Introduction to OO Perl with Moose

Introduction to
OO Perl
With Moose

Dave Cross
Magnum Solutions Ltd
dave@mag-sol.com

What We Will Cover
 Introduction to Object Oriented
programming
 Overview of Moose
 Object Attributes
 Subclasses
 Further information

London Perl Workshop
24th November 2012 2

What is OOP?
 “Traditional” programming is procedural
 Subroutines work on variables
 my $twelve = regenerate($eleven);
 Variables are dumb
 Just stores for data


What is OOP?
 Object Oriented programming inverts this
 Variables are objects
 Objects can carry out certain processes
 Called methods
 my $twelve = $eleven->regenerate();
 Objects are intelligent
 Objects know what methods they can carry
out


Some Concepts
 A Class is a type of intelligent variable
 e.g. Time Lord
 An Object is an instance of a class
 e.g. The Doctor
 A Method is an action that an object does
 e.g. Regenerate
 An Attribute is a piece of data in an object
 e.g. Name


Some Concepts
 A class contains a number of methods
 An object is of a particular class
 The class defines the behaviour of an object
 An object has many attributes
 A class can also have attributes


Methods
 Methods can be either class methods or
object methods
 Class methods are called on a class
 my $doctor = TimeLord->new;
 Object methods are called on an object
 $doctor->regenerate;


Constructors
 All classes need a constructor method
 Creates a new object of that class
 Usually a class method
 Often called new
 my $doctor = TimeLord->new;


Constructors

 A Class might have multiple constructors
 my $doctor = TimeLord->new;
 my $flesh_dr =
TimeLord->clone($doctor);
 A constructor might be an object method
 my $flesh_dr = $doctor->clone;


Accessors & Mutators

 Access object attributes with an accessor
method
 say “The time lord's name is “,
$doctor->get_name;
 Change an attribute with a mutator method
 $doctor->set_age(
$doctor->get_age + 1
);


Accessor/Mutators

 Accessors and mutators are often the same
method
 say “The time lord's name is “,
$doctor->name;
 $doctor->age($doctor->age + 1);
 Checks number of parameters
 Reacts appropriately


Accessor/Mutators
 Which to choose?
 Perl Best Practices says get_foo/set_foo
 I like one method called foo
 No firm rules
 Pick one
 Stick with it


Subclasses
 A subclass is a specialisation of a class
 “Alien” is a class
 “Dalek” is one possible subclass
 Avoid reimplementing shared methods


OO Perl
 Three rules of OO Perl
 A class is a package
 An object is reference
 A method is a subroutine


A Class is a Package
 Same as any other package
 Contains subroutines
 Methods
 Contains variables
 Class attributes


An Object is a Reference
 Usually a reference to a hash
 Hash keys are attribute names
 Hash values are attribute values
 Actually a “blessed” hash
 So it knows what class it is


A Method is a Subroutine
 Just like any other subroutine
 Some rules on parameters
 First parameter is class name or object
reference
 Some differences in calling
 Arrow notation
 $doctor->name()


Simple Class
 package Alien;
sub new {
my ($class, $name) = @_;
my $self = { name => $name };
return bless $self, $class;
}


Simple Class
 sub name {
my ($self, $name) = @_;
if (defined $name) {
$self->{name} = $name;
}
return $self->{name};
}
1;


Using Our Class
 use Alien;
my $alien = Alien->new('Mork');
say $alien->name; # prints Mork
$alien->name('Mork from Ork');
say $alien->name;
# prints Mork from Ork


Moose
 Moose is a Modern Object System for Perl 5
 Based on Perl 6 object system
 More powerful
 More flexible
 Easier


Simple Moose Class
 package Alien;
use Moose;

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

no Moose;
__PACKAGE__->meta->make_immutable;


What's Going On?
 use Moose;
 Loads Moose environment
 Makes our class a subclass of Moose::Object
 Turns on use strict and use warnings


Declarative Attributes
 has name => (
is => 'rw',
isa => 'Str',
);
 Creates an attribute called 'name'
 Makes it read/write
 Must be a string


Read/Write Attributes
 Moose creates method to access/alter
attributes
 $alien->name('Strax');
say $alien->name;
 The 'is' property controls how they work
 'rw' : read and write
 'ro' : read only


Private Attributes
 Use is => 'bare' for attributes that aren't
readable
 No methods are created
 Direct hash access
 $alien->{name} =
'Commander Strax';


Housekeeping
 Moose classes carry a lot of baggage
 We can (and should) turn some of it off
 no Moose;
 Remove Moose exports from your namespace
 See also namespace::autoclean
 __PACKAGE__->meta->make_immutable;
 No more changes to class definition
 Performance improvements


Using Our Class
 From the user's perspective, nothing changes
 Use it just like other Perl classes
 use Alien;
my $strax = Alien->new(
name => 'Strax'
);
say $strax->name;
 Named parameters are good


Subclassing
 A subclass is a specialisation of a superclass
 More specific behaviour
 New attributes
 New methods
 Overriding superclass methods

24th November 2012

Subclassing
 Not all aliens are the same
 package Dalek;
use Moose;
extends 'Alien';
has accuracy => (
isa => 'Num',
is => 'rw',
);

24th November 2012

Subclassing
 sub exterminate {
my $self = shift;
say “EX-TERM-IN-ATE”;
if (rand < $self->accuracy) {
say “$_[0] has been exterminated”;
return 1;
} else {
return;
}
}

24th November 2012

Using Subclasses
 use Dalek;

my $karn = Dalek->new(
name => 'Karn', accuracy => 0.9,
);

say $karn->name;
$karn->exterminate('The Doctor');

24th November 2012

Overriding Methods
 Daleks have a different way of using names
 A Dalek's name is always “Dalek
Something”
 Need to override the name method from
Alien
 But we still want to get the name itself from
Alien's method

24th November 2012

Method Modifiers
 Moose has a declarative way to modify
methods from your superclass
 before : run this code before the superclass
method
 after : run this code after the superclass
method
 around : run this code around the superclass
method

24th November 2012

Dalek Names
 around name => sub {
my $orig = shift;
my $class = shift;

return 'Dalek ' .
$class->$orig(@_);
};

24th November 2012

Declarative Attributes
 Attributes are declared in a class using the
has keyword
 This is different to “classic” Perl OO
 Where attributes are created by the presence of
accessor methods
 Attributes have a number of properties
 Properties define the attribute

24th November 2012

Properties
 has name => (
isa => 'Str',
is => 'rw',
);
 'isa' and 'is' are properties
 Many other options exist

24th November 2012

is
 is : defines if you can read or write the
attribute
 Actually defines whether accessor method is
created
And how it works
 $obj->ro_attr('Some value');
 “Cannot assign a value to a read-only
accessor”
 Use is => 'bare' for private attributes
 No accessor created
24th November 2012

Accessor Name
 “is” is actually a shortcut for two other
properties
 reader and writer
 has name => (
reader => 'get_name',
writer => 'set_name',
);

24th November 2012

Accessor Name
 Now we don't have a method called name
 say $obj->name; # Error
 Need to use get_name
 say $obj->get_name;
 And set_name
 $obj->set_name('New Name');

24th November 2012

Best Practices
 What is best practice
 One method (name)
 Two methods (get_name, set_name)
 Who cares?
 Choose one
 And stick with it
 Perl Best Practices says two methods
 See MooseX::FollowPBP

24th November 2012

Required Attributes
 By default Moose attributes are optional
 Make them mandatory with required
 has name => (
required => 1,
);
 my $alien = Alien->new;
 “Attribute (name) is required at constructor
Alien::new”

24th November 2012

Attribute Defaults
 Set a default for missing attributes
 has accuracy => (
default => 0.5,
);
 Or a subroutine reference
default => sub { rand },
);

24th November 2012

Attribute Builder
 Define a builder method instead of a default
subroutine
builder => '_build_accuracy',
);
 sub _build_accuracy {
return rand;
}
 Easier to subclass

24th November 2012

Attribute Types
 Set the type of an attribute with isa
isa => 'Num',
);
 Validation checks run as value is set

24th November 2012

Defining Types
 Accuracy should be less than 1
 To give the Doctor a chance
 Define your own type
 subtype 'Accuracy'
=> as 'Num'
=> where { $_ < 1 };

24th November 2012

Using Types
isa => 'Accuracy',
);
 my $dalek = Dalek->new(
accuracy => 1
);
 “Attribute (accuracy) does not pass the type
constraint because: Validation failed for
'Accuracy' with value 1 at constructor
Dalek::new”

24th November 2012

Aggregate Attributes
 You can define aggregate attributes
 isa => 'ArrayRef'
 Reference to array (elements are any type)
 isa => 'ArrayRef[Int]'
 Reference to array (elements are integers)

24th November 2012

Array Example
 Daleks like to keep track of their victims
 has victims (
is => 'rw',
isa => 'ArrayRef[Str]',
default => sub { [] },
);
 And in the exterminate method
 push $self->victims, $_[0];

24th November 2012

Array Example
 sub brag {
my $self = shift;
if (@{$self->victims}) {
say $self->name, ' has killed ',
scalar @{$self->victims},
' enemies of the Daleks';
say 'Their names are: ',
join(', ',
@{$self->victims});
} else {
say $self->name,
' has nothing to brag about';
}
}
24th November 2012

Hash Attributes
 Moose also supports hash ref attributes
 has some_attribute => (
isa => 'HashRef[Str]',
is => 'rw',
);

24th November 2012

More Types
 Attributes can also be objects
 has useragent => (
is => 'rw',
isa => 'LWP::UserAgent',
);
 Or a union of types
 has output => (
is 'rw',
isa => 'Object | Filehandle',
);

24th November 2012

Lazy Attributes
 Some attributes are rarely used
 And can be complex to construct
 It's a waste of resources to build them before
they are needed
 Mark them as lazy
 And define a build method


Lazy Attributes
 has useragent => (
is => 'LWP::UserAgent',
lazy => 1,
builder => '_build_ua',
);
 sub _build_ua {
return LWP::UserAgent->new(...);
}
 $self->useragent->get(...);
# creates object


Triggers
 A trigger is a subroutine that is called when
an attribute's value changes
 Subroutine is passed the old and new values
 has name => (
trigger => &name_change,
);
 sub name_change {
my ($self, $new, $old) = @_;
warn
“Name changed from $old to $new”;
}


Constructors
 A constructor is a special type of method
 It is usually a class method
 It returns a new object
 Moose classes prefer named parameters
 my $karn = Dalek->new(
name => 'Karn', accuracy => 0.99,
);


Default Constructor
 The default Moose constructor builds an
object from its parameters
 Checks for mandatory attributes
 Checks type constraints
 Returns an object


Different Behaviour
 Some constructors need to do other
processing
 Not just build an object
 Sometimes it's convenient not to use named
parameters
 Use BUILD and BUILDARGS to override
Moose's default behaviour


BUILDARGS
 More flexible parameters
 Take a parameter list convert it to named
parameters
 Commonly Daleks only need a name
 my $karn = Dalek->new(
name => 'Karn'
);
 Why not simplify?
 my $karn = Dalek->new('Karn');


Dalek Construction
 We can use BUILDARGS to build a list of
named parameters
 around BUILDARGS => sub {
my $orig = shift;
my $class = shift;
if (@_ == 1 and !ref $_[0]) {
return
$class->$orig({name => $_[0]});
} else {
return $class->$orig(@_);
}
}


Announcing Your Dalek
 When a new Dalek is created we want to
announce its name
 We can use the BUILD method
 After a new object is constructed, the
BUILD method is called
 Use it to carry out any additional processing


BUILD Example
 sub BUILD {
my $self = shift;
say $self->name . ' is born.';
}
 This method is called every time a new
Dalek object is created
 Called after the object is constructed
 But before the new method returns


More Moose
 Moose does a lot more
 We have only scratched the surface
 Good documentation
 CPAN
 Add-on modules
 MooseX::*


More Moose
 Type hierarchy
 Type coercion
 Method modifiers
 Method delegation
 Overriding attributes
 Roles and traits
 Meta programming


Perl School
 Object Oriented Programming with Perl
and Moose
 8th December
 Google Campus, London
 Full day course
 £30
 http://perlschool.co.uk/


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Introduction to OO Perl with Moose

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