Another pass at the FAQ for legibility

[gitmo/Moose.git] / lib / Moose / Cookbook / FAQ.pod

=pod

=head1 NAME

Moose::Cookbook::FAQ - Frequently asked questions about Moose

=head1 FREQUENTLY ASKED QUESTIONS

=head2 Module Stability

=head3 Is Moose "production ready"?

Yes! Many sites with household names are using Moose to build
high-traffic services. Countless others are using Moose in
production.

As of this writing, Moose is a dependency of several hundred CPAN
modules. L<http://cpants.perl.org/dist/used_by/Moose>

=head3 Is Moose's API stable?

Yes. The sugary API, the one 95% of users will interact with, is
B<very stable>. Any changes will be B<100% backwards compatible>.

The meta API is less set in stone. We reserve the right to tweak
parts of it to improve efficiency or consistency. This will not be
done lightly. We do perform deprecation cycles. We I<really>
do not like making ourselves look bad by breaking your code.
Submitting test cases is the best way to ensure that your code is not
inadvertantly broken by refactoring.

=head3 I heard Moose is slow, is this true?

Again, this one is tricky, so Yes I<and> No.

Firstly, I<nothing> in life is free, and some Moose features
do cost more than others. It is also the policy of Moose to
B<only charge you for the features you use>, and to do our
absolute best to not place any extra burdens on the execution
of your code for features you are not using. Of course using
Moose itself does involve some overhead, but it is mostly
compile time. At this point we do have some options available
for getting the speed you need.

Currently we provide the option of making your classes immutable
as a means of boosting speed. This will mean a slightly larger compile
time cost, but the runtime speed increase (especially in object
construction) is pretty significant.

We are regularly converting the hotspots of L<Class::MOP> to XS.
Florian Ragwitz and Yuval Kogman are currently working on a way
to compile your accessors and instances directly into C, so that
everyone can enjoy blazing fast OO.

=head3 When will Moose 1.0 be ready?

Moose is ready now! Stevan Little declared 0.18, released in March 2007,
to be "ready to use".

=head2 Constructors

=head3 How do I write custom constructors with Moose?

Ideally, you should never write your own C<new> method, and should
use Moose's other features to handle your specific object construction
needs. Here are a few scenarios, and the Moose way to solve them;

If you need to call initialization code post instance construction,
then use the C<BUILD> method. This feature is taken directly from
Perl 6. Every C<BUILD> method in your inheritance chain is called
(in the correct order) immediately after the instance is constructed.
This allows you to ensure that all your superclasses are initialized
properly as well. This is the best approach to take (when possible)
because it makes subclassing your class much easier.

If you need to affect the constructor's parameters prior to the
instance actually being constructed, you have a number of options.

To change the parameter processing as a whole, you can use
the C<BUILDARGS> method. The default implementation accepts key/value
pairs or a hash reference. You can override it to take positional args,
or any other format

To change the handling of individual parameters, there are I<coercions>
(See the L<Moose::Cookbook::Basics::Recipe5> for a complete example and
explanation of coercions). With coercions it is possible to morph
argument values into the correct expected types. This approach is the
most flexible and robust, but does have a slightly higher learning
curve.

=head3 How do I make non-Moose constructors work with Moose?

Usually the correct approach to subclassing a non-Moose class is
delegation.  Moose makes this easy using the C<handles> keyword,
coercions, and C<lazy_build>, so subclassing is often not the
ideal route.

That said, the default Moose constructor is inherited from
L<Moose::Object>. When inheriting from a non-Moose class, the
inheritance chain to L<Moose::Object> is broken. The simplest way
to fix this is to simply explicitly inherit from L<Moose::Object>
yourself.

However, this does not always fix the issue of actually calling the Moose
constructor. Fortunately, the modules L<MooseX::NonMoose> and
L<MooseX::Alien> aim to make subclassing non-Moose classes easier.

If neither extension fills your specific needs, you can use
L<Class::MOP::Class/new_object>. This low-level constructor accepts the
special C<__INSTANCE__> parameter, allowing you to instantiate your Moose
attributes:

  package My::HTML::Template;
  use Moose;

  # explicit inheritance
  extends 'HTML::Template', 'Moose::Object';

  # explicit constructor
  sub new {
      my $class = shift;
      # call HTML::Template's constructor
      my $obj = $class->SUPER::new(@_);
      return $class->meta->new_object(
          # pass in the constructed object
          # using the special key __INSTANCE__
          __INSTANCE__ => $obj,
          @_, # pass in the normal args
      );
  }

Of course, this only works if both your Moose class and the
inherited non-Moose class use the same instance type (typically
HASH refs).

Note that this doesn't call C<BUILDALL> automatically, you must do that
yourself.

Other techniques can be used as well, such as creating the object
using C<Moose::Object::new>, but calling the inherited non-Moose
class's initialization methods (if available).

In short, there are several ways to extend non-Moose classes. It is
best to evaluate each case based on the class you wish to extend,
and the features you wish to employ. As always, both IRC and the
mailing list are great ways to get help finding the best approach.

=head2 Accessors

=head3 How do I tell Moose to use get/set accessors?

The easiest way to accomplish this is to use the C<reader> and
C<writer> attribute options:

  has 'bar' => (
      isa    => 'Baz',
      reader => 'get_bar',
      writer => 'set_bar',
  );

Moose will still take advantage of type constraints, triggers, etc.
when creating these methods.

If you do not like this much typing, and wish it to be a default for your
classes, please see L<MooseX::FollowPBP>. This extension will allow you to
write:

  has 'bar' => (
      isa => 'Baz',
      is  => 'rw',
  );

Moose will create separate C<get_bar> and C<set_bar> methods
instead of a single C<bar> method.

NOTE: This B<cannot> be set globally in Moose, as that would break
other classes which are built with Moose. You can still save on typing
by defining a new L<MyApp::Moose> that exports Moose's sugar and then
turns on L<MooseX::FollowPBP>. See L<Moose::Cookbook::Extending::Recipe4>.

=head3 How can I inflate/deflate values in accessors?

Well, the first question to ask is if you actually need both inflate
and deflate.

If you only need to inflate, then we suggest using coercions. Here is
some basic sample code for inflating a L<DateTime> object:

  class_type 'DateTime';

  coerce 'DateTime'
      => from 'Str'
        => via { DateTime::Format::MySQL->parse_datetime($_) };

  has 'timestamp' => (is => 'rw', isa => 'DateTime', coerce => 1);

This creates a custom type for L<DateTime> objects, then attaches
a coercion to that type. The C<timestamp> attribute is then told
to expect a C<DateTime> type, and to try to coerce it. When a C<Str>
type is given to the C<timestamp> accessor, it will attempt to
coerce the value into a C<DateTime> object using the code in found
in the C<via> block.

For a more comprehensive example of using coercions, see the
L<Moose::Cookbook::Basics::Recipe5>.

If you need to deflate your attribute's value, the current best practice
is to add an C<around> modifier to your accessor:

  # a timestamp which stores as
  # seconds from the epoch
  has 'timestamp' => (is => 'rw', isa => 'Int');

  around 'timestamp' => sub {
      my $next = shift;
      my ($self, $timestamp) = @_;
      # assume we get a DateTime object ...
      $next->($self, $timestamp->epoch);
  };

It is also possible to do deflation using coercion, but this tends
to get quite complex and require many subtypes. An example of this
is outside the scope of this document, ask on #moose or send a mail
to the list.

Still another option is to write a custom attribute metaclass, which
is also outside the scope of this document, but we would be happy to
explain it on #moose or the mailing list.

=head2 Method Modifiers

=head3 How can I affect the values in C<@_> using C<before>?

You can't, actually: C<before> only runs before the main method,
and it cannot easily affect the method's execution.

You similarly can't use C<after> to affect the return value of a
method.

We limit C<before> and C<after> because this lets you write more
concise code. You do not have to worry about passing C<@_> to the
original method, or forwarding its return value (being careful to
preserve context).

The C<around> method modifier has neither of these limitations, but
is a little more verbose.

=head3 Can I use C<before> to stop execution of a method?

Yes, but only if you throw an exception. If this is too drastic a
measure then we suggest using C<around> instead. The C<around> method
modifier is the only modifier which can gracefully prevent execution
of the main method. Here is an example:

    around 'baz' => sub {
        my $next = shift;
        my ($self, %options) = @_;
        unless ($options->{bar} eq 'foo') {
            return 'bar';
        }
        $next->($self, %options);
    };

By choosing not to call the C<$next> method, you can stop the
execution of the main method.

=head2 Type Constraints

=head3 How can I provide a custom error message for a type constraint?

Use the C<message> option when building the subtype:

  subtype 'NaturalLessThanTen'
      => as 'Natural'
      => where { $_ < 10 }
      => message { "This number ($_) is not less than ten!" };

This C<message> block will be called when a value fails to pass the
C<NaturalLessThanTen> constraint check.

=head3 Can I turn off type constraint checking?

Not yet. This option will likely come in a future release.

=head2 Roles

=head3 How do I get Moose to call BUILD in all my composed roles?

See L<Moose::Cookbook::WTF> and specifically the
B<Why is BUILD not called for my composed roles?> question in the
B<Roles> section.

=head3 What are Traits, and how are they different from Roles?

In Moose, a trait is almost exactly the same thing as a role, except
that traits typically register themselves, which allows you to refer
to them by a short name ("Big" vs "MyApp::Role::Big").

In Moose-speak, a I<Role> is usually composed into a I<class> at
compile time, whereas a I<Trait> is usually composed into an instance
of a class at runtime to add or modify the behavior of
B<just that instance>.

Outside the context of Moose, traits and roles generally mean exactly the
same thing. The original paper called them Traits, however Perl 6 will call
them Roles.

=head1 AUTHOR

Stevan Little E<lt>stevan@iinteractive.comE<gt>

=head1 COPYRIGHT AND LICENSE

Copyright 2006-2009 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