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


Module Stability

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 $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.

Is Moose's API stable?

Yes and No. The external API, the one 90% of users will interact with, is very stable and any changes will be 100% backwards compatible. The introspection API is 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.

I heard Moose is slow, is this true?

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

First let me say that nothing in life is free, and that 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 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 Module::Compile, and the idea of compiling highly optimized .pmc files. In addition, we have mapped out some core methods as candidates for conversion to XS.

When will Moose 1.0 be ready?

It is right now, I declared 0.18 to be "ready to use".


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 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.

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

However, this does not always fix the issue of actually calling the Moose constructor. Fortunately "new_object" in Class::MOP::Class, the low level constructor, accepts the special __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 BUILDALL automatically, you must do that yourself.

Other techniques can be used as well, such as creating the object using 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.


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. 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 Moose::Policy, and more specifically Moose::Policy::FollowPBP. This will allow you to write:

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

And have Moose 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.

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 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 DateTime objects, then attaches a coercion to that subtype. 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, the current best practice is to add an 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.

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. What you want is an around method.

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 I 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 have a custom error message for a type constraint?

Use the 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 NaturalLessThanTen constraint check.

Can I turn off type constraint checking?

Not yet, but soon. This option will likely be coming in the next release.


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

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

What are Traits, and how are they different to 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.


Stevan Little <stevan@iinteractive.com>


Copyright 2006-2008 by Infinity Interactive, Inc.


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