Moose::Manual::FAQ(3) User Contributed Perl DocumentationMoose::Manual::FAQ(3)NAME
Moose::Manual::FAQ - Frequently asked questions about Moose
VERSION
version 2.0604
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) { ... }
AUTHOR
Moose is maintained by the Moose Cabal, along with the help of many
contributors. See "CABAL" in Moose and "CONTRIBUTORS" in Moose for
details.
COPYRIGHT AND LICENSE
This software is copyright (c) 2012 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.
perl v5.16.2 2012-09-19 Moose::Manual::FAQ(3)
