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


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.

no Moose and immutabilize

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

  package Person;

  use Moose;

  # extends, roles, attributes, etc.

  # methods

  no Moose;



The no Moose bit is simply good code hygiene, and making classes immutable speeds up a lot of things, most notably object construction.

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 ;)


If you override the BUILDARGS method in your class, make sure to play nice and call SUPER::BUILDARGS 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.

Use lazy_build

Lazy is good, and often solves initialization ordering problems. It's also good for deferring work that may never have to be done. If you're going to be lazy, use lazy_build to save yourself some typing and standardize names.

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.

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.

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 MooseX::AttributeHelpers instead of auto_deref

The auto_deref feature is a bit troublesome. Directly exposing a complex attribute is ugly. Instead, consider using MooseX::AttributeHelpers to define an API that exposes those pieces of functionality that need exposing. Then you can expose just the functionality that you want.

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".

If you're intending to package your types up for re-use using MooseX::Types later, avoid using characters that are invalid in perl identifiers such as a space or period.

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 at length 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.


Yuval (nothingmuch) Kogman

Dave Rolsky <>


Copyright 2009 by Infinity Interactive, Inc.

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