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