Introduction to Moose

YAPC 2009

Moose Summed Up

Declarative OO sugar
Introspectable
Extensible (MooseX::* on CPAN)

Moose Background

Created by Stevan Little, first released in 2006
Moose builds on Perl 5's native OO
Borrows ideas from other languages, notably Perl 6
Provides semantics for common operations

Part 0: Moose Concepts

Classes

Classes have ...
- Attributes
- Methods
- Superclasses
- Method modifiers
- Constructor and destructor
- One metaclass object
Classes do roles

Class Example

package Person;
use Moose;

Poof, a Moose-based class!

Attributes

Aka property, slot, field, member variable
A piece of data owned by an object

Attributes

Attributes have ...
- Access-control (read-only vs read-write)
- An optional type
- Accessor methods
- Delegation methods
- Optional default value
- Many more features
Stored in the object, but don't worry about that

Attribute Example

package Person;
use Moose;

has first_name => ( is => 'rw' );

Methods

Nothing fancy here, just Perl subroutines

package Person;
use Moose;

sub greet { ... }

Roles

Classes do (or consume) roles
Similar to mixins and Java interfaces

Roles

Like classes, can have attributes, methods, do roles
Roles can require methods
Roles are composed (flattened) into classes

Role Example

package HasPermissions;
use Moose::Role;

has is_admin => ( is => 'rw' );

Role Example

And then ...

package Person;
use Moose;

with 'HasPermissions';

Method Modifiers

AKA advice
"Before foo(), do this first"
"Do this after foo()
"Put this code around foo()"

Before and After

before 'foo'
    => sub { warn 'About to call foo()' };

after  'foo'
    => sub { warn 'Leaving foo()' };

Around

around 'foo' => sub {
    my $real_foo = shift;
    my $self     = shift;

    warn 'Just before foo()';
    my @return =
        $self->$real_foo( @_, bar => 42 );

    return ( @return, 'modify return values' );
};

Type Constraints

NOT A FULL-BLOWN TYPE SYSTEM!
But still darn useful
Constrain attribute values
Coerce from other types

Type Constraint Example

package Person;
use Moose;

has weight => (
    is  => 'ro',
    isa => 'Int',
);

# kaboom
Person->new( weight => 'fat' );

Delegation

Attributes can define delegations
Lets you hide some implementation details
Fewer objects to chase around

Delegation

package Person;
use Moose;

has blog_uri => (
    is      => 'rw',
    isa     => 'URI',
    handles => { 'blog_hostname' => 'host' },
);

$person->blog_hostname;
# really calls $person->blog_uri->host

Constructors

Moose creates new() for you
Provide an optional BUILDARGS() and BUILD()

Destructors

Provide an optional DEMOLISH()

Moose Meta-API

Answers questions like ...
- What methods does this class have?
- What are its parents?
- What attributes does it have (including inherited attributes)?
- What roles does it do?
- Much, much, more

Moose Meta-API

Not just for introspection ...
- Add methods, attributes, roles, etc
- Extend and alter core features

Why Moose?

A quick bit of propaganda ...

With Moose

package Person;
use Moose;

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

Without Moose

package Person;
use strict;
use warnings;
use Carp 'confess';

sub new {
    my $class = shift;
    my %args  = @_;
    my $self  = {};

    if (exists $args{last_name}) {
        confess "Attribute (last_name) does not pass the type constraint because: "
                . "Validation failed for 'Str' with value $args{last_name}"
            if ref($args{last_name});
        $self->{last_nane} = $args{last_name};
    }

    return bless $self, $class;
}

sub last_name {
    my $self = shift;

    if (@_) {
        my $value = shift;
        confess "Attribute (last_name) does not pass the type constraint because: "
                . "Validation failed for 'Str' with value $value"
            if ref($value);
        $self->{last_name} = $value;
    }

    return $self->{last_name};
}

Side by side

package Person;
use Moose;

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

package Person;
use strict;
use warnings;
use Carp 'confess';

sub new {
    my $class = shift;
    my %args  = @_;
    my $self  = {};

    if (exists $args{last_name}) {
        confess "Attribute (last_name) does not pass the type constraint because: "
                . "Validation failed for 'Str' with value $args{last_name}"
            if ref($args{last_name});
        $self->{last_nane} = $args{last_name};
    }

    return bless $self, $class;
}

sub last_name {
    my $self = shift;

    if (@_) {
        my $value = shift;
        confess "Attribute (last_name) does not pass the type constraint because: "
                . "Validation failed for 'Str' with value $value"
            if ref($value);
        $self->{last_name} = $value;
    }

    return $self->{last_name};
}

Side by side

package Person;
use Moose;

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

package Person;
use strict;
use warnings;
use Carp 'confess';

sub new {
    my $class = shift;
    my %args  = @_;
    my $self  = {};

    if (exists $args{last_name}) {
        confess "Attribute (last_name) does not pass the type constraint because: "
                . "Validation failed for 'Str' with value $args{last_name}"
            if ref($args{last_name});
        $self->{last_nane} = $args{last_name};
    }

    return bless $self, $class;
}

sub last_name {
    my $self = shift;

    if (@_) {
        my $value = shift;
        confess "Attribute (last_name) does not pass the type constraint because: "
                . "Validation failed for 'Str' with value $value"
            if ref($value);
        $self->{last_name} = $value;
    }

    return $self->{last_name};
}

Side by side

package Person;
use Moose;

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

package Person;
use strict;
use warnings;
use Carp 'confess';

sub new {
    my $class = shift;
    my %args  = @_;
    my $self  = {};

    if (exists $args{last_name}) {
        confess "Attribute (last_name) does not pass the type constraint because: "
                . "Validation failed for 'Str' with value $args{last_name}"
            if ref($args{last_name});
        $self->{last_nane} = $args{last_name};
    }

    return bless $self, $class;
}

sub last_name {
    my $self = shift;

    if (@_) {
        my $value = shift;
        confess "Attribute (last_name) does not pass the type constraint because: "
                . "Validation failed for 'Str' with value $value"
            if ref($value);
        $self->{last_name} = $value;
    }

    return $self->{last_name};
}

Side by side

package Person;
use Moose;

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

package Person;
use strict;
use warnings;
use Carp 'confess';

sub new {
    my $class = shift;
    my %args  = @_;
    my $self  = {};

    if (exists $args{last_name}) {
        confess "Attribute (last_name) does not pass the type constraint because: "
                . "Validation failed for 'Str' with value $args{last_name}"
            if ref($args{last_name});
        $self->{last_nane} = $args{last_name};
    }

    return bless $self, $class;
}

sub last_name {
    my $self = shift;

    if (@_) {
        my $value = shift;
        confess "Attribute (last_name) does not pass the type constraint because: "
                . "Validation failed for 'Str' with value $value"
            if ref($value);
        $self->{last_name} = $value;
    }

    return $self->{last_name};
}

Side by side

package Person;
use Moose;

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

package Person;
use strict;
use warnings;
use Carp 'confess';

sub new {
    my $class = shift;
    my %args  = @_;
    my $self  = {};

    if (exists $args{last_name}) {
        confess "Attribute (last_name) does not pass the type constraint because: "
                . "Validation failed for 'Str' with value $args{last_name}"
            if ref($args{last_name});
        $self->{last_nane} = $args{last_name};
    }

    return bless $self, $class;
}

sub last_name {
    my $self = shift;

    if (@_) {
        my $value = shift;
        confess "Attribute (last_name) does not pass the type constraint because: "
                . "Validation failed for 'Str' with value $value"
            if ref($value);
        $self->{last_name} = $value;
    }

    return $self->{last_name};
}

Side by side

package Person;
use Moose;

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

package Person;
use strict;
use warnings;
use Carp 'confess';

sub new {
    my $class = shift;
    my %args  = @_;
    my $self  = {};

    if (exists $args{last_name}) {
        confess "Attribute (last_name) does not pass the type constraint because: "
                . "Validation failed for 'Str' with value $args{last_name}"
            if ref($args{last_name});
        $self->{last_nane} = $args{last_name};
    }

    return bless $self, $class;
}

sub last_name {
    my $self = shift;

    if (@_) {
        my $value = shift;
        confess "Attribute (last_name) does not pass the type constraint because: "
                . "Validation failed for 'Str' with value $value"
            if ref($value);
        $self->{last_name} = $value;
    }

    return $self->{last_name};
}

Side by side

5 lines 21 lines

92 characters 741 characters

package Person;
use Moose;

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

package Person;
use strict;
use warnings;
use Carp 'confess';

sub new {
    my $class = shift;
    my %args  = @_;
    my $self  = {};

    if (exists $args{last_name}) {
        confess "Attribute (last_name) does not pass the type constraint because: "
                . "Validation failed for 'Str' with value $args{last_name}"
            if ref($args{last_name});
        $self->{last_nane} = $args{last_name};
    }

    return bless $self, $class;
}

sub last_name {
    my $self = shift;

    if (@_) {
        my $value = shift;
        confess "Attribute (last_name) does not pass the type constraint because: "
                . "Validation failed for 'Str' with value $value"
            if ref($value);
        $self->{last_name} = $value;
    }

    return $self->{last_name};
}

Typo?

sub new {
    my $class = shift;
    my %args  = @_;
    my $self  = {};

    if (exists $args{last_name}) {
        confess "Attribute (last_name) does not pass the type constraint because: "
                . "Validation failed for 'Str' with value $args{last_name}"
            if ref($args{last_name});
        $self->{last_nane} = $args{last_name};
    }

    return bless $self, $class;
}

Typo?

if (exists $args{last_name}) {
    confess "Attribute (last_name) does not pass the type constraint because: "
            . "Validation failed for 'Str' with value $args{last_name}"
        if ref($args{last_name});
    $self->{last_nane} = $args{last_name};
}

Typo?

$self->{last_nane} = $args{last_name};

Typo?

$self->{last_nane}

Why Moose?

package Person;
use Moose;

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

Part 1: Moose Classes

Moose Classes

Moose classes are Perl packages which use Moose

Moose.pm and Your Class

package Person;
use Moose;

Moose.pm provides declarative sugar
Turns on strict and warnings
Creates metaclasses for your class: Person->meta
Moose classes automatically inherit from Moose::Object

What `Moose::Object` Provides

Constructor - new()
Calls your BUILDARGS() and/or BUILD()
Calls your DEMOLISH during object destruction

BUILDARGS

Takes @_, returns a hash reference of attribute names/value
Accepts a hash or hashref; throws otherwise
Provide your own for other cases
Always call $class->SUPER::BUILDARGS(@_) as a fallback!

BUILDARGS Example

package Person;
use Moose;

sub BUILDARGS {
    my $class = shift;

    if ( @_ == 1 && ! ref $_[0] ) {
        return { ssn => $_[0] };
    }

    return $class->SUPER::BUILDARGS(@_);
}

Person->new('123-45-6789')

BUILD

Called after object is created, before new returns
Chance to do more complex validation, set complex attributes
Called in reverse inheritance order, parents to children
Return value is ignored

BUILD Example

package Person;
use Moose;

sub BUILD {
    my $self = shift;

    if ( $self->country_of_residence
         eq 'USA' ) {
        die 'All US residents'
            . ' must have an SSN'
            unless $self->has_ssn;
    }
}

DEMOLISH

Like DESTROY, but Moose makes sure all DEMOLISH methods in a hierarchy are called
Called in normal inheritance order, children to parents

extends

extends is sugar for declaring parent classes

package Employee;
use Moose;
extends 'Person';

extends

Each call to extends resets your parents

Wrong

package EvilEmployee;
use Moose;
extends 'Person';
extends 'Thief';

Right

package EvilEmployee;
use Moose;
extends 'Person', 'Thief';

Extending un-Moose-y Parents

package My::LWP;
use Moose;
extends 'LWP';

No Moose::Object, so ...
- No attribute-handling new()
- No BUILDARGS() or BUILD()
- No DEMOLISH()
But see MooseX::NonMoose for a workaround

`override` and `super`

override is another method modifier
An alternative to Perl's SUPER::

`override` and `super`

package Employee;
use Moose;

extends 'Person';

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

    die "Pay me first" unless $self->got_paid;
    super();
};

Caveat `super`

Mostly like $self->SUPER::work(@_)
But cannot change @_!
Binds the parent's method at compile time

Minimal Attributes

has 'foo'
Use is => 'ro' or is => 'rw'
Attributes without "is" have no accessors

Read-write attributes

package Person;
use Moose;

has first_name => ( is => 'rw' );

my $person =
    Person->new( first_name => 'Dave' );

$person->first_name('Stevan');
print $person->first_name; # Stevan

Read-only attributes

package Person;
use Moose;

has first_name => ( is => 'ro' );

my $person =
    Person->new( first_name => 'Dave' );

$person->first_name('Stevan');
print $person->first_name; # Dave

There is More to Come

Attributes have a lot of features

Cleaning Up Moose Droppings

package Person;
use Moose;

# true
Person->can('extends');

Not very hygienic

Cleaning Up Moose Droppings

package Person;
use Moose;

...

no Moose;

# false
Person->can('extends');

No Moose

no Moose at the end of a package is a best practice
Just do it

Immutability

Stevan's Incantation of Fleet-Footedness

package Person;
use Moose;

__PACKAGE__->meta->make_immutable;

What `make_immutable` does

Magic
Uses eval to "inline" a constructor
Memoizes a lot of meta-information
Makes loading your class slower
Makes object creation much faster

When to Immutabilize?

Almost always
Startup time vs execution time

Classes Summary

use Moose
Class->meta
Moose::Object base class
extends, override, and super
Simple attributes: has, is => 'ro', & is => 'rw'
no Moose
__PACKAGE__->meta->make_immutable

Questions?

Exercises

# cd exercises
$ perl bin/prove -lv t/00-prereq.t

Missing anything? Install it. (see tarballs/)

# perl bin/prove -lv t/01-classes.t

Iterate til this passes all its tests

Part 2: Roles

Just What Is a Role?

Mixin? Interface? Trait?
Yes ... and more!

Roles Can Have State and Behavior

package HasPermissions;
use Moose::Role;

# state
has access_level => ( is => 'rw' );

# behavior
sub can_access {
    my $self     = shift;
    my $required = shift;

    return $self->access_level >= $required;
}

Roles Can Define Interfaces

package Printable;
use Moose::Role;

requires 'as_string';

Roles Can Do All Three

package Printable;
use Moose::Role;

requires 'as_string';

has has_been_printed => ( is => 'rw'  );

sub print {
    my $self = shift;
    print $self->as_string;
    $self->has_been_printed(1);
}

Classes Consume Roles

package Person;
use Moose;

with 'HasPermissions';

Classes Consume Roles

my $person = Person->new(
    first_name => 'Kenichi',
    last_name => 'Asai',
    access_level => 42,
);

print $person->full_name
    . ' has '
    . $person->can_access(42)
        ? 'great power'
        : 'little power';

Roles in Practice

Consuming a role =~ inlining the role

In Other Words ...

package Person;
use Moose;

with 'Printable';

In Other Words ...

package Person;
use Moose;

with 'Printable';

has has_been_printed => ( is => 'rw'  );

sub print {
    my $self = shift;
    print $self->as_string;
    $self->has_been_printed(1);
}

Except

Role consumption is introspectable

if ( Person->does('Printable') ) { ... }

# or ...

if ( Person->meta->does('Printable') ) { ... }

These Names Are the Same

What if a role and class define the same method?
A class's local methods win over the role's
The role's methods win over the class's inherited methods

Conflicts Between Roles

Two roles with a method of the same name
Generates a compile-time error when consumed by a class

Conflict Example

package IsFragile;
use Moose::Role;

sub break { ... }

package CanBreakdance;
use Moose::Role;

sub break { ... }

Conflict Example

package FragileDancer;
use Moose;

with 'IsFragile', 'CanBreakdance';

Only one with!

Conflict Resolution

The consuming class must resolve the conflict by implementing the method
Can use some combination of method exclusion and aliasing

Method Aliasing

package FragileDancer;
use Moose;

with 'IsFragile' =>
         { alias =>
               { break => 'break_bone' } },
     'CanBreakdance' =>
         { alias =>
               { break => 'break_it_down' } };

Renames the roles' methods
Still conflicts, need to exclude as well

Method Exclusion

package FragileDancer;
use Moose;

with 'IsFragile' =>
         { alias =>
               { break => 'break_bone' },
           exclude => 'break' },
     'CanBreakdance' =>
         { alias =>
               { break => 'break_dance' },
           exclude => 'break' };

And then ...

package FragileDancer;
use Moose;

sub break {
    my $self = shift;

    $self->break_dance;
    if ( rand(1) < 0.5 ) {
        $self->break_bone;
    }
}

Still Full of Fail

Roles are also about semantics!
We've fulfilled the letter and lost the spirit
Roles have a meaning
Think twice before blindly aliasing and excluding methods!

Hot Role-on-Role Action

package Comparable;
use Moose::Role;

requires 'compare';

Hot Role-on-Role Action

package TestsEquality;
use Moose::Role;

with 'Comparable';

sub is_equal {
    my $self = shift;
    return $self->compare(@_) == 0;
}

And then ...

package Integer;
use Moose;

with 'TestsEquality';

# Satisfies the Comparable role
sub compare { ... }

Integer->does('TestsEquality'); # true
Integer->does('Comparable'); # also true!

Name Conflicts Between Roles

package HasSubProcess;
use Moose::Role;

sub execute { ... }

package Killer;
use Moose::Role;

with 'HasSubProcess';

sub execute { ... }

Delayed Conflict

package StateOfTexas;
with 'Killer';

StateOfTexas must implement its own execute
But loading the Killer role by itself does not cause an error

Roles as Interfaces

Roles can require methods of their consumers
Compile-time checks
Method must exist when the role is consumed

The Attribute Gotcha

package HasSize;
use Moose::Role;

requires 'size';

package Shirt;
use Moose;

with 'HasSize';

has size => ( is => 'ro' );

The Attribute Gotcha Workaround

package HasSize;
use Moose::Role;

requires 'size';

package Shirt;
use Moose;

has size => ( is => 'ro' );

with 'HasSize';

Compile-time Is a Lie

Really, it's package load time
That's run-time, but before the "real" run-time
Moose does not rewire Perl, it's just sugar!
(but MooseX::Declare does rewire Perl)

Enforcing Roles

package Comparison;
use Moose;

has [ 'left', 'right' ] => (
    is   => 'ro',
    does => 'Comparable',
);

A sneak peek at type constraints

Roles Can Be Applied to Objects

use Moose::Util qw( apply_all_roles );

my $fragile_person = Person->new( ... );
apply_all_roles( $fragile_person, 'IsFragile' );

Does not change the Person class
Works with non-Moose classes, great for monkey-patching!

Roles Are Dirty Too

Once again, clean up those Moose droppings

package Comparable;
use Moose::Role;

requires 'compare';

no Moose::Role;

But roles cannot be made immutable

The Zen of Roles

Roles represent discrete units of ...
- state
- behavior
- interface
Roles are shareable between unrelated classes
Roles are what a class does, not what it is
Roles add functionality, inheritance specializes

Abstract Examples

Human @ISA Animal
Human does Toolmaker (as does Chimpanzee)
Car @ISA Vehicle
Car does HasEngine

Real Examples

Objects representing SQL database components and queries
- Schema, Table, Column, ColumnAlias
- Select, Insert, Update, Delete

Real Examples

Column and ColumnAlias both do ColumnLike
ColumnLike things can be used in certain parts of queries
All queries do HasWhereClause
Select does Comparable and Selectable (for subselects)
A where clause requires its components to do Comparable

Roles Summary

Roles can define an interface with requires
Roles can have state (attributes) and behavior (methods)
Roles can mix interface, state, & behavior
Roles are composed (flattened) into classes
Roles can do other roles
Roles can be used as a type in APIs (must do Comparable)

Questions?

Exercises

# cd exercises
# perl bin/prove -lv t/02-roles.t

Iterate til this passes all its tests

Part 3: Basic Attributes

Attributes Are Huge

Moose's biggest feature
The target of many MooseX modules

Quick Review

Declared with has
Read-only or read-write

package Shirt;
use Moose;

has 'color'     => ( is => 'ro' );
has 'is_ripped' => ( is => 'rw' );

Required-ness

Required means "must be passed to the constructor"
But can be undef

Required-ness

package Person;
use Moose;

has first_name => (
    is       => 'ro',
    required => 1,
);

Person->new( first_name => undef ); # ok
Person->new(); # kaboom

Default and Builder

Attributes can have defaults
Simple non-referecne scalars (number, string)
Subroutine reference
A builder method

Default

Can be a non-reference scalar (including undef)

package Person;
use Moose;

has bank => (
    is      => 'rw',
    default => 'Spire FCU',
);

Default

Can be a subroutine reference

package Person;
use Moose;

has bank => (
    is      => 'rw',
    default =>
        sub { Bank->new(
                  name => 'Spire FCU' ) },
);

Default as a Subroutine Reference

Called as a method on the object
Called anew for each object

Why No Other Reference Types?

package Person;
use Moose;

has bank => (
    is      => 'rw',
    default => Bank->new(
                   name => 'Spire FCU' ),
);

Now every person shares the same Bank object!

Defaulting to an Empty Reference

package Person;
use Moose;

has packages => (
    is      => 'rw',
    default => sub { [] },
);

What if I Want to Share?

package Person;
use Moose;

my $highlander_bank =
    Bank->new( name => 'Spire FCU' );

has bank => (
    is      => 'rw',
    default => sub { $highlander_bank },
);

Builder

A method name which returns the default

Builder

package Person;
use Moose;

has bank => (
    is      => 'rw',
    builder => '_build_bank',
);

sub _build_bank {
    my $self = shift;
    return Bank->new( name => 'Spire FCU' );
}

Default vs Builder

Use default for simple scalars
Use default to return empty references
Use default for very trivial subroutine references
Use builder for everything else

Builder Bonuses

Can be overridden and method modified, because it's called by name
Roles can require a builder

Role Requires Builder

package HasBank;
use Moose::Role;

requires '_build_bank';

has bank => (
    is      => 'ro',
    builder => '_build_bank',
);

Lazy, Good for Nothing Attributes

Normally, defaults are generated during object construction
This can be expensive
We want to default to $self->size * 2, but attribute initialization order is unpredictable
Use lazy attributes!

The Power of Dynamic Defaults

package Person;
use Moose;

has shoe_size => (
    is => 'ro',
);

The Power of Dynamic Defaults

has shoes => (
    is      => 'ro',
    lazy    => 1,
    builder => '_build_shoes',
);

sub _build_shoes {
    my $self = shift;

    return Shoes->new(
        size => $_[0]->shoe_size );
}

Lazy is Good

Lazy defaults are executed when the attribute is read
Can see other object attributes
Still need to watch out for circular laziness

Clearer and Predicate

Attributes can have a value, including undef, or not
Can clear the value with a clearer method
Can check for the existence of a value with a predicate method
By default, these methods are not created

Clearer and Predicate

package Person;
use Moose;

has account => (
    is        => 'ro',
    lazy      => 1,
    builder   => '_build_account',
    clearer   => '_clear_account',
    predicate => 'has_account',
);

Clearer and Lazy Defaults

Lazy defaults are good for computed attributes
Clear the attribute when the source data changes
Recalculated at next access

Renaming constructor arguments

By default, constructor names = attribute names
Use init_arg to change this
Set init_arg => undef to make it unconstructable

Some `init_arg` examples

package Person;
use Moose;

has shoe_size => (
    is       => 'ro',
    init_arg => 'foot_size',
);

Person->new( shoe_size => 13 );

my $person =
    Person->new( foot_size => 13 );
print $person->shoe_size;

Some `init_arg` examples

package Person;
use Moose;

has shoes => (
    is       => 'ro',
    init_arg => undef,
);

Person->new( shoes => Shoes->new );

Why Set `init_arg => undef`?

Use this with a lazy default for attributes-as-cache
Compute the value as needed
Ensure that it is always generated correctly (not set by constructor)
Use triggers or method modifiers (coming soon) to clear the value

Attribute Inheritance

By default, subclasses inherit attribute as-is
Can change some attribute parameters in subclasses
- default
- builder
- required
- lazy
- others we've not yet covered

Attribute Inheritance Example

package Employee;
use Moose;

extends 'Person';

has '+first_name' => (
    default => 'Joe',
);

Attribute Inheritance Warning

An attribute is a contract about a class's API
Don't break that contract in a subclass
Especially important in the context of types

Changing Accessor Names

package Person;
use Moose;

has first_name => (
    accessor => 'first_name',
);

The long-hand version of is => 'rw'

Changing Accessor Names

package Person;
use Moose;

has first_name => (
    reader => 'first_name',
    writer => undef,
);

The long-hand version of is => 'ro'

Changing Accessor Names

package Person;
use Moose;

has first_name => (
    reader => 'get_first_name',
    writer => 'set_first_name,
);

Changing Accessor Names

package Person;
use Moose;

has first_name => (
    is     => 'rw',
    writer => '_first_name',
);

Can also mix-and-match

ETOOMUCHTYPING

MooseX::FollowPBP
get_foo and set_foo
MooseX::SemiAffordanceAccessor
foo and set_foo

ETOOMUCHTYPING

package Person;
use Moose;
use MooseX::SemiAffordanceAccessor;

has first_name => (
    is => 'rw',
);

Creates first_name and set_first_name

Basic Attributes Summary

Attributes can be required
Attributes can have a default or builder
Attributes with a default or builder can be lazy
Attributes can have a clearer and/or predicate
An attribute's constructor name can be changed with init_arg
A subclass can alter its parents' attributes
Attribute accessor names can be changed

Questions?

Exercises

# cd exercises
# perl bin/prove -lv t/03-basic-attributes.t

Iterate til this passes all its tests

Part 4: Method Modifiers

What is a Method Modifier

Apply to an existing method
... from a parent class, the current class, or a role
Roles can provide modifiers that are applied at composition time

What is a Method Modifier

"Inject" behavior
Add behavior to generated methods (accessors, delegations)
Provide roles which modify existing behavior

Before and After

Simplest modifiers - before and after
Guess when they run!

Uses for `before`

As a pre-call check

package Person;
use Moose;

before work => sub {
    my $self = shift;
    die 'I have no job!'
        unless $self->has_title;
};

Uses for `before`

Logging/Debugging

package Person;
use Moose;

before work => sub {
    my $self = shift;
    return unless $DEBUG;

    warn "Called work on ", $self->full_name,
         "with the arguments: [@_]\n";
};

Uses for `after`

Also works for logging/debugging
Post-X side-effects (recording audit info)

package Person;
use Moose;

after work => sub {
    my $self = shift;
    $self->work_count(
        $self->work_count + 1 );
};

Other Uses

Modifiers are useful for adding behavior to generated methods

Other Uses Example

has password => (
     is      => 'rw',
     clearer => 'clear_password',
);

has hashed_password => (
     is      => 'ro',
     builder => '_build_hashed_password',
     clearer => '_clear_hashed_password',
);

after clear_password => sub {
    my $self = shift;
    $self->_clear_hashed_password;
};

`before` and `after` Limitations

Cannot alter method parameters
Cannot alter return value
But can throw an exception

The `around` Modifier

The big gun
Can alter parameters and/or return values
Can skip calling the wrapped method entirely

The power of `around`

around insert => sub {
    my $orig = shift;
    my $self = shift;

    $self->_validate_insert(@_);

    my $new_user =
        $self->$orig(
            $self->_munge_insert(@_) );

    $new_user->_assign_uri;

    return $new_user;
};

Modifier Order

Before runs order from last to first
After runs in order from first to last
Around runs in order from last to first

Modifier Order Illustrated

before 2
 before 1
  around 2
   around 1
    wrapped method
   around 1
  around 2
 after 1
after 2

Modifiers in Roles

Roles can use these modifiers
Very powerful!

Modifiers in Roles

package IsUnreliable;
use Moose::Role;

requires 'run';

around run => sub {
    my $orig = shift;
    my $self = shift;

    return if rand(1) < 0.5;

    return $self->$orig(@_);
};

Augment and Inner

Inverted super
From least- to most-specific
Grandparent to parent to child
Not allowed in roles

Augment and Inner

package Document;

sub xml { '<doc>' . inner() . '</doc>' }

package Report;
extends 'Document';

augment xml => { title() . inner() . summary() };

package TPSReport;
extends 'Report';

augment xml => { tps_xml() . inner() };

Augment and Inner

When we call $tps->xml ...
- Document->xml
- Report->xml
- TPSReport->xml

Augment and Inner Usage

Call inner() to "fill in the blank"
Requires designing for subclassing
Call inner() in the terminal class, just in case

Method Modifiers Summary

Use before and after for ...
- logging
- pre- or post-validation
- to add behavior to generated methods
These two modifiers cannot change parameters or return values

Method Modifiers Summary

Use around to ...
- alter parameters passed to the original method
- alter the return value of the original method
- not call the original method at all (or call a different method)

Method Modifiers Summary

When using modifiers in a role, require the modified method
Use augment and inner to invert the normal subclassing flow ...
- Least- to most-specific (parents to children)
- Build in "insertability" (stick more stuff in the "middle")
Always call inner in the most specific subclass to allow for future extension

Questions?

Exercises

# cd exercises
# perl bin/prove -lv t/04-method-modifiers.t

Iterate til this passes all its tests

Part 5: Types

Part 6: Advanced Attributes

Weak References

A weak reference lets you avoid circular references
Weak references do not increase the reference count

Circular Reference Illustrated

my $foo = {};
my $bar = { foo => $foo };
$foo->{bar} = $bar;

Neither $foo nor $bar go out of scope
(until the program exits)

Weakening Circular References

use Scalar::Util qw( weaken );

my $foo = {};
my $bar = { foo => $foo };
$foo->{bar} = $bar;
weaken $foo->{bar}

When $bar goes out of scope, $foo->{bar} becomes undef

Circular References in Attributes

package Person;
use Moose;

has name   => ( is => 'ro' );
has friend => ( is => 'rw' );

my $alice = Person->new( name => 'Alice' );
my $bob   = Person->new( name => 'Bob' );
$bob->friend($alice);
$alice->friend($bob);

The Fix

package Person;
use Moose;

has name   => ( is => 'ro' );
has friend => ( is => 'rw', weak_ref => 1 );

my $alice = Person->new( name => 'Alice' );
my $bob   = Person->new( name => 'Bob' );
$bob->friend($alice);
$alice->friend($bob);

Under the Hood

A weak_ref attribute calls weaken ...
- during object construction
- when the attribute is set via a writer

Triggers

A code reference run after an attribute is set
Like an after modifier, but makes intentions clearer

Gross

after salary_level => {
    my $self = shift;
    return unless @_;
    $self->clear_salary;
};

Use a Trigger Instead

Cleaner

has salary_level => (
    is      => 'rw',
    trigger => sub { $_[0]->clear_salary },
);

Delegation

Attributes can be objects
Delegation transparently calls methods on those objects

Delegation Examples

package Person;

has lungs => (
    is      => 'ro',
    isa     => 'Lungs',
    handles => [ 'inhale', 'exhale' ],
);

Creates $person->inhale and ->exhale methods
Internally calls $person->lungs->inhale

Why Delegation?

Reduce the number of classes exposed
Re-arrange class internals -
turn a method into an attribute with delegation
Provide convenenience methods

Moose's `handles` Parameter

Accepts many arguments ...
- Array reference - list of methods to delegate as-is
- Hash reference - map of method names
- Regex - delegates all matching methods
- Role name - delegates all methods in the role
- Sub reference - does something complicated ;)

Array Reference

Takes each method name and creates a simple delegation from the delegating class to the delegatee attribute

Hash Reference

Mapping of names in the delegating class to the delegatee class

package Person;
use Moose;

has account => (
    is      => 'ro',
    isa     => 'BankAccount',
    handles => {
        receive_money => 'deposit',
        give_money    => 'withdraw',
    },
);

Hash Reference Detailed

    handles => {
        receive_money => 'deposit',
        give_money    => 'withdraw',
    },

$person->receive_money = $person->account->deposit
$person->give_money = $person->account->withdraw

Regex

package Person;
use Moose;

has name => (
    is      => 'ro',
    isa     => 'Name',
    handles => qr/.*/,
);

Creates a delegation for every method in the Name class
Excludes meta and methods inherited from Moose::Object

Role Name

package Auditor;
use Moose::Role;

sub record_change  { ... }
sub change_history { ... }

package Account;
use Moose;

has history => (
    is      => 'ro',
    does    => 'Auditor',
    handles => 'Auditor',
);

Role Name Detailed

Account gets delegate methods for each method in the Auditor role
- record_history
- change_history

Traits and Metaclasses

The ultimate in customization
Per attribute metaclasses
Per attribute roles applied to the attribute metaclass
Change the meta-level behavior

Traits and Metaclasses

The default metaclass is Moose::Meta::Attribute
Controls accessor generation, defaults, delegation, etc.
Adding a role to this metaclass (or replacing it) allows for infinite customization

Traits and Metaclasses

Can add/alter/remove attribute parameter (from has)
Can change behavior of created attribute

Simple Trait Example

package Person;
use Moose;
use MooseX::LabeledAttributes;

has ssn => (
    traits => [ 'Labeled' ],
    is     => 'ro',
    isa    => 'Str',
    label  => 'Social Security Number',
);

print Person->meta
            ->get_attribute('ssn')->label;

Simple Metaclass Example

package Person;
use Moose;
use MooseX::LabeledAttributes;

has ssn => (
    metaclass =>
        'MooseX::Meta::Attribute::Labeled',
    is        => 'ro',
    isa       => 'Str',
    label     => 'Social Security Number',
);

print Person->meta
            ->get_attribute('ssn')->label;

Traits vs Metaclass

Can apply any mix of traits to an attribute
But just one metaclass
Traits (aka roles) can cooperate
Metaclasses require you to pick just one

Advanced Attributes Summary

Use weak_ref to avoid circular references
Use trigger to do an action post-attribute write
Use delegations to hide "internal" objects
Traits and metaclasses let you extend Moose's core attribute features

Questions?

Exercises

# cd exercises
# perl bin/prove -lv t/06-advanced-attributes.t

Iterate til this passes all its tests

Part 7: Introspection

Part 8: A Tour of MooseX

Part 9: Writing Moose Extensions

Introduction to Moose

Introduction to Moose

YAPC 2009

Moose Summed Up

Moose Background

Part 0: Moose Concepts

Classes

Class Example

Attributes

Attributes

Attribute Example

Methods

Roles

Roles

Role Example

Role Example

Method Modifiers

Before and After

Around

Type Constraints

Type Constraint Example

Delegation

Delegation

Constructors

Destructors

Moose Meta-API

Moose Meta-API

Why Moose?

With Moose

Without Moose

Side by side

Side by side

Side by side

Side by side

Side by side

Side by side

Side by side

Typo?

Typo?

Typo?

Typo?

Why Moose?

Part 1: Moose Classes

Moose Classes

Moose.pm and Your Class

What Moose::Object Provides

BUILDARGS

BUILDARGS Example

BUILD

BUILD Example

DEMOLISH

extends

extends

Wrong

Right

Extending un-Moose-y Parents

override and super

override and super

Caveat super

Minimal Attributes

Read-write attributes

Read-only attributes

There is More to Come

Cleaning Up Moose Droppings

Cleaning Up Moose Droppings

No Moose

Immutability

What make_immutable does

When to Immutabilize?

Classes Summary

Questions?

Exercises

Part 2: Roles

Just What Is a Role?

Roles Can Have State and Behavior

Roles Can Define Interfaces

Roles Can Do All Three

Classes Consume Roles

Classes Consume Roles

Roles in Practice

What `Moose::Object` Provides

`override` and `super`

`override` and `super`

Caveat `super`

What `make_immutable` does

Some `init_arg` examples

Some `init_arg` examples

Why Set `init_arg => undef`?