Rename Basics::Recipe1 to Basics::Point_AttributesAndSubclassing

[gitmo/Moose.git] / lib / Moose / Cookbook / Basics / BankAccount_MethodModifiersAndSubclassing.pod

package Moose::Cookbook::Basics::BankAccount_MethodModifiersAndSubclassing;

# ABSTRACT: Demonstrates the use of method modifiers in a subclass

__END__


=pod

=head1 SYNOPSIS

  package BankAccount;
  use Moose;

  has 'balance' => ( isa => 'Int', is => 'rw', default => 0 );

  sub deposit {
      my ( $self, $amount ) = @_;
      $self->balance( $self->balance + $amount );
  }

  sub withdraw {
      my ( $self, $amount ) = @_;
      my $current_balance = $self->balance();
      ( $current_balance >= $amount )
          || confess "Account overdrawn";
      $self->balance( $current_balance - $amount );
  }

  package CheckingAccount;
  use Moose;

  extends 'BankAccount';

  has 'overdraft_account' => ( isa => 'BankAccount', is => 'rw' );

  before 'withdraw' => sub {
      my ( $self, $amount ) = @_;
      my $overdraft_amount = $amount - $self->balance();
      if ( $self->overdraft_account && $overdraft_amount > 0 ) {
          $self->overdraft_account->withdraw($overdraft_amount);
          $self->deposit($overdraft_amount);
      }
  };

=head1 DESCRIPTION

The first recipe demonstrated how to build very basic Moose classes,
focusing on creating and manipulating attributes. The objects in that
recipe were very data-oriented, and did not have much in the way of
behavior (i.e. methods). In this recipe, we expand upon the concepts
from the first recipe to include some real behavior. In particular, we
show how you can use a method modifier to implement new behavior for a
method.

The classes in the SYNOPSIS show two kinds of bank account. A simple
bank account has one attribute, the balance, and two behaviors,
depositing and withdrawing money.

We then extend the basic bank account in the CheckingAccount
class. This class adds another attribute, an overdraft account. It
also adds overdraft protection to the withdraw method. If you try to
withdraw more than you have, the checking account attempts to
reconcile the difference by withdrawing money from the overdraft
account. (1)

The first class, B<BankAccount>, introduces a new attribute feature, a
default value:

  has 'balance' => ( isa => 'Int', is => 'rw', default => 0 );

This says that a B<BankAccount> has a C<balance> attribute, which has
an C<Int> type constraint, a read/write accessor, and a default value
of C<0>. This means that every instance of B<BankAccount> that is
created will have its C<balance> slot initialized to C<0>, unless some
other value is provided to the constructor.

The C<deposit> and C<withdraw> methods should be fairly
self-explanatory, as they are just plain old Perl 5 OO. (2)

As you know from the first recipe, the keyword C<extends> sets a
class's superclass. Here we see that B<CheckingAccount> C<extends>
B<BankAccount>. The next line introduces yet another new attribute
feature, class-based type constraints:

  has 'overdraft_account' => ( isa => 'BankAccount', is => 'rw' );

Up until now, we have only seen the C<Int> type constraint, which (as
we saw in the first recipe) is a builtin type constraint. The
C<BankAccount> type constraint is new, and was actually defined the
moment we created the B<BankAccount> class itself. In fact, Moose
creates a corresponding type constraint for every class in your
program (3).

This means that in the first recipe, constraints for both C<Point> and
C<Point3D> were created. In this recipe, both C<BankAccount> and
C<CheckingAccount> type constraints are created automatically. Moose
does this as a convenience so that your classes and type constraint
can be kept in sync with one another. In short, Moose makes sure that
it will just DWIM (4).

In B<CheckingAccount>, we see another method modifier, the C<before>
modifier.

  before 'withdraw' => sub {
      my ( $self, $amount ) = @_;
      my $overdraft_amount = $amount - $self->balance();
      if ( $self->overdraft_account && $overdraft_amount > 0 ) {
          $self->overdraft_account->withdraw($overdraft_amount);
          $self->deposit($overdraft_amount);
      }
  };

Just as with the C<after> modifier from the first recipe, Moose will
handle calling the superclass method (in this case C<<
BankAccount->withdraw >>).

The C<before> modifier will (obviously) run I<before> the code from
the superclass is run. Here, C<before> modifier implements overdraft
protection by first checking if there are available funds in the
checking account. If not (and if there is an overdraft account
available), it transfers the amount needed into the checking
account (5).

As with the method modifier in the first recipe, we could use
C<SUPER::> to get the same effect:

  sub withdraw {
      my ( $self, $amount ) = @_;
      my $overdraft_amount = $amount - $self->balance();
      if ( $self->overdraft_account && $overdraft_amount > 0 ) {
          $self->overdraft_account->withdraw($overdraft_amount);
          $self->deposit($overdraft_amount);
      }
      $self->SUPER::withdraw($amount);
  }

The benefit of taking the method modifier approach is we do not need
to remember to call C<SUPER::withdraw> and pass it the C<$amount>
argument when writing C<< CheckingAccount->withdraw >>.

This is actually more than just a convenience for forgetful
programmers. Using method modifiers helps isolate subclasses from
changes in the superclasses. For instance, if B<<
BankAccount->withdraw >> were to add an additional argument of some
kind, the version of B<< CheckingAccount->withdraw >> which uses
C<SUPER::withdraw> would not pass that extra argument correctly,
whereas the method modifier version would automatically pass along all
arguments correctly.

Just as with the first recipe, object instantiation uses the C<new>
method, which accepts named parameters.

  my $savings_account = BankAccount->new( balance => 250 );

  my $checking_account = CheckingAccount->new(
      balance           => 100,
      overdraft_account => $savings_account,
  );

And as with the first recipe, a more in-depth example can be found in
the F<t/recipes/moose_cookbook_basics_recipe2.t> test file.

=head1 CONCLUSION

This recipe expanded on the basic concepts from the first recipe with
a more "real world" use case.

=head1 FOOTNOTES

=over 4

=item (1)

If you're paying close attention, you might realize that there's a
circular loop waiting to happen here. A smarter example would have to
make sure that we don't accidentally create a loop between the
checking account and its overdraft account.

=item (2)

Note that for simple methods like these, which just manipulate some
single piece of data, it is often not necessary to write them at all.
For instance, C<deposit> could be implemented via the C<inc> native
delegation for counters - see
L<Moose::Meta::Attribute::Native::Trait::Counter> for more specifics,
and L<Moose::Meta::Attribute::Native> for a broader overview.

=item (3)

In reality, this creation is sensitive to the order in which modules
are loaded. In more complicated cases, you may find that you need to
explicitly declare a class type before the corresponding class is
loaded.

=item (4)

Moose does not attempt to encode a class's is-a relationships within
the type constraint hierarchy. Instead, Moose just considers the class
type constraint to be a subtype of C<Object>, and specializes the
constraint check to allow for subclasses. This means that an instance
of B<CheckingAccount> will pass a C<BankAccount> type constraint
successfully. For more details, please refer to the
L<Moose::Util::TypeConstraints> documentation.

=item (5)

If the overdraft account does not have the amount needed, it will
throw an error. Of course, the overdraft account could also have
overdraft protection. See note 1.

=back

=head1 ACKNOWLEDGMENT

The BankAccount example in this recipe is directly taken from the
examples in this chapter of "Practical Common Lisp":

L<http://www.gigamonkeys.com/book/object-reorientation-generic-functions.html>

=begin testing

my $savings_account;

{
    $savings_account = BankAccount->new( balance => 250 );
    isa_ok( $savings_account, 'BankAccount' );

    is( $savings_account->balance, 250, '... got the right savings balance' );
    is(
        exception {
            $savings_account->withdraw(50);
        },
        undef,
        '... withdrew from savings successfully'
    );
    is( $savings_account->balance, 200,
        '... got the right savings balance after withdrawal' );

    $savings_account->deposit(150);
    is( $savings_account->balance, 350,
        '... got the right savings balance after deposit' );
}

{
    my $checking_account = CheckingAccount->new(
        balance           => 100,
        overdraft_account => $savings_account
    );
    isa_ok( $checking_account, 'CheckingAccount' );
    isa_ok( $checking_account, 'BankAccount' );

    is( $checking_account->overdraft_account, $savings_account,
        '... got the right overdraft account' );

    is( $checking_account->balance, 100,
        '... got the right checkings balance' );

    is(
        exception {
            $checking_account->withdraw(50);
        },
        undef,
        '... withdrew from checking successfully'
    );
    is( $checking_account->balance, 50,
        '... got the right checkings balance after withdrawal' );
    is( $savings_account->balance, 350,
        '... got the right savings balance after checking withdrawal (no overdraft)'
    );

    is(
        exception {
            $checking_account->withdraw(200);
        },
        undef,
        '... withdrew from checking successfully'
    );
    is( $checking_account->balance, 0,
        '... got the right checkings balance after withdrawal' );
    is( $savings_account->balance, 200,
        '... got the right savings balance after overdraft withdrawal' );
}

{
    my $checking_account = CheckingAccount->new(
        balance => 100

            # no overdraft account
    );
    isa_ok( $checking_account, 'CheckingAccount' );
    isa_ok( $checking_account, 'BankAccount' );

    is( $checking_account->overdraft_account, undef,
        '... no overdraft account' );

    is( $checking_account->balance, 100,
        '... got the right checkings balance' );

    is(
        exception {
            $checking_account->withdraw(50);
        },
        undef,
        '... withdrew from checking successfully'
    );
    is( $checking_account->balance, 50,
        '... got the right checkings balance after withdrawal' );

    isnt(
        exception {
            $checking_account->withdraw(200);
        },
        undef,
        '... withdrawal failed due to attempted overdraft'
    );
    is( $checking_account->balance, 50,
        '... got the right checkings balance after withdrawal failure' );
}

=end testing

=cut