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

VERSION
    version 2.2200

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. See
    <http://moose.iinteractive.com/about.html#organizations> for a partial list.

    As of this writing, Moose is a dependency of several hundred CPAN modules.
    <https://metacpan.org/requires/module/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 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:

      MyClass->meta->make_immutable();

  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::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.

  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.

    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';
            }
            $self->$next(%options);
        };

    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.

  Roles
   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
    <http://www.iam.unibe.ch/~scg/Research/Traits/>).

    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;
              $self->init_height;
          }
      );

      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) { ... }

AUTHORS
    *   Stevan Little <stevan AT cpan.org>

    *   Dave Rolsky <autarch AT urth.org>

    *   Jesse Luehrs <doy AT cpan.org>

    *   Shawn M Moore <sartak AT cpan.org>

    *   יובל קוג'מן (Yuval Kogman) <nothingmuch AT woobling.org>

    *   Karen Etheridge <ether AT cpan.org>

    *   Florian Ragwitz <rafl AT debian.org>

    *   Hans Dieter Pearcey <hdp AT cpan.org>

    *   Chris Prather <chris AT prather.org>

    *   Matt S Trout <mstrout AT cpan.org>

COPYRIGHT AND LICENSE
    This software is copyright (c) 2006 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.