merge trunk to pluggable errors

[gitmo/Moose.git] / lib / Moose / Intro.pod

=pod

=head1 NAME

Moose::Intro - What is Moose, and how do I use it?

=head1 WHAT IS MOOSE?

Moose is a I<complete> object system for Perl 5. If you've used a
modern object-oriented language (which Perl 5 definitely isn't), you
know they provide keywords for attribute declaration, object
construction, and inheritance. These keywords are part of the
language, and you don't care how they are implemented.

Moose aims to do the same thing for Perl 5 OO. We can't actually
create new keywords, but we do offer "sugar" that looks a lot like
them. More importantly, with Moose, you I<declaritively define> your
class, without needing to know about blessed hashrefs, accessor
methods, and so on.

Moose lets you focus on the I<logical> structure of your classes, so
you can focus on "what" rather than "how". With Moose, a class
definition should read like a list of very concise English sentences.

Moose is built in top of C<Class::MOP>, a meta-object protocol (aka
MOP). Using the MOP, Moose provides complete introspection for all
Moose-using classes. This means you can ask classes about their
attributes, parents, children, methods, etc., all using a well-defined
API. The MOP abstracts away tedious digging about in the Perl symbol
table, looking at C<@ISA> vars, and all the other crufty Perl tricks
we know and love (?).

Moose is based in large part on the Perl 6 object system, as well as
drawing on the best ideas from CLOS, Smalltalk, and many other
languages.

=head1 WHY MOOSE?

Moose makes Perl 5 OO both simpler and more powerful. It encapsulates
all the tricks of Perl 5 power users in high-level declarative APIs
which are easy to use, and don't require any special knowledge of how
Perl works under the hood.

Moose makes Perl 5 OO fun, accessible, and powerful. And if you want
to dig about in the guts, Moose lets you do that too, by using and
extending its powerful introspection API.

=head1 AN EXAMPLE

  package Person;

  use Moose;

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

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

This is a I<complete and usable> class definition!

  package User;

  use DateTime;
  use Moose;

  extends 'Person';

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

  has 'last_login' => (
      is      => 'rw',
      isa     => 'DateTime',
      handles => { 'date_of_last_login' => 'date' },
  );

  sub login {
      my $self = shift;
      my $pw   = shift;

      return 0 if $pw ne $self->password;

      $self->last_login( DateTime->now() );

      return 1;
  }

We'll leave the line-by-line explanation of this code to other
documentation, but you can see how Moose reduces common OO idioms to
simple declarative constructs.

=head2 Where's the Constructor?

One point of confusion that might come up with Moose is how it handles
object construction. B<You should not define a C<new()> method for
your classes!>

Moose will provide one for you. It will accept a hash or hash
reference of named parameters matching your attributes. This is just
another way in which Moose keeps your from worrying I<how> classes are
implemented. Simply define a class and you're ready to start creating
objects!

=head1 MOOSE CONCEPTS (VS "OLD SCHOOL" Perl)

In the past, you may not have thought too much about the difference
between packages and classes, attributes and methods, constructors vs
methods, etc. Part of what the MOP provides is well-defined
introspection features for each of those things, and in turn Moose
provides I<distinct> sugar for each of them. Moose also introduces
concepts that are uncommon (or entirely new) like roles, method
modifiers, and declarative delegation.

Knowing what these concepts mean in Moose-speak, and how they used to
be done in old school Perl 5 OO is a good way to start learning to use
Moose.

=head2 Class

When you say "use Moose" in a package, you are defining your package
as a class. At its simplest, a class will consist simply of attributes
and/or methods. It can also include roles, method modifiers, and more.

A class I<has> zero or more B<attributes>.

A class I<has> zero or more B<methods>.

A class I<may have> one or more superclasses (aka parent classes). A
class inherits from its superclass(es).

A class may I<have> B<method modifiers>. These modifiers can apply to
its own methods or methods that are inherited from its ancestors.

A class may I<do> one or more B<roles>.

A class I<has> a B<constructor> and a B<destructor>. These are
provided for you "for free" by Moose.

The B<constructor> accepts named parameters corresponding to the
class's attributes and uses them to initialize an B<object instances>.

A class I<has> a B<metaclass>, which in turn has B<meta-attributes>,
B<meta-methods>, and B<meta-roles>. This metaclass I<describes> the
class.

A class is usually analogous to a category of nouns, like "People" or
"Users".

  package Person;

  use Moose;
  # now it's a Moose class!

=head2 Attribute

An attribute is a property of the class that defines it. It I<always>
has a name, and it I<may have> a number of other defining
characteristics.

These characteristics may include a read/write flag, a B<type>,
accessor method names, B<delegations>, a default value, and more.

Attributes I<are not> methods, but defining them causes various
accessor methods to be created. At a minimum, a normal attribute will
always have a reader accessor method. Many attributes have things like
a writer method, clearer method, and predicate method ("has it been
set?").

An attribute may also define B<delegation>s, which will create
additional methods based on the delegation specification.

By default, Moose stores attributes in the object instance, which is a
hashref, I<but this is invisible to the author of a Moose-base class>!
It is best to think of Moose attributes as "properties" of the
I<opaque> B<object instance>. These properties are accessed through
well-defined accessor methods.

An attribute is usually analagous to specific feature of something in
the class's category. For example, People have first and last
names. Users have passwords and last login datetimes.

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

=head2 Method

A method is very straightforward. Any subroutine you define in your
class is a method.

Methods correspond to verbs, and are what your objects can do. For
example, a User can login.

  sub login { ... }

=head2 Roles

A role is something that a class I<does>. For example, a Machine class
might do the Breakable role, and a so could a Bone class. A role is
used to define some concept that cuts across multiple unrelated
classes, like "breakability", or "has a color".

A role I<has> zero or more B<attributes>.

A role I<has> zero or more B<methods>.

A role I<has> zero or more B<method modifiers>.

A role I<has> zero or more B<required methods>.

A required method is not implemented by the role. Instead, a required
method says "to use this Role you must implement this method".

Roles are I<composed> into classes (or other roles). When a role is
composed into a class, its attributes and methods are "flattened" into
the class. Roles I<do not> show up in the inheritance hierarchy. When
a role is composed, it's attributes and methods appear as if they were
defined I<in the consuming class>.

Role are somewhat like mixins or interfaces in other OO languages.

  package Breakable;

  use Moose::Role;

  has is_broken => (
      is  => 'rw',
      isa => 'Bool',
  );

  requires 'break';

  before 'break' => {
      my $self = shift;

      $self->is_broken(1);
  };

=head2 Method Modifiers

A method modifier is a way of defining an action to be taken when a
named method is called. Think of it as a hook on the named method. For
example, you could say "before calling C<login()>, call this modifier
first". Modifiers come in different flavors like "before", "after",
"around", and "augment", and you can apply more than one modifier to
a single method.

Method modifiers are often used as an alternative to overriding a
method in a parent class. They are also used in roles as a way of
modifying methods in the consuming class.

Under the hood, a method modifier is just a plain old Perl subroutine
that gets called before or after (or around, etc.) some named method.

  before 'login' => sub {
      my $self = shift;
      my $pw   = shift;

      warn "Called login() with $pw\n";
  };

=head2 Type

Moose also comes with a (miniature) type system. This allows you to
define types for attributes. Moose has a set of built-in types based
on what Perl provides, such as "Str", "Num", "Bool", "HashRef", etc.

In addition, every class name in your application can also be used as
a type name. We saw an example using "DateTime" earlier.

Finally, you can define your own types, either as subtypes or entirely
new types, with their own constraints. For example, you could define a
type "PosInt", a subtype of "Int" which only allows positive numbers.

=head2 Delegation

Moose attributes provide declarative syntax for defining
delegations. A delegation is a method which delegates the real work to
some attribute of the class.

You saw this in the User example, where we defined a delegation for
the C<date_of_last_login()> method. Under the hood, this simple calls
C<date()> on the User object's C<last_login> attribute.

=head2 Constructor

A constructor creates an B<object instance> for the class. In old
school Perl, this was usually done by defining a method called
C<new()> which in turn called C<bless> on a reference.

With Moose, this C<new()> method is created for you, and it simply
does the right thing. You should never need to define your own
constructor!

Sometimes you want to do something whenever an object is created. In
those cases, you can provide a C<BUILD()> method in your class. Moose
will call this for you after creating a new object.

=head2 Destructor

This is a special method called when an object instance goes out of
scope. You can specialize what your class does in this method if you
need to, but you usually don't.

With old school Perl 5, this is the C<DESTROY()> method, but with
Moose it is the C<DEMOLISH()> method.

=head2 Object Instance

An object instance is a specific noun in the class's "category". For
example, one specific Person or User. An instance is created by the
class's B<constructor>.

An instance has values for its attributes. For example, a specific
person has a first and last name,

In old school Perl 5, this is often a blessed hash reference. With
Moose, you should never need to know what your object instance
actually is. (ok, it's usually a blessed hashref with Moose too)

=head2 Moose VS Old School Summary

=over 4

=item * Class

A package with no introspection other than mucking about in the symbol
table.

With Moose, you get well-defined declaration and introspection.

=item * Attributes

Hand-written accessor methods, symbol table hackery, or a helper
module like C<Class::Accessor>.

With Moose, these are declaritively defined, and distinct from
methods.

=item * Method

These are pretty much the same in Moose as in old school Perl.

=item * Roles

C<Class::Trait> or C<Class::Role>, or maybe C<mixin.pm>.

With Moose, they're part of the core feature set, and are
introspectable like everything else.

=item * Method Modifiers

Could only be done through serious symbol table wizardry, and you
probably never saw this before (at least in Perl 5).

=item * Type

Hand-written parameter checking in your C<new()> method and accessors.

With Moose, you define types declaratively, and then use them by name
in your attributes.

=item * Delegation

C<Class::Delegation> or C<Class::Delegator>, but probably even more
hand-written code.

With Moose, this is also declarative.

=item * Constructor

A C<new()> method which calls C<bless> on a reference.

Comes for free when you define a class with Moose.

=item * Destructor

A C<DESTROY()> method.

With Moose, this is called C<DEMOLISH()>.

=item * Object Instance

A blessed reference, usually a hash reference.

With Moose, this is an opaque thing which has a bunch of attributes
and methods, as defined by its class.

=item * Immutabilization

Moose comes with a feature called "immutabilization". When you make
your class immutable, it means you're done adding methods, attributes,
roles, etc. This lets Moose optimize your class with a bunch of
extremely dirty in-place code generation tricks that speed up things
like object construction and so on.

=back

=head1 META WHAT?

A metaclass is a class that describes classes. With Moose, every class
you define gets a C<meta()> method. It returns a L<Moose::Meta::Class>
object, which has an introspection API that can tell you about the
class it represents.

  my $meta = User->meta();

  for my $attribute ( $meta->compute_all_applicable_attributes ) {
      print $attribute->name(), "\n";

      if ( $attribute->has_type_constraint ) {
          print "  type: ", $attribute->type_constraint->name, "\n";
      }
  }

  for my $method ( $meta->compute_all_applicable_methods ) {
      print $method->name, "\n";
  }

Almost every concept we defined earlier has a meta class, so we have
L<Moose::Meta::Class>, L<Moose::Meta::Attribute>,
L<Moose::Meta::Method>, L<Moose::Meta::Role>,
L<Moose::Meta::TypeConstraint>, L<Moose::Meta::Instance>, and so on.

=head1 BUT I NEED TO DO IT MY WAY!

One of the great things about Moose, is that if you dig down and find
that it does something the "wrong way", you can change it by extending
a metaclass. For example, you can have arrayref based objects, you can
make your constructors strict (no unknown params allowed!), you can
define a naming scheme for attribute accessors, you can make a class a
Singleton, and much, much more.

Many of these extensions require surprisingly small amounts of code,
and once you've done it once, you'll never have to hand-code "your way
of doing things" again. Instead you ll just load your favorite
extensions.

  package MyWay::User;

  use Moose;
  use MooseX::StrictConstructor
  use MooseX::MyWay;

  has ...;


=head1 JUSTIFICATION

If you're still still asking yourself "Why do I need this?", then this
section is for you.

=over 4

=item Another object system!?!?

Yes, I know there has been an explosion recently of new ways to
build objects in Perl 5, most of them based on inside-out objects
and other such things. Moose is different because it is not a new
object system for Perl 5, but instead an extension of the existing
object system.

Moose is built on top of L<Class::MOP>, which is a metaclass system
for Perl 5. This means that Moose not only makes building normal
Perl 5 objects better, but it also provides the power of metaclass
programming.

=item Is this for real? Or is this just an experiment?

Moose is I<based> on the prototypes and experiments Stevan did for the
Perl 6 meta-model. However, Moose is B<NOT> an experiment or
prototype; it is for B<real>.

=item Is this ready for use in production?

Yes.

Moose has been used successfully in production environments by several
people and companies. There are Moose applications which have been in
production with little or no issue now for well over two years. We
consider it highly stable and we are commited to keeping it stable.

Of course, in the end, you need to make this call yourself. If you
have any questions or concerns, please feel free to email Stevan, the
moose@perl.org list, or just stop by irc.perl.org#moose and ask away.

=item Is Moose just Perl 6 in Perl 5?

No. While Moose is very much inspired by Perl 6, it is not itself Perl
6.  Instead, it is an OO system for Perl 5. Stevan built Moose because
he was tired of writing the same old boring Perl 5 OO code, and
drooling over Perl 6 OO. So instead of switching to Ruby, he wrote
Moose :)

=item Wait, I<post> modern, I thought it was just I<modern>?

Stevan read Larry Wall's talk from the 1999 Linux World entitled
"Perl, the first postmodern computer language" in which he talks about
how he picked the features for Perl because he thought they were cool
and he threw out the ones that he thought sucked. This got him
thinking about how we have done the same thing in Moose. For Moose, we
have "borrowed" features from Perl 6, CLOS (LISP), Smalltalk, Java,
BETA, OCaml, Ruby and more, and the bits we didn't like (cause they
sucked) we tossed aside. So for this reason (and a few others) Stevan
has re-dubbed Moose a I<postmodern> object system.

Nuff Said.

=back

=head1 WHAT NEXT?

So you're sold on Moose. Time to learn how to really use it.

We recommend that you start with the L<Moose::Cookbook>. If you work
your way through all the recipes under the basics section, you should
have a pretty good sense of how Moose works, and all of its basic OO
features.

After that, check out the Role recipes. If you're really curious, go
on and read the Meta and Extending recipes, but those are mostly there
for people who want to be Moose wizards and change how Moose works.

If you want to see how Moose would translate directly old school Perl
5 OO code, check out the L<Moose::Unsweetened>.

=head1 AUTHOR

Dave Rolsky E<lt>autarch@urth.orgE<gt> and Stevan Little
E<lt>stevan@iinteractive.comE<gt>

=head1 COPYRIGHT AND LICENSE

Copyright 2008 by Infinity Interactive, Inc.

L<http://www.iinteractive.com>

This library is free software; you can redistribute it and/or modify
it under the same terms as Perl itself.

=cut