Provided by: libmoose-perl_2.1005-1_amd64 
NAME
Moose::Cookbook::Basics::BankAccount_MethodModifiersAndSubclassing - Demonstrates the use
of method modifiers in a subclass
VERSION
version 2.1005
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);
}
};
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, BankAccount, introduces a new attribute feature, a default value:
has 'balance' => ( isa => 'Int', is => 'rw', default => 0 );
This says that a BankAccount has a "balance" attribute, which has an "Int" type
constraint, a read/write accessor, and a default value of 0. This means that every
instance of BankAccount that is created will have its "balance" slot initialized to 0,
unless some other value is provided to the constructor.
The "deposit" and "withdraw" methods should be fairly self-explanatory, as they are just
plain old Perl 5 OO. \fIs0(2)
As you know from the first recipe, the keyword "extends" sets a class's superclass. Here
we see that CheckingAccount "extends" 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 "Int" type constraint, which (as we saw in the first
recipe) is a builtin type constraint. The "BankAccount" type constraint is new, and was
actually defined the moment we created the 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 "Point" and "Point3D" were
created. In this recipe, both "BankAccount" and "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 \fIs0(4).
In CheckingAccount, we see another method modifier, the "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 "after" modifier from the first recipe, Moose will handle calling the
superclass method (in this case "BankAccount->withdraw").
The "before" modifier will (obviously) run before the code from the superclass is run.
Here, "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 "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
"SUPER::withdraw" and pass it the $amount argument when writing
"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
BankAccount->withdraw were to add an additional argument of some kind, the version of
CheckingAccount->withdraw which uses "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 "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
t/recipes/moose_cookbook_basics_recipe2.t test file.
CONCLUSION
This recipe expanded on the basic concepts from the first recipe with a more "real world"
use case.
FOOTNOTES
(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.
(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, "deposit" could
be implemented via the "inc" native delegation for counters - see
Moose::Meta::Attribute::Native::Trait::Counter for more specifics, and
Moose::Meta::Attribute::Native for a broader overview.
(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.
(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 "Object", and specializes the constraint check to allow for subclasses.
This means that an instance of CheckingAccount will pass a "BankAccount" type
constraint successfully. For more details, please refer to the
Moose::Util::TypeConstraints documentation.
(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.
ACKNOWLEDGMENT
The BankAccount example in this recipe is directly taken from the examples in this chapter
of "Practical Common Lisp":
<http://www.gigamonkeys.com/book/object-reorientation-generic-functions.html>
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::BankAccount_MethodModifiersAndSubclassing(3pm)