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


version 2.0802


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. See for a partial list.

As of this writing, Moose is a dependency of several hundred CPAN modules.

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 inadvertently 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. This can be done with the following code:



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::HTTP_SubtypesAndCoercion 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::DateTime_ExtendingNonMooseParent for an example of how to do it, or take a look at "MooseX::NonMoose" in Moose::Manual::MooseX.


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.

If you like bar and set_bar, see MooseX::SemiAffordanceAccessor.

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::Mooseish_MooseSugar.

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::HTTP_SubtypesAndCoercion.

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 = shift;

      return $self->$next unless @_;

      # assume we get a DateTime object ...
      my $timestamp = shift;
      return $self->$next( $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.

Alternatively, the MooseX::Mangle extension provides the mangle_args function, which does allow you to affect @_.

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';

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

Alternatively, the MooseX::Mangle extension provides the guard function, which will conditionally prevent execution of the original method.

Why can't I see return values in an after modifier?

As with the before modifier, the after modifier is simply called after the main method. It is passed the original contents of @_ and not the return values of the main method.

Again, the arguments are too lengthy as to why this has to be. And as with before I recommend using an around modifier instead. Here is some sample code:

  around 'foo' => sub {
      my $next = shift;
      my ($self, @args) = @_;
      my @rv = $next->($self, @args);
      # do something silly with the return values
      return reverse @rv;

Alternatively, the MooseX::Mangle extension provides the mangle_return function, which allows modifying the return values of the original 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?

There's no support for it in the core of Moose yet. This option may come in a future release.

Meanwhile there's a MooseX extension that allows you to do this on a per-attribute basis, and if it doesn't do what you it's easy to write one that fits your use case.

My coercions stopped working with recent Moose, why did you break it?

Moose 0.76 fixed a case where coercions were being applied even if the original constraint passed. This has caused some edge cases to fail where people were doing something like

    subtype 'Address', as 'Str';
    coerce 'Address', from 'Str', via { get_address($_) };

This is not what they intended, because the type constraint Address is too loose in this case. It is saying that all strings are Addresses, which is obviously not the case. The solution is to provide a where clause that properly restricts the type constraint:

    subtype 'Address', as 'Str', where { looks_like_address($_) };

This will allow the coercion to apply only to strings that fail to look like an Address.


Why is BUILD not called for my composed roles?

BUILD is never called in composed roles. The primary reason is that roles are not order sensitive. Roles are composed in such a way that the order of composition does not matter (for information on the deeper theory of this read the original traits papers here

Because roles are essentially unordered, it would be impossible to determine the order in which to execute the BUILD methods.

As for alternate solutions, there are a couple.

  • Using a combination of lazy and default in your attributes to defer initialization (see the Binary Tree example in the cookbook for a good example of lazy/default usage Moose::Cookbook::Basics::BinaryTree_AttributeFeatures)

  • Use attribute triggers, which fire after an attribute is set, to facilitate initialization. These are described in the Moose docs, and examples can be found in the test suite.

In general, roles should not require initialization; they should either provide sane defaults or should be documented as needing specific initialization. One such way to "document" this is to have a separate attribute initializer which is required for the role. Here is an example of how to do this:

  package My::Role;
  use Moose::Role;

  has 'height' => (
      is      => 'rw',
      isa     => 'Int',
      lazy    => 1,
      default => sub {
          my $self = shift;

  requires 'init_height';

In this example, the role will not compose successfully unless the class provides a init_height method.

If none of those solutions work, then it is possible that a role is not the best tool for the job, and you really should be using classes. Or, at the very least, you should reduce the amount of functionality in your role so that it does not require initialization.

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, but Perl 6 will call them roles.

Can an attribute-generated method (e.g. an accessor) satisfy requires?

Yes, just be sure to consume the role after declaring your attribute. "Required Attributes" in Moose::Manual::Roles provides an example:

  package Breakable;
  use Moose::Role;
  requires 'stress';

  package Car;
  use Moose;
  has 'stress' => ( is  => 'rw', isa => 'Int' );
  with 'Breakable';

If you mistakenly consume the Breakable role before declaring your stress attribute, you would see an error like this:

  'Breakable' requires the method 'stress' to be implemented by 'Car' at...

Moose and Subroutine Attributes

Why don't subroutine attributes I inherited from a superclass work?

Currently when subclassing a module is done at runtime with the extends keyword, but attributes are checked at compile time by Perl. To make attributes work, you must place extends in a BEGIN block so that the attribute handlers will be available at compile time, like this:

  BEGIN { extends qw/Foo/ }

Note that we're talking about Perl's subroutine attributes here, not Moose attributes:

  sub foo : Bar(27) { ... }


Moose is maintained by the Moose Cabal, along with the help of many contributors. See "CABAL" in Moose and "CONTRIBUTORS" in Moose for details.


This software is copyright (c) 2013 by Infinity Interactive, Inc..

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.