Objects

When you design a program, you have to approach it from many levels. At the bottom, you have specific details about the problem you're solving. At the top levels, you have to organize the code so it makes sense. Our only hope to manage this complexity is to exploit abstraction (treating similar things similarly) and encapsulation (grouping related details together).

Functions alone are insufficient for large problems. Several techniques group functions into units of related behaviors. One popular technique is object orientation (OO), or object oriented programming (OOP), where programs work with objects—discrete, unique entities with their own identities.

Moose

Perl's default object system is but minimal but flexible. Its syntax is a little clunky, and it exposes how an object system works. You can build great things on top of it, but you or someone else will have to build a lot of code to get what some other languages provide for free.

Moose is a complete object system for Perl. It's a complete distribution available from the CPAN—not a part of the core language, but worth installing and using regardless. Moose offers both a simpler way to use an object system as well as advanced features of languages such as Smalltalk and Common Lisp.

Moose objects work with plain vanilla Perl. Within your programs, you can mix and match objects written with Perl's default object system and Moose. Of course, you have to write a lot more code to get what Moose provides for you.

Classes

A Moose object is a concrete instance of a class, which is a template describing data and behavior specific to the object. A class generally belongs to a package (Packages), which provides its name:

    package Cat
    {
        use Moose;
    }

This Cat class appears to do nothing, but that's all Moose needs to make a class. Create objects (or instances) of the Cat class with the syntax:

    my $brad = Cat->new;
    my $jack = Cat->new;

Just as an arrow dereferences a reference, this arrow calls a method on Cat.

Methods

A method is a function associated with a class. A function may belong to a namespace; you've seen that. Similarly, a method belongs to a class.

When you call a method, you do so with an invocant. When you call new() on Cat, the name of the class, Cat, is new()'s invocant. Think of this as sending a message to a class: "do whatever new() does." In this case, sending the new message—calling the new() method—returns a new object of the Cat class.

When you call a method on an object, that object is the invocant:

    my $choco = Cat->new;
    $choco->sleep_on_keyboard;

A method's first argument is its invocant ($self, by convention). Suppose a Cat can meow():

    package Cat
    {
        use Moose;

        sub meow
        {
            my $self = shift;
            say 'Meow!';
        }
    }

Now any Cat instance can wake you up in the morning because it hasn't eaten yet:

    # the cat always meows three times at 6 am
    my $fuzzy_alarm = Cat->new;
    $fuzzy_alarm->meow for 1 .. 3;

Every object can have its own distinct data. (More on this briefly.) Methods which read or write the data of their invocants are instance methods; they depend on the presence of an appropriate invocant to work correctly. Methods (such as meow()) which do not access instance data are class methods. You may invoke class methods on classes and class and instance methods on instances, but you cannot invoke instance methods on classes.

Constructors, which create instances, are obviously class methods. Moose provides a default constructor for you, named new(). That's why you can create a Cat object with the minimal class declaration earlier.

Class methods are effectively namespaced global functions. Without access to instance data, they have few advantages over namespaced functions. Most OO code uses instance methods to read and write instance data.

Attributes

Every Perl object is unique. Objects can contain private data associated with each unique object—you may hear this data called attributes, instance data, or object state. Define an attribute by declaring it as part of the class:

    package Cat
    {
        use Moose;

        has 'name', is => 'ro', isa => 'Str';
    }

Moose exports the has() function for you to use to declare an attribute. In English, this code reads "Cat objects have a name attribute. It's read-only, and is a string." The first argument, 'name', is the attribute's name. The is => 'ro' pair of arguments declares that this attribute is read only, so you cannot modify the attribute's value after you've set it. Finally, the isa => 'Str' pair declares that the value of this attribute can only be a string.

In this example, Moose creates an accessor method named name() and allows you to pass a name parameter to Cat's constructor:

    for my $name (qw( Tuxie Petunia Daisy ))
    {
        my $cat = Cat->new( name => $name );
        say "Created a cat for ", $cat->name;
    }

        has 'name' => ( is => 'ro', isa => 'Str' );

    has( 'name', 'is', 'ro', 'isa', 'Str' );

    has 'name' => (
        is         => 'ro',
        isa        => 'Str',

        # advanced Moose options; perldoc Moose
        init_arg   => undef,
        lazy_build => 1,
    );

Moose will complain if you try to set name to a value which isn't a string. Attributes do not need to have types. In that case, anything goes:

    package Cat
    {
        use Moose;

        has 'name', is => 'ro', isa => 'Str';
        has 'age',  is => 'ro';
    }

    my $invalid = Cat->new( name => 'bizarre',
                            age  => 'purple' );

If you add a type to your attribute declaration, Moose will attempt to validate the values assigned to that attribute. Sometimes this strictness is invaluable.

If you mark an attribute as readable and writable (with is => rw), Moose will create a mutator method which can change that attribute's value:

    package Cat
    {
        use Moose;

        has 'name', is => 'ro', isa => 'Str';
        has 'age',  is => 'ro', isa => 'Int';
        has 'diet', is => 'rw';
    }

    my $fat = Cat->new( name => 'Fatty',
                        age  => 8,
                        diet => 'Sea Treats' );

    say $fat->name, ' eats ', $fat->diet;

    $fat->diet( 'Low Sodium Kitty Lo Mein' );
    say $fat->name, ' now eats ', $fat->diet;

An ro accessor used as a mutator will throw the exception Cannot assign a value to a read-only accessor at ....

Using ro or rw is a matter of design, convenience, and purity. Moose enforces no particular philosophy here. Some people suggest making all instance data ro such that you must pass instance data into the constructor (Immutability). In the Cat example, age() might still be an accessor, but the constructor could take the year of the cat's birth and calculate the age itself based on the current year. This approach consolidates validation code and ensure that all created objects have valid data.

Instance data shows some of the value of object orientation. An object contains related data and can perform behaviors with that data. A class describes that data and those behaviors. You can have multiple independent objects with separate instance data and treat all of those objects the same way; they will behave differently depending on their instance data.

Encapsulation

Moose allows you to declare which attributes class instances possess (a cat has a name) as well as the attributes of those attributes (you cannot change a cat's name; you can only read it). Moose itself decides how to store those attributes. You can change that if you like, but allowing Moose to manage your storage encourages encapsulation: hiding the internal details of an object from external users of that object.

Consider a change to how Cats manage their ages. Instead of passing a value for an age to the constructor, pass in the year of the cat's birth and calculate the age as needed:

    package Cat
    {
        use Moose;

        has 'name',        is => 'ro', isa => 'Str';
        has 'diet',        is => 'rw';
        has 'birth_year',  is => 'ro', isa => 'Int';

        sub age
        {
            my $self = shift;
            my $year = (localtime)[5] + 1900;

            return $year - $self->birth_year;
        }
    }

While the syntax for creating Cat objects has changed, the syntax for using Cat objects has not. Outside of Cat, age() behaves as it always has. How it works is a detail hidden inside the Cat class.

Calculating ages has another advantage. A default attribute value will do the right thing when someone creates a new Cat object without passing a birth year:

    package Cat
    {
        use Moose;

        has 'name', is => 'ro', isa => 'Str';
        has 'diet', is => 'rw', isa => 'Str';

        has 'birth_year',
            is      => 'ro',
            isa     => 'Int',
            default => sub { (localtime)[5] + 1900 };
    }

The default keyword on an attribute takes a function reference You can use a simple value such as a number or string directly, but use a function reference for anything more complex. which returns the default value for that attribute when constructing a new object. If the code creating an object passes no constructor value for that attribute, the object gets the default value:

    my $kitten = Cat->new( name => 'Choco' );

... and that kitten will have an age of 0 until next year.

Polymorphism

Encapsulation is useful, but the real power of object orientation is much broader. A well-designed OO program can manage many types of data. When well-designed classes encapsulate specific details of objects into the appropriate places, something curious happens: the code often becomes less specific.

Moving the details of what the program knows about individual Cats (the attributes) and what the program knows that Cats can do (the methods) into the Cat class means that code that deals with Cat instances can happily ignore how Cat does what it does.

Consider a function which displays details of an object:

    sub show_vital_stats
    {
        my $object = shift;

        say 'My name is ', $object->name;
        say 'I am ',       $object->age;
        say 'I eat ',      $object->diet;
    }

It's obvious (in context) that this function works if you pass it a Cat object. It's less obvious that it will do the right thing for any object with the appropriate three accessors, no matter how that object provides those accessors and no matter what kind of object it is: Cat, Caterpillar, or Catbird. show_vital_stats() cares that an invocant is valid only in that it supports three methods, name(), age(), and diet() which take no arguments and each return something which can concatenate in a string context. You may have a hundred different classes in your code, none of which have any obvious relationships, but they will work with this function if they conform to this expected behavior.

This property is called polymorphism. It means that you can substitute an object of one class for an object of another class if they provide the same external interface.

Consider how you might enumerate a zoo's worth of animals without this polymorphic function. The benefit of genericity should be obvious. As well, any specific details about how to calculate the age of an ocelot or octopus can belong in the relevant class—where it matters most.

Of course, the mere existence of a method called name() or age() does not by itself imply the behavior of that object. A Dog object may have an age() which is an accessor such that you can discover $rodney is 11 but $lucky is 6. A Cheese object may have an age() method that lets you control how long to stow $cheddar to sharpen it. age() may be an accessor in one class but not in another:

    # how old is the cat?
    my $years = $zeppie->age;

    # store the cheese in the warehouse for six months
    $cheese->age;

Sometimes it's useful to know what an object does and what that means.

Roles

A role is a named collection of behavior and state See the P6 design documents on roles at http://feather.perl6.nl/syn/S14.html and research on Smalltalk traits at http://scg.unibe.ch/research/traits for copious details.. While a class organizes behaviors and state into a template for objects, a role organizes a named collection of behaviors and state. You can instantiate a class, but not a role. A role is something a class does.

Given an Animal which has an age and a Cheese which can age, one difference may be that Animal does the LivingBeing role, while the Cheese does the Storable role:

    package LivingBeing
    {
        use Moose::Role;

        requires qw( name age diet );
    }

The requires keyword provided by Moose::Role allows you to list methods that this role needs from its composing classes. In other words, anything which does this role must supply the name(), age(), and diet() methods. The Cat class must explicitly mark that it performs the role:

    package Cat
    {
        use Moose;

        has 'name', is => 'ro', isa => 'Str';
        has 'diet', is => 'rw', isa => 'Str';

        has 'birth_year',
            is      => 'ro',
            isa     => 'Int',
            default => sub { (localtime)[5] + 1900 };

        with 'LivingBeing';

        sub age { ... }
    }

The with line causes Moose to compose the LivingBeing role into the Cat class. Composition ensures all of the attributes and methods of the role are part of the class. LivingBeing requires any composing class to provide methods named name(), age(), and diet(). Cat satisfies these constraints. If LivingBeing were composed into a class which did not provide those methods, Moose would throw an exception.

Now all Cat instances will return a true value when queried if they provide the LivingBeing role. Cheese objects should not:

    say 'Alive!' if $fluffy->DOES('LivingBeing');
    say 'Moldy!' if $cheese->DOES('LivingBeing');

This design technique separates the capabilities of classes and objects from the implementation of those classes and objects. The birth year calculation behavior of the Cat class could itself be a role:

    package CalculateAge::From::BirthYear
    {
        use Moose::Role;

        has 'birth_year',
            is      => 'ro',
            isa     => 'Int',
            default => sub { (localtime)[5] + 1900 };

        sub age
        {
            my $self = shift;
            my $year = (localtime)[5] + 1900;

            return $year - $self->birth_year;
        }
    }

Extracting this role from Cat makes the useful behavior available to other classes. Now Cat can compose both roles:

    package Cat
    {
        use Moose;

        has 'name', is => 'ro', isa => 'Str';
        has 'diet', is => 'rw';

        with 'LivingBeing',
             'CalculateAge::From::BirthYear';
    }

Notice how the age() method of CalculateAge::From::BirthYear satisfies the requirement of the LivingBeing role. Notice also that any check that Cat performs LivingBeing returns a true value. Extracting age() into a role has only changed the details of how Cat calculates an age. It's still a LivingBeing. Cat can choose to implement its own age or get it from somewhere else. All that matters is that it provides an age() which satisfies the LivingBeing constraint.

Remember how polymorphism means that you can treat multiple objects with the same behavior in the same way? An object may implement the same behavior in multiple ways. This is allomorphism. Pervasive allomorphism can reduce the size of your classes and increase the code shared between them. It also allows you to name specific and discrete collections of behaviors—very useful for testing for capabilities instead of implementations.

To compare roles to other design techniques such as mixins, multiple inheritance, and monkeypatching, see http://www.modernperlbooks.com/mt/2009/04/the-why-of-perl-roles.html.

Roles and DOES()

When you compose a role into a class, the class and its instances will return a true value when you call DOES() on them:

    say 'This Cat is alive!'
        if $kitten->DOES( 'LivingBeing' );

Inheritance

Perl's object system supports inheritance, which establishes a parent and child relationship between two classes such that one specializes the other. The child class behaves the same way as its parent—it has the same number and types of attributes and can use the same methods. It may have additional data and behavior, but you may substitute any instance of a child where code expects its parent. In one sense, a subclass provides the role implied by the existence of its parent class.

Consider a LightSource class which provides two public attributes (enabled and candle_power) and two methods (light and extinguish):

    package LightSource
    {
        use Moose;

        has 'candle_power', is      => 'ro',
                            isa     => 'Int',
                            default => 1;

        has 'enabled', is      => 'ro',
                       isa     => 'Bool',
                       default => 0,
                       writer  => '_set_enabled';

        sub light
        {
            my $self = shift;
            $self->_set_enabled( 1 );
        }

        sub extinguish
        {
            my $self = shift;
            $self->_set_enabled( 0 );
        }
    }

(Note that enabled's writer option creates a private accessor usable within the class to set the value.)

Inheritance and Attributes

A subclass of LightSource could define an industrial-strength super candle which provides a hundred times the amount of light:

    package SuperCandle
    {
        use Moose;

        extends 'LightSource';

        has '+candle_power', default => 100;
    }

extends takes a list of class names to use as parents of the current class. If that were the only line in this class, SuperCandle objects would behave in the same ways as LightSource objects. A SuperCandle instance would have both the candle_power and enabled attributes as well as the light() and extinguish() methods.

The + at the start of an attribute name (such as candle_power) indicates that the current class does something special with that attribute. Here the super candle overrides the default value of the light source, so any new SuperCandle created has a light value of 100 regular candles.

When you invoke light() or extinguish() on a SuperCandle object, Perl will look in the SuperCandle class for the method. If there's no method by that name in the child class, Perl will look at each parent in turn, recursively. In this case, those methods are in the LightSource class.

Attribute inheritance works similarly (see perldoc Class::MOP).

Method Dispatch Order

A method call always involves a dispatch strategy. This strategy controls how Perl selects the appropriate method. This may seem obvious, given how simple the Cat class is, but much of the power of OO comes from method dispatch.

Method dispatch order (or method resolution order or MRO) is obvious for single-parent classes. Look in the object's class, then its parent, and so on until you find the method or run out of parents. Classes which inherit from multiple parents (multiple inheritance)—Hovercraft extends both Boat and Car—require trickier dispatch. Reasoning about multiple inheritance is complex. Avoid multiple inheritance when possible.

Perl uses a depth-first method resolution strategy. It searches the class of the first named parent and all of that parent's parents recursively before searching the classes of the current class's immediate parents. The mro pragma (Pragmas) provides alternate strategies, including the C3 MRO strategy which searches a given class's immediate parents before searching any of their parents.

See perldoc mro for more details.

Inheritance and Methods

As with attributes, subclasses may override methods. Imagine a light that you cannot extinguish:

    package Glowstick
    {
        use Moose;

        extends 'LightSource';

        sub extinguish {}
    }

Calling extinguish() on a glowstick does nothing, even though LightSource's method does something. Method dispatch will find the subclass's method. You may not have meant to do this. When you do, use Moose's override to express your intention clearly.

Within an overridden method, Moose's super() allows you to call the overridden method:

    package LightSource::Cranky
    {
        use Carp 'carp';
        use Moose;

        extends 'LightSource';

        override light => sub
        {
            my $self = shift;

            carp "Can't light a lit light source!"
                if $self->enabled;

            super();
        };

        override extinguish => sub
        {
            my $self = shift;

            carp "Can't extinguish unlit light source!"
                unless $self->enabled;

            super();
        };
    }

This subclass adds a warning when trying to light or extinguish a light source that already has the current state. The super() function dispatches to the nearest parent's implementation of the current method, per the normal Perl method resolution order. (See perldoc Moose::Manual::MethodModifiers for more dispatch options.)

Inheritance and isa()

Perl's isa() method returns true if its invocant is or extends a named class. That invocant may be the name of a class or an instance of an object:

    say 'Looks like a LightSource'
        if $sconce->isa( 'LightSource' );

    say 'Hominidae do not glow'
        unless $chimpy->isa( 'LightSource' );

Moose and Perl OO

Moose provides many features beyond Perl's default OO. While you can build everything you get with Moose yourself (Blessed References), or cobble it together with a series of CPAN distributions, Moose is worth using. It is a coherent whole, with good documentation. Many important projects use it successfully. Its development community is mature and attentive.

Moose takes care of constructors, destructors, accessors, and encapsulation. You must do the work of declaring what you want, but what you get back is safe and easy to use. Moose objects can extend and work with objects from the vanilla Perl system.

Moose also allows metaprogramming—manipulating your objects through Moose itself. If you've ever wondered which methods are available on a class or an object or which attributes an object supports, this information is available:

    my $metaclass = Monkey::Pants->meta;

    say 'Monkey::Pants instances have the attributes:';

    say $_->name for $metaclass->get_all_attributes;

    say 'Monkey::Pants instances support the methods:';

    say $_->fully_qualified_name
        for $metaclass->get_all_methods;

You can even see which classes extend a given class:

    my $metaclass = Monkey->meta;

    say 'Monkey is the superclass of:';

    say $_ for $metaclass->subclasses;

See perldoc Class::MOP::Class for more information about metaclass operations and perldoc Class::MOP for Moose metaprogramming information.

Moose and its meta-object protocol (or MOP) offers the possibility of a better syntax for declaring and working with classes and objects in Perl. This is valid code:

    use MooseX::Declare;

    role LivingBeing { requires qw( name age diet ) }

    role CalculateAge::From::BirthYear { has 'birth_year',
            is      => 'ro',
            isa     => 'Int',
            default => sub { (localtime)[5] + 1900 };

        method age
        {
            return (localtime)[5] + 1900
                                  - $self->birth_year;
        }
    }

    class Cat with LivingBeing
              with CalculateAge::From::BirthYear
    {
        has 'name', is => 'ro', isa => 'Str';
        has 'diet', is => 'rw';
    }

The MooseX::Declare CPAN distribution uses Devel::Declare to add new Moose-specific syntax. The class, role, and method keywords reduce the amount of boilerplate necessary to write good object oriented code in Perl. Note specifically the declarative nature of this example, as well as the lack of my $self = shift; in age().

If you're using a Perl of 5.14 or newer, Devel::Declare is less useful; Perl itself supports a pluggable keyword system. In that case, a syntax-warping module such as MooseX::Method::Signatures or Moops may be more to your taste.

While Moose is not a part of the Perl core, its popularity ensures that it's available on many OS distributions. Perl distributions such as Strawberry Perl and ActivePerl also include it. Even though Moose is a CPAN module and not a core library, its cleanliness and simplicity make it essential to modern Perl programming.

Blessed References

Perl's core object system is deliberately minimal. It has only three rules:

• A class is a package.
• A method is a function.
• A (blessed) reference is an object.

You can build anything else out of those three rules. This minimalism can be impractical for larger projects—in particular, the possibilities for greater abstraction through metaprogramming (Code Generation) are awkward and limited. Moose (Moose) is a better choice for modern programs larger than a couple of hundred lines, although lots of legacy code still uses Perl's default OO.

You've seen the first two rules already. The bless builtin associates the name of a class with a reference. That reference is now a valid invocant, and Perl will perform method dispatch on it, using the associated class.

A constructor is a method which creates and blesses a reference. By convention, constructors have the name new(), but this is not a requirement. Constructors are also almost always class methods.

bless takes two operands, a reference and a class name, and evaluates to the reference. The reference may be any valid reference, empty or not. The class does not have to exist yet. You may even use bless outside of a constructor or a class ... but you're violating encapsulation to expose the details of object construction outside of a constructor.. A constructor can be as simple as:

    sub new
    {
        my $class = shift;
        bless {}, $class;
    }

By design, this constructor receives the class name as the method's invocant. You may also hard-code the name of a class, at the expense of flexibility. A parametric constructor allows reuse through inheritance, delegation, or exporting.

The type of reference used is relevant only to how the object stores its own instance data. It has no other effect on the resulting object. Hash references are most common, but you can bless any type of reference:

    my $array_obj  = bless [],          $class;
    my $scalar_obj = bless \$scalar,    $class;
    my $func_obj   = bless \&some_func, $class;

Moose classes define object attributes declaratively, but Perl's default OO is lax. A class representing basketball players which stores jersey number and position might use a constructor like:

    package Player
    {
        sub new
        {
            my ($class, %attrs) = @_;
            bless \%attrs, $class;
        }
    }

... and create players with:

    my $joel   = Player->new( number => 10,
                            position => 'center' );

    my $damian = Player->new( number  => 0,
                             position => 'guard' );

The class's methods can access object attributes as hash elements directly:

    sub format
    {
        my $self = shift;
        return '#'       . $self->{number}
             . ' plays ' . $self->{position};
    }

... but so can any other code, so any change to the object's internal representation may break other code. Accessor methods are safer:

    sub number   { return shift->{number}   }
    sub position { return shift->{position} }

... and now you're starting to write yourself what Moose gives you for free. Better yet, Moose encourages people to use accessors instead of direct access by hiding the accessor generation code. Goodbye, temptation.

Method Lookup and Inheritance

Given a blessed reference, a method call of the form:

    my $number = $joel->number;

... looks up the name of the class associated with the blessed reference $joel—in this case, Player. Next, Perl looks for a function Remember that Perl makes no distinction between functions in a namespace and methods. named number() in Player. If no such function exists and if Player extends a parent class, Perl looks in the parent class (and so on and so on) until it finds a number(). If Perl finds number(), it calls that method with $joel as an invocant.

Moose provides extends to track inheritance relationships, but Perl uses a package global variable named @ISA. The method dispatcher looks in each class's @ISA to find the names of its parent classes. If InjuredPlayer extends Player, you might write:

    package InjuredPlayer
    {
        @InjuredPlayer::ISA = 'Player';
    }

The parent pragma (Pragmas) is cleaner Older code may use the base pragma, but parent superseded base in Perl 5.10.:

    package InjuredPlayer
    {
        use parent 'Player';
    }

Moose has its own metamodel which stores extended inheritance information. This allows Moose to provide additional metaprogramming opportunities.

You may inherit from multiple parent classes:

    package InjuredPlayer;
    {
        use parent qw( Player Hospital::Patient );
    }

... though the caveats about multiple inheritance and method dispatch complexity apply. Consider instead roles (Roles) or Moose method modifiers.

AUTOLOAD

If there is no applicable method in the invocant's class or any of its superclasses, Perl will next look for an AUTOLOAD() function (AUTOLOAD) in every class according to the selected method resolution order. Perl will invoke any AUTOLOAD() it finds to provide or decline the desired method.

AUTOLOAD() makes multiple inheritance much more difficult to understand.

Method Overriding and SUPER

As with Moose, you may override methods in basic Perl OO. Unlike Moose, Perl provides no mechanism for indicating your intent to override a parent's method. Worse yet, any function you predeclare, declare, or import into the child class may override a method in the parent class by having the same name. Even if you forget to use the override system of Moose, at least it exists. Basic Perl OO offers no such protection.

To override a method in a child class, declare a method with the same name as the method in the parent. Within an overridden method, call the parent method with the SUPER:: dispatch hint:

    sub overridden
    {
        my $self = shift;
        warn 'Called overridden() in child!';
        return $self->SUPER::overridden( @_ );
    }

The SUPER:: prefix to the method name tells the method dispatcher to dispatch to an overridden method of the appropriate name. You can provide your own arguments to the overridden method, but most code reuses @_. Be careful to shift off the invocant if you do.

Strategies for Coping with Blessed References

Blessed references may seem minimal and tricky and confusing. They are. Moose is much easier to use, so use it whenever possible. If you do find yourself maintaining code which uses blessed references, or if you can't convince your team to use Moose in full yet, you can work around some of the problems of blessed references with discipline.

• Do not mix functions and methods in the same class.
• Use a single .pm file for each class, unless the class is a small, self-contained helper used from a single place.
• Follow standards of vanilla Perl OO, such as naming constructors new() and using $self as the invocant name in your documentation.
• Use accessor methods pervasively, even within methods in your class. A module such as Class::Accessor helps to avoid repetitive boilerplate.
• Avoid AUTOLOAD() where possible. If you must use it, use function forward declarations (Declaring Functions) to avoid ambiguity.
• Expect that someone, somewhere will eventually need to subclass (or delegate to or reimplement the interface of) your classes. Make it easier for them by not assuming details of the internals of your code, by using the two-argument form of bless, and by breaking your classes into the smallest responsible units of code.
• Use helper modules such as Role::Tiny to allow better use and reuse.

Reflection

Reflection (or introspection) is the process of asking a program about itself as it runs. By treating code as data you can manage code in the same way that you manage data. This is a principle behind code generation (Code Generation).

Moose's Class::MOP (Class::MOP) simplifies many reflection tasks for object systems. If you use Moose, its metaprogramming system will help you. If not, several other Perl idioms help you inspect and manipulate running programs.

Checking that a Module Has Loaded

If you know the name of a module, you can check that Perl believes it has loaded that module by looking in the %INC hash. When Perl loads code with use or require, it stores an entry in %INC where the key is the file path of the module to load and the value is the full path on disk to that module. In other words, loading Modern::Perl effectively does:

    $INC{'Modern/Perl.pm'} =
        '.../lib/site_perl/5.12.1/Modern/Perl.pm';

The details of the path will vary depending on your installation. To test that Perl has successfully loaded a module, convert the name of the module into the canonical file form and test for that key's existence within %INC:

    sub module_loaded
    {
        (my $modname = shift) =~ s!::!/!g;
        return exists $INC{ $modname . '.pm' };
    }

As with @INC, any code anywhere may manipulate %INC. Some modules (such as Test::MockObject or Test::MockModule) manipulate %INC for good reasons. Depending on your paranoia level, you may check the path and the expected contents of the package yourself.

The Class::Load CPAN module's is_class_loaded() function does all of this for you without making you manipulate %INC.

Checking that a Package Exists

To check that a package exists somewhere in your program—if some code somewhere has executed a package directive with a given name—check that the package inherits from UNIVERSAL. Anything which extends UNIVERSAL must somehow provide the can() method. If no such package exists, Perl will throw an exception about an invalid invocant, so wrap this call in an eval block:

    say "$pkg exists" if eval { $pkg->can( 'can' ) };

An alternate approach is to grovel through Perl's symbol tables. You're on your own here.

Checking that a Class Exists

Because Perl makes no strong distinction between packages and classes, the best you can do without Moose is to check that a package of the expected class name exists. You can check that the package can() provide new(), but there is no guarantee that any new() found is either a method or a constructor.

Checking a Module Version Number

Modules do not have to provide version numbers, but every package inherits the VERSION() method from the universal parent class UNIVERSAL (The UNIVERSAL Package):

    my $version = $module->VERSION;

VERSION() returns the given module's version number, if defined. Otherwise it returns undef. If the module does not exist, the method will likewise return undef.

Checking that a Function Exists

To check whether a function exists in a package, call can() as a class method on the package name:

    say "$func() exists" if $pkg->can( $func );

Perl will throw an exception unless $pkg is a valid invocant; wrap the method call in an eval block if you have any doubts about its validity. Beware that a function implemented in terms of AUTOLOAD() (AUTOLOAD) may report the wrong answer if the function's package has not predeclared the function or overridden can() correctly. This is a bug in the other package.

Use this technique to determine if a module's import() has imported a function into the current namespace:

    say "$func() imported!" if __PACKAGE__->can( $func );

As with checking for the existence of a package, you can root around in symbol tables yourself, if you have the patience for it.

Checking that a Method Exists

There is no foolproof way for reflection to distinguish between a function or a method.

Rooting Around in Symbol Tables

A symbol table is a special type of hash where the keys are the names of package global symbols and the values are typeglobs. A typeglob is an internal data structure which can contain any or all of a scalar, an array, a hash, a filehandle, and a function.

Access a symbol table as a hash by appending double-colons to the name of the package. For example, the symbol table for the MonkeyGrinder package is available as %MonkeyGrinder::.

You can test the existence of specific symbol names within a symbol table with the exists operator (or manipulate the symbol table to add or remove symbols, if you like). Yet be aware that certain changes to the Perl core have modified the details of what typeglobs store and when and why.

See the "Symbol Tables" section in perldoc perlmod for more details, then prefer the other techniques in this section for reflection. If you really must manipulate symbol tables and typeglobs, consider using the Package::Stash CPAN module instead.

Advanced OO Perl

Creating and using objects in Perl with Moose (Moose) is easy. Designing good programs is not. You must balance between designing too little and too much. Only practical experience can help you understand the most important design techniques, but several principles can guide you.

Favor Composition Over Inheritance

Novice OO designs often overuse inheritance to reuse code and to exploit polymorphism. The result is a deep class hierarchy with responsibilities scattered in the wrong places. Maintaining this code is difficult—who knows where to add or edit behavior? What happens when code in one place conflicts with code declared elsewhere?

Inheritance is only one of many tools for OO programmers. It's not always the right tool; it's often the wrong tool. A Car may extend Vehicle::Wheeled (an is-a relationship), but Car may better contain several Wheel objects as instance attributes (a has-a relationship).

Decomposing complex classes into smaller, focused entities (whether classes or roles) improves encapsulation and reduces the possibility that any one class or role does too much. Smaller, simpler, and better encapsulated entities are easier to understand, test, and maintain.

Single Responsibility Principle

When you design your object system, consider the responsibilities of each entity. For example, an Employee object may represent specific information about a person's name, contact information, and other personal data, while a Job object may represent business responsibilities. Separating these entities in terms of their responsibilities allows the Employee class to consider only the problem of managing information specific to who the person is and the Job class to represent what the person does. (Two Employees may have a Job-sharing arrangement, for example, or one Employee may have the CFO and the COO Jobs.)

When each class has a single responsibility, you improve the encapsulation of class-specific data and behaviors and reduce coupling between classes.

Don't Repeat Yourself

Complexity and duplication complicate development and maintenance. The DRY principle (Don't Repeat Yourself) is a reminder to seek out and to eliminate duplication within the system. Duplication exists in data as well as in code. Instead of repeating configuration information, user data, and other artifacts within your system, create a single, canonical representation of that information from which you can generate the other artifacts.

This principle helps to reduce the possibility that important parts of your system will get unsynchronized. It also helps you to find the optimal representation of the system and its data.

Liskov Substitution Principle

The Liskov substitution principle suggests that you should be able to substitute a specialization of a class or a role for the original without violating the API of the original. In other words, an object should be as or more general with regard to what it expects and at least as specific about what it produces as the object it replaces.

Imagine two classes, Dessert and its child class PecanPie. If the classes follow the Liskov substitution principle, you can replace every use of Dessert objects with PecanPie objects in the test suite, and everything should pass See Reg Braithwaite's "IS-STRICTLY-EQUIVALENT-TO-A" for more details, http://weblog.raganwald.com/2008/04/is-strictly-equivalent-to.html..

Subtypes and Coercions

Moose allows you to declare and use types and extend them through subtypes to form ever more specialized descriptions of what your data represents and how it behaves. These type annotations help verify that the data on which you want to work in specific functions or methods is appropriate and even to specify mechanisms by which to coerce data of one type to data of another type.

For example, you may wish to allow people to provide dates to a Ledger entry as strings while representing them as DateTime instances internally. You can do this by creating a Date type and adding a coercion from string types. See Moose::Util::TypeConstraints and MooseX::Types for more information.

Immutability

OO novices often treat objects as if they were bundles of records which use methods to get and set internal values. This simple technique leads to the unfortunate temptation to spread the object's responsibilities throughout the entire system.

With a well-designed object, you tell it what to do, not how to do it. As a rule of thumb, if you find yourself accessing object instance data (even through accessor methods) outside of the object itself, you may have too much access to an object's internals.

You can prevent this inappropriate access by making your objects immutable. Provide the necessary data to their constructors, then disallow any modifications of this information from outside the class. Expose no methods to mutate instance data. Once you've constructed such an object, you know it's always in a valid state. You can never modify its data to put it in an invalid state.

This takes tremendous discipline, but the resulting systems are robust, testable, and maintainable. Some designs go as far as to prohibit the modification of instance data within the class itself, though this is much more difficult to achieve.