms as the Perl 5 programming language system itself.

=cut