NAME¶
Moose::Manual::FAQ - Frequently asked questions about Moose
VERSION¶
version 2.0603
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.