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Moose::Manual::BestPractices - Get the most out of Moose


version 2.2207


Moose has a lot of features, and there's definitely more than one way to do it. However, we think that picking a subset of these features and using them consistently makes everyone's life easier.

Of course, as with any list of "best practices", these are really just opinions. Feel free to ignore us.

namespace::autoclean and immutabilize

We recommend that you remove the Moose sugar and end your Moose class definitions by making your class immutable.

  package Person;

  use Moose;
  use namespace::autoclean;

  # extends, roles, attributes, etc.

  # methods



The use namespace::autoclean bit is simply good code hygiene, as it removes imported symbols from your class's namespace at the end of your package's compile cycle, including Moose keywords. Once the class has been built, these keywords are not needed. (This is preferred to placing no Moose at the end of your package).

The make_immutable call allows Moose to speed up a lot of things, most notably object construction. The trade-off is that you can no longer change the class definition.

Never override new

Overriding new is a very bad practice. Instead, you should use a BUILD or BUILDARGS methods to do the same thing. When you override new, Moose can no longer inline a constructor when your class is immutabilized.

There are two good reasons to override new. One, you are writing a MooseX extension that provides its own Moose::Object subclass and a subclass of Moose::Meta::Method::Constructor to inline the constructor. Two, you are subclassing a non-Moose parent.

If you know how to do that, you know when to ignore this best practice ;)

Always call the original/parent BUILDARGS

If you override the BUILDARGS method in your class, make sure to play nice and call super() to handle cases you're not checking for explicitly.

The default BUILDARGS method in Moose::Object handles both a list and hashref of named parameters correctly, and also checks for a non-hashref single argument.

Provide defaults whenever possible, otherwise use required

When your class provides defaults, this makes constructing new objects simpler. If you cannot provide a default, consider making the attribute required.

If you don't do either, an attribute can simply be left unset, increasing the complexity of your object, because it has more possible states that you or the user of your class must account for.

Use builder instead of default most of the time

Builders can be inherited, they have explicit names, and they're just plain cleaner.

However, do use a default when the default is a non-reference, or when the default is simply an empty reference of some sort.

Also, keep your builder methods private.

Be lazy

Lazy is good, and often solves initialization ordering problems. It's also good for deferring work that may never have to be done. Make your attributes lazy unless they're required or have trivial defaults.

Consider keeping clearers and predicates private

Does everyone really need to be able to clear an attribute? Probably not. Don't expose this functionality outside your class by default.

Predicates are less problematic, but there's no reason to make your public API bigger than it has to be.

Avoid lazy_build

As described above, you rarely actually need a clearer or a predicate. lazy_build adds both to your public API, which exposes you to use cases that you must now test for. It's much better to avoid adding them until you really need them - use explicit lazy and builder options instead.

Default to read-only, and consider keeping writers private

Making attributes mutable just means more complexity to account for in your program. The alternative to mutable state is to encourage users of your class to simply make new objects as needed.

If you must make an attribute read-write, consider making the writer a separate private method. Narrower APIs are easy to maintain, and mutable state is trouble.

In order to declare such attributes, provide a private writer parameter:

    has pizza => (
        is     => 'ro',
        isa    => 'Pizza',
        writer => '_pizza',

Think twice before changing an attribute's type in a subclass

Down this path lies great confusion. If the attribute is an object itself, at least make sure that it has the same interface as the type of object in the parent class.

Don't use the initializer feature

Don't know what we're talking about? That's fine.

Use Moose::Meta::Attribute::Native traits instead of auto_deref

The auto_deref feature is a bit troublesome. Directly exposing a complex attribute is ugly. Instead, consider using Moose::Meta::Attribute::Native traits to define an API that only exposes the necessary pieces of functionality.

Always call inner in the most specific subclass

When using augment and inner, we recommend that you call inner in the most specific subclass of your hierarchy. This makes it possible to subclass further and extend the hierarchy without changing the parents.

Namespace your types

Use some sort of namespacing convention for type names. We recommend something like "MyApp::Type::Foo". We also recommend considering MooseX::Types.

Do not coerce Moose built-ins directly

If you define a coercion for a Moose built-in like ArrayRef, this will affect every application in the Perl interpreter that uses this type.

    # very naughty!
    coerce 'ArrayRef'
        => from Str
        => via { [ split /,/ ] };

Instead, create a subtype and coerce that:

    subtype 'My::ArrayRef' => as 'ArrayRef';

    coerce 'My::ArrayRef'
        => from 'Str'
        => via { [ split /,/ ] };

Do not coerce class names directly

Just as with Moose built-in types, a class type is global for the entire interpreter. If you add a coercion for that class name, it can have magical side effects elsewhere:

    # also very naughty!
    coerce 'HTTP::Headers'
        => from 'HashRef'
        => via { HTTP::Headers->new( %{$_} ) };

Instead, we can create an "empty" subtype for the coercion:

    subtype 'My::HTTP::Headers' => as class_type('HTTP::Headers');

    coerce 'My::HTTP::Headers'
        => from 'HashRef'
        => via { HTTP::Headers->new( %{$_} ) };

Use coercion instead of unions

Consider using a type coercion instead of a type union. This was covered in Moose::Manual::Types.

Define all your types in one module

Define all your types and coercions in one module. This was also covered in Moose::Manual::Types.


Following these practices has a number of benefits.

It helps ensure that your code will play nice with others, making it more reusable and easier to extend.

Following an accepted set of idioms will make maintenance easier, especially when someone else has to maintain your code. It will also make it easier to get support from other Moose users, since your code will be easier to digest quickly.

Some of these practices are designed to help Moose do the right thing, especially when it comes to immutabilization. This means your code will be faster when immutabilized.

Many of these practices also help get the most out of meta programming. If you used an overridden new to do type coercion by hand, rather than defining a real coercion, there is no introspectable metadata. This sort of thing is particularly problematic for MooseX extensions which rely on introspection to do the right thing.


  • Stevan Little <>

  • Dave Rolsky <>

  • Jesse Luehrs <>

  • Shawn M Moore <>

  • יובל קוג'מן (Yuval Kogman) <>

  • Karen Etheridge <>

  • Florian Ragwitz <>

  • Hans Dieter Pearcey <>

  • Chris Prather <>

  • Matt S Trout <>


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