Provided by: libmoose-perl_2.2009-1_amd64 bug

NAME

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

VERSION

       version 2.2009

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.little@iinteractive.com>

       •   Dave Rolsky <autarch@urth.org>

       •   Jesse Luehrs <doy@tozt.net>

       •   Shawn M Moore <code@sartak.org>

       •   יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>

       •   Karen Etheridge <ether@cpan.org>

       •   Florian Ragwitz <rafl@debian.org>

       •   Hans Dieter Pearcey <hdp@weftsoar.net>

       •   Chris Prather <chris@prather.org>

       •   Matt S Trout <mst@shadowcat.co.uk>

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.