NAME

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

FREQUENTLY ASKED QUESTIONS

Module Stability

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. http://cpants.perl.org/dist/used_by/Moose

Is Moose's API stable?

Yes. The sugary API, the one 95% of users will interact with, is very stable. Any changes will be 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 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.

I heard Moose is slow, is this true?

Again, this one is tricky, so Yes and No.

Firstly, nothing in life is free, and some Moose features do cost more than others. It is also the policy of Moose to 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 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.

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".

Constructors

How do I write custom constructors with Moose?

Ideally, you should never write your own 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 BUILD method. This feature is taken directly from Perl 6. Every 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 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 coercions (See the 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.

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 handles keyword, coercions, and lazy_build, so subclassing is often not the ideal route.

That said, if you really need to inherit from a non-Moose class, see Moose::Cookbook::Basics::Recipe12 for an example of how to do it.

Accessors

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

The easiest way to accomplish this is to use the reader and 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 MooseX::FollowPBP. This extension will allow you to write:

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

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

NOTE: This 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 MyApp::Moose that exports Moose's sugar and then turns on MooseX::FollowPBP. See Moose::Cookbook::Extending::Recipe4.

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

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

If you need to deflate your attribute's value, the current best practice is to add an 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.

Method Modifiers

How can I affect the values in @_ using before?

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

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

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

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

Can I use 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 around instead. The 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 $next method, you can stop the execution of the main method.

Type Constraints

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

Use the message option when building the subtype:

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

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

Can I turn off type constraint checking?

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

Roles

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

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

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 Role is usually composed into a class at compile time, whereas a Trait is usually composed into an instance of a class at runtime to add or modify the behavior of 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.

AUTHOR

Stevan Little <stevan@iinteractive.com>

COPYRIGHT AND LICENSE

Copyright 2006-2009 by Infinity Interactive, Inc.

http://www.iinteractive.com

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

cpanm Moose

perl -MCPAN -e shell
install Moose