update FAQ on constructor stuff

[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. I have several medium-to-large-ish web applications in 
production using Moose, they have been running without 
issue now for well over a year. 

At C<$work> we are re-writing our core offering to use Moose, 
so it's continued development is assured. 

Several other people on #moose either have apps in production 
which use Moose, or are in the process of deploying sites 
which use Moose. 

=head3 Is Moose's API stable?

Yes and No. The external API, the one 90% of users will interact
with, is B<very stable> and any changes B<will be 100% backwards 
compatible>. The introspection API is I<mostly> stable; I still 
reserve the right to tweak that if needed, but I will do my 
absolute best to maintain backwards compatibility here as well.

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

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

First let me say that I<nothing> in life is free, and that 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 have 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. This is not very well 
documented yet, so please ask on the list or on #moose for more
information.

We are also discussing and experimenting with L<Module::Compile>,
and the idea of compiling highly optimized C<.pmc> files. In
addition, we have mapped out some core methods as candidates for
conversion to XS.

=head3 When will Moose 1.0 be ready?

It is right now, I declared 0.18 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 sub classing 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.

First, there are I<coercions> (See the L<Moose::Cookbook::Recipe5> 
for a complete example and explaination 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.

Second, using an C<around> method modifier on C<new> can be an 
effective way to affect the contents of C<@_> prior to letting 
Moose deal with it. This carries with it the extra burden for 
your subclasses, in that they have to be sure to explicitly 
call your C<new> and/or work around your C<new> to get to the 
version from L<Moose::Object>. 

The last approach is to use the standard Perl technique of calling 
the C<SUPER::new> within your own custom version of C<new>. This, 
of course, brings with it all the issues of the C<around> solution 
as well as any issues C<SUPER::> might add.

In short, try to use C<BUILD> and coercions, they are your best 
bets.

=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 constructors 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 L<Class::MOP::Class/new_object>, the 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). 

It is also entirely possible to just rely on HASH autovivification
to create the slots needed for Moose based attributes, although this
does restrict use of construction time attribute features somewhat.

In short, there are several ways to go about this, 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. Here is some example code:

  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 class, please see L<Moose::Policy>, and more specifically
L<Moose::Policy::FollowPBP>. This will allow you to write:

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

And have Moose create seperate 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.

=head3 How can I get Moose to inflate/deflate values in the accessor?

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

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

  subtype 'DateTime'
      => as 'Object'
      => where { $_->isa('DateTime') };
      
  coerce 'DateTime'
      => from 'Str'
        => via { DateTime::Format::MySQL->parse_datetime($_) };
        
  has 'timestamp' => (is => 'rw', isa => 'DateTime', coerce => 1);

This creates a custom subtype for L<DateTime> objects, then attaches 
a coercion to that subtype. 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::Recipe5>.

If you need to deflate your attribute, the current best practice is to 
add an C<around> modifier to your accessor. Here is some example code:

  # 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 I 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. What you want is 
an C<around> method. 

=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 I 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 have a custom error message for a type constraint?

Use the C<message> option when building the subtype, like so:

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

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

=head3 Can I turn off type constraint checking?

Not yet, but soon. This option will likely be coming in the next 
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<How come BUILD 
is not called for my composed roles?> question in the B<Roles> section.

=head1 AUTHOR

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

=head1 COPYRIGHT AND LICENSE

Copyright 2006-2008 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