Moo(3pm) User Contributed Perl Documentation Moo(3pm)
NAME
Moo - Minimalist Object Orientation (with Moose compatibility)
SYNOPSIS
package Cat::Food;
use Moo;
use strictures 2;
use namespace::clean;
sub feed_lion {
my $self = shift;
my $amount = shift || 1;
$self->pounds( $self->pounds - $amount );
}
has taste => (
is => 'ro',
);
has brand => (
is => 'ro',
isa => sub {
die "Only SWEET-TREATZ supported!" unless $_[0] eq 'SWEET-TREATZ'
},
);
has pounds => (
is => 'rw',
isa => sub { die "$_[0] is too much cat food!" unless $_[0] < 15 },
);
1;
And elsewhere:
my $full = Cat::Food->new(
taste => 'DELICIOUS.',
brand => 'SWEET-TREATZ',
pounds => 10,
);
$full->feed_lion;
say $full->pounds;
DESCRIPTION
"Moo" is an extremely light-weight Object Orientation system. It allows
one to concisely define objects and roles with a convenient syntax that
avoids the details of Perl's object system. "Moo" contains a subset of
Moose and is optimised for rapid startup.
"Moo" avoids depending on any XS modules to allow for simple
deployments. The name "Moo" is based on the idea that it provides
almost -- but not quite -- two thirds of Moose. As such, the
Moose::Manual can serve as an effective guide to "Moo" aside from the
MOP and Types sections.
Unlike Mouse this module does not aim at full compatibility with
Moose's surface syntax, preferring instead to provide full
interoperability via the metaclass inflation capabilities described in
"MOO AND MOOSE".
For a full list of the minor differences between Moose and Moo's
surface syntax, see "INCOMPATIBILITIES WITH MOOSE".
WHY MOO EXISTS
If you want a full object system with a rich Metaprotocol, Moose is
already wonderful.
But if you don't want to use Moose, you may not want "less
metaprotocol" like Mouse offers, but you probably want "no
metaprotocol", which is what Moo provides. "Moo" is ideal for some
situations where deployment or startup time precludes using Moose and
Mouse:
• A command line or CGI script where fast startup is essential
• code designed to be deployed as a single file via App::FatPacker
• A CPAN module that may be used by others in the above situations
"Moo" maintains transparent compatibility with Moose so if you install
and load Moose you can use Moo classes and roles in Moose code without
modification.
Moo -- Minimal Object Orientation -- aims to make it smooth to upgrade
to Moose when you need more than the minimal features offered by Moo.
MOO AND MOOSE
If Moo detects Moose being loaded, it will automatically register
metaclasses for your Moo and Moo::Role packages, so you should be able
to use them in Moose code without modification.
Moo will also create Moose type constraints for Moo classes and roles,
so that in Moose classes "isa => 'MyMooClass'" and "isa => 'MyMooRole'"
work the same as for Moose classes and roles.
Extending a Moose class or consuming a Moose::Role will also work.
Extending a Mouse class or consuming a Mouse::Role will also work. But
note that we don't provide Mouse metaclasses or metaroles so the other
way around doesn't work. This feature exists for Any::Moose users
porting to Moo; enabling Mouse users to use Moo classes is not a
priority for us.
This means that there is no need for anything like Any::Moose for Moo
code - Moo and Moose code should simply interoperate without problem.
To handle Mouse code, you'll likely need an empty Moo role or class
consuming or extending the Mouse stuff since it doesn't register true
Moose metaclasses like Moo does.
If you need to disable the metaclass creation, add:
no Moo::sification;
to your code before Moose is loaded, but bear in mind that this switch
is global and turns the mechanism off entirely so don't put this in
library code.
MOO AND CLASS::XSACCESSOR
If a new enough version of Class::XSAccessor is available, it will be
used to generate simple accessors, readers, and writers for better
performance. Simple accessors are those without lazy defaults, type
checks/coercions, or triggers. Simple readers are those without lazy
defaults. Readers and writers generated by Class::XSAccessor will
behave slightly differently: they will reject attempts to call them
with the incorrect number of parameters.
MOO VERSUS ANY::MOOSE
Any::Moose will load Mouse normally, and Moose in a program using Moose
- which theoretically allows you to get the startup time of Mouse
without disadvantaging Moose users.
Sadly, this doesn't entirely work, since the selection is load order
dependent - Moo's metaclass inflation system explained above in "MOO
AND MOOSE" is significantly more reliable.
So if you want to write a CPAN module that loads fast or has only pure
perl dependencies but is also fully usable by Moose users, you should
be using Moo.
For a full explanation, see the article
<https://shadow.cat/blog/matt-s-trout/moo-versus-any-moose> which
explains the differing strategies in more detail and provides a direct
example of where Moo succeeds and Any::Moose fails.
PUBLIC METHODS
Moo provides several methods to any class using it.
new
Foo::Bar->new( attr1 => 3 );
or
Foo::Bar->new({ attr1 => 3 });
The constructor for the class. By default it will accept attributes
either as a hashref, or a list of key value pairs. This can be
customized with the "BUILDARGS" method.
does
if ($foo->does('Some::Role1')) {
...
}
Returns true if the object composes in the passed role.
DOES
if ($foo->DOES('Some::Role1') || $foo->DOES('Some::Class1')) {
...
}
Similar to "does", but will also return true for both composed roles
and superclasses.
meta
my $meta = Foo::Bar->meta;
my @methods = $meta->get_method_list;
Returns an object that will behave as if it is a Moose metaclass object
for the class. If you call anything other than "make_immutable" on it,
the object will be transparently upgraded to a genuine
Moose::Meta::Class instance, loading Moose in the process if required.
"make_immutable" itself is a no-op, since we generate metaclasses that
are already immutable, and users converting from Moose had an
unfortunate tendency to accidentally load Moose by calling it.
LIFECYCLE METHODS
There are several methods that you can define in your class to control
construction and destruction of objects. They should be used rather
than trying to modify "new" or "DESTROY" yourself.
BUILDARGS
around BUILDARGS => sub {
my ( $orig, $class, @args ) = @_;
return { attr1 => $args[0] }
if @args == 1 && !ref $args[0];
return $class->$orig(@args);
};
Foo::Bar->new( 3 );
This class method is used to transform the arguments to "new" into a
hash reference of attribute values.
The default implementation accepts a hash or hash reference of named
parameters. If it receives a single argument that isn't a hash
reference it will throw an error.
You can override this method in your class to handle other types of
options passed to the constructor.
This method should always return a hash reference of named options.
FOREIGNBUILDARGS
sub FOREIGNBUILDARGS {
my ( $class, $options ) = @_;
return $options->{foo};
}
If you are inheriting from a non-Moo class, the arguments passed to the
parent class constructor can be manipulated by defining a
"FOREIGNBUILDARGS" method. It will receive the same arguments as
"BUILDARGS", and should return a list of arguments to pass to the
parent class constructor.
BUILD
sub BUILD {
my ($self, $args) = @_;
die "foo and bar cannot be used at the same time"
if exists $args->{foo} && exists $args->{bar};
}
On object creation, any "BUILD" methods in the class's inheritance
hierarchy will be called on the object and given the results of
"BUILDARGS". They each will be called in order from the parent classes
down to the child, and thus should not themselves call the parent's
method. Typically this is used for object validation or possibly
logging.
DEMOLISH
sub DEMOLISH {
my ($self, $in_global_destruction) = @_;
...
}
When an object is destroyed, any "DEMOLISH" methods in the inheritance
hierarchy will be called on the object. They are given boolean to
inform them if global destruction is in progress, and are called from
the child class upwards to the parent. This is similar to "BUILD"
methods but in the opposite order.
Note that this is implemented by a "DESTROY" method, which is only
created on on the first construction of an object of your class. This
saves on overhead for classes that are never instantiated or those
without "DEMOLISH" methods. If you try to define your own "DESTROY",
this will cause undefined results.
IMPORTED SUBROUTINES
extends
extends 'Parent::Class';
Declares a base class. Multiple superclasses can be passed for multiple
inheritance but please consider using roles instead. The class will be
loaded but no errors will be triggered if the class can't be found and
there are already subs in the class.
Calling extends more than once will REPLACE your superclasses, not add
to them like 'use base' would.
with
with 'Some::Role1';
or
with 'Some::Role1', 'Some::Role2';
Composes one or more Moo::Role (or Role::Tiny) roles into the current
class. An error will be raised if these roles cannot be composed
because they have conflicting method definitions. The roles will be
loaded using the same mechanism as "extends" uses.
has
has attr => (
is => 'ro',
);
Declares an attribute for the class.
package Foo;
use Moo;
has 'attr' => (
is => 'ro'
);
package Bar;
use Moo;
extends 'Foo';
has '+attr' => (
default => sub { "blah" },
);
Using the "+" notation, it's possible to override an attribute.
has [qw(attr1 attr2 attr3)] => (
is => 'ro',
);
Using an arrayref with multiple attribute names, it's possible to
declare multiple attributes with the same options.
The options for "has" are as follows:
"is"
required, may be "ro", "lazy", "rwp" or "rw".
"ro" stands for "read-only" and generates an accessor that dies if
you attempt to write to it - i.e. a getter only - by defaulting
"reader" to the name of the attribute.
"lazy" generates a reader like "ro", but also sets "lazy" to 1 and
"builder" to "_build_${attribute_name}" to allow on-demand generated
attributes. This feature was my attempt to fix my incompetence when
originally designing "lazy_build", and is also implemented by
MooseX::AttributeShortcuts. There is, however, nothing to stop you
using "lazy" and "builder" yourself with "rwp" or "rw" - it's just
that this isn't generally a good idea so we don't provide a shortcut
for it.
"rwp" stands for "read-write protected" and generates a reader like
"ro", but also sets "writer" to "_set_${attribute_name}" for
attributes that are designed to be written from inside of the class,
but read-only from outside. This feature comes from
MooseX::AttributeShortcuts.
"rw" stands for "read-write" and generates a normal getter/setter by
defaulting the "accessor" to the name of the attribute specified.
"isa"
Takes a coderef which is used to validate the attribute. Unlike
Moose, Moo does not include a basic type system, so instead of doing
"isa => 'Num'", one should do
use Scalar::Util qw(looks_like_number);
...
isa => sub {
die "$_[0] is not a number!" unless looks_like_number $_[0]
},
Note that the return value for "isa" is discarded. Only if the sub
dies does type validation fail.
Sub::Quote aware
Since Moo does not run the "isa" check before "coerce" if a coercion
subroutine has been supplied, "isa" checks are not structural to your
code and can, if desired, be omitted on non-debug builds (although if
this results in an uncaught bug causing your program to break, the
Moo authors guarantee nothing except that you get to keep both
halves).
If you want Moose compatible or MooseX::Types style named types, look
at Type::Tiny.
To cause your "isa" entries to be automatically mapped to named
Moose::Meta::TypeConstraint objects (rather than the default
behaviour of creating an anonymous type), set:
$Moo::HandleMoose::TYPE_MAP{$isa_coderef} = sub {
require MooseX::Types::Something;
return MooseX::Types::Something::TypeName();
};
Note that this example is purely illustrative; anything that returns
a Moose::Meta::TypeConstraint object or something similar enough to
it to make Moose happy is fine.
"coerce"
Takes a coderef which is meant to coerce the attribute. The basic
idea is to do something like the following:
coerce => sub {
$_[0] % 2 ? $_[0] : $_[0] + 1
},
Note that Moo will always execute your coercion: this is to permit
"isa" entries to be used purely for bug trapping, whereas coercions
are always structural to your code. We do, however, apply any
supplied "isa" check after the coercion has run to ensure that it
returned a valid value.
Sub::Quote aware
If the "isa" option is a blessed object providing a "coerce" or
"coercion" method, then the "coerce" option may be set to just 1.
"handles"
Takes a string
handles => 'RobotRole'
Where "RobotRole" is a role that defines an interface which becomes
the list of methods to handle.
Takes a list of methods
handles => [ qw( one two ) ]
Takes a hashref
handles => {
un => 'one',
}
"trigger"
Takes a coderef which will get called any time the attribute is set.
This includes the constructor, but not default or built values. The
coderef will be invoked against the object with the new value as an
argument.
If you set this to just 1, it generates a trigger which calls the
"_trigger_${attr_name}" method on $self. This feature comes from
MooseX::AttributeShortcuts.
Note that Moose also passes the old value, if any; this feature is
not yet supported.
Sub::Quote aware
"default"
Takes a coderef which will get called with $self as its only argument
to populate an attribute if no value for that attribute was supplied
to the constructor. Alternatively, if the attribute is lazy,
"default" executes when the attribute is first retrieved if no value
has yet been provided.
If a simple scalar is provided, it will be inlined as a string. Any
non-code reference (hash, array) will result in an error - for that
case instead use a code reference that returns the desired value.
Note that if your default is fired during new() there is no guarantee
that other attributes have been populated yet so you should not rely
on their existence.
Sub::Quote aware
"predicate"
Takes a method name which will return true if an attribute has a
value.
If you set this to just 1, the predicate is automatically named
"has_${attr_name}" if your attribute's name does not start with an
underscore, or "_has_${attr_name_without_the_underscore}" if it does.
This feature comes from MooseX::AttributeShortcuts.
"builder"
Takes a method name which will be called to create the attribute -
functions exactly like default except that instead of calling
$default->($self);
Moo will call
$self->$builder;
The following features come from MooseX::AttributeShortcuts:
If you set this to just 1, the builder is automatically named
"_build_${attr_name}".
If you set this to a coderef or code-convertible object, that
variable will be installed under "$class::_build_${attr_name}" and
the builder set to the same name.
"clearer"
Takes a method name which will clear the attribute.
If you set this to just 1, the clearer is automatically named
"clear_${attr_name}" if your attribute's name does not start with an
underscore, or "_clear_${attr_name_without_the_underscore}" if it
does. This feature comes from MooseX::AttributeShortcuts.
NOTE: If the attribute is "lazy", it will be regenerated from
"default" or "builder" the next time it is accessed. If it is not
lazy, it will be "undef".
"lazy"
Boolean. Set this if you want values for the attribute to be grabbed
lazily. This is usually a good idea if you have a "builder" which
requires another attribute to be set.
"required"
Boolean. Set this if the attribute must be passed on object
instantiation.
"reader"
The name of the method that returns the value of the attribute. If
you like Java style methods, you might set this to "get_foo"
"writer"
The value of this attribute will be the name of the method to set the
value of the attribute. If you like Java style methods, you might
set this to "set_foo".
"weak_ref"
Boolean. Set this if you want the reference that the attribute
contains to be weakened. Use this when circular references, which
cause memory leaks, are possible.
"init_arg"
Takes the name of the key to look for at instantiation time of the
object. A common use of this is to make an underscored attribute
have a non-underscored initialization name. "undef" means that
passing the value in on instantiation is ignored.
"moosify"
Takes either a coderef or array of coderefs which is meant to
transform the given attributes specifications if necessary when
upgrading to a Moose role or class. You shouldn't need this by
default, but is provided as a means of possible extensibility.
before
before foo => sub { ... };
See "before method(s) => sub { ... };" in Class::Method::Modifiers for
full documentation.
around
around foo => sub { ... };
See "around method(s) => sub { ... };" in Class::Method::Modifiers for
full documentation.
after
after foo => sub { ... };
See "after method(s) => sub { ... };" in Class::Method::Modifiers for
full documentation.
SUB QUOTE AWARE
"quote_sub" in Sub::Quote allows us to create coderefs that are
"inlineable," giving us a handy, XS-free speed boost. Any option that
is Sub::Quote aware can take advantage of this.
To do this, you can write
use Sub::Quote;
use Moo;
use namespace::clean;
has foo => (
is => 'ro',
isa => quote_sub(q{ die "Not <3" unless $_[0] < 3 })
);
which will be inlined as
do {
local @_ = ($_[0]->{foo});
die "Not <3" unless $_[0] < 3;
}
or to avoid localizing @_,
has foo => (
is => 'ro',
isa => quote_sub(q{ my ($val) = @_; die "Not <3" unless $val < 3 })
);
which will be inlined as
do {
my ($val) = ($_[0]->{foo});
die "Not <3" unless $val < 3;
}
See Sub::Quote for more information, including how to pass lexical
captures that will also be compiled into the subroutine.
CLEANING UP IMPORTS
Moo will not clean up imported subroutines for you; you will have to do
that manually. The recommended way to do this is to declare your
imports first, then "use Moo", then "use namespace::clean". Anything
imported before namespace::clean will be scrubbed. Anything imported
or declared after will be still be available.
package Record;
use Digest::MD5 qw(md5_hex);
use Moo;
use namespace::clean;
has name => (is => 'ro', required => 1);
has id => (is => 'lazy');
sub _build_id {
my ($self) = @_;
return md5_hex($self->name);
}
1;
For example if you were to import these subroutines after
namespace::clean like this
use namespace::clean;
use Digest::MD5 qw(md5_hex);
use Moo;
then any "Record" $r would have methods such as "$r->md5_hex()",
"$r->has()" and "$r->around()" - almost certainly not what you intend!
Moo::Roles behave slightly differently. Since their methods are
composed into the consuming class, they can do a little more for you
automatically. As long as you declare your imports before calling "use
Moo::Role", those imports and the ones Moo::Role itself provides will
not be composed into consuming classes so there's usually no need to
use namespace::clean.
On namespace::autoclean: Older versions of namespace::autoclean would
inflate Moo classes to full Moose classes, losing the benefits of Moo.
If you want to use namespace::autoclean with a Moo class, make sure you
are using version 0.16 or newer.
INCOMPATIBILITIES WITH MOOSE
TYPES
There is no built-in type system. "isa" is verified with a coderef; if
you need complex types, Type::Tiny can provide types, type libraries,
and will work seamlessly with both Moo and Moose. Type::Tiny can be
considered the successor to MooseX::Types and provides a similar API,
so that you can write
use Types::Standard qw(Int);
has days_to_live => (is => 'ro', isa => Int);
API INCOMPATIBILITIES
"initializer" is not supported in core since the author considers it to
be a bad idea and Moose best practices recommend avoiding it. Meanwhile
"trigger" or "coerce" are more likely to be able to fulfill your needs.
No support for "super", "override", "inner", or "augment" - the author
considers augment to be a bad idea, and override can be translated:
override foo => sub {
...
super();
...
};
around foo => sub {
my ($orig, $self) = (shift, shift);
...
$self->$orig(@_);
...
};
The "dump" method is not provided by default. The author suggests
loading Devel::Dwarn into "main::" (via "perl -MDevel::Dwarn ..." for
example) and using "$obj->$::Dwarn()" instead.
"default" only supports coderefs and plain scalars, because passing a
hash or array reference as a default is almost always incorrect since
the value is then shared between all objects using that default.
"lazy_build" is not supported; you are instead encouraged to use the
"is => 'lazy'" option supported by Moo and MooseX::AttributeShortcuts.
"auto_deref" is not supported since the author considers it a bad idea
and it has been considered best practice to avoid it for some time.
"documentation" will show up in a Moose metaclass created from your
class but is otherwise ignored. Then again, Moose ignores it as well,
so this is arguably not an incompatibility.
Since "coerce" does not require "isa" to be defined but Moose does
require it, the metaclass inflation for coerce alone is a trifle insane
and if you attempt to subtype the result will almost certainly break.
Handling of warnings: when you "use Moo" we enable strict and warnings,
in a similar way to Moose. The authors recommend the use of
"strictures", which enables FATAL warnings, and several extra pragmas
when used in development: indirect, multidimensional, and
bareword::filehandles.
Additionally, Moo supports a set of attribute option shortcuts intended
to reduce common boilerplate. The set of shortcuts is the same as in
the Moose module MooseX::AttributeShortcuts as of its version 0.009+.
So if you:
package MyClass;
use Moo;
use strictures 2;
The nearest Moose invocation would be:
package MyClass;
use Moose;
use warnings FATAL => "all";
use MooseX::AttributeShortcuts;
or, if you're inheriting from a non-Moose class,
package MyClass;
use Moose;
use MooseX::NonMoose;
use warnings FATAL => "all";
use MooseX::AttributeShortcuts;
META OBJECT
There is no meta object. If you need this level of complexity you need
Moose - Moo is small because it explicitly does not provide a
metaprotocol. However, if you load Moose, then
Class::MOP::class_of($moo_class_or_role)
will return an appropriate metaclass pre-populated by Moo.
IMMUTABILITY
Finally, Moose requires you to call
__PACKAGE__->meta->make_immutable;
at the end of your class to get an inlined (i.e. not horribly slow)
constructor. Moo does it automatically the first time ->new is called
on your class. ("make_immutable" is a no-op in Moo to ease migration.)
An extension MooX::late exists to ease translating Moose packages to
Moo by providing a more Moose-like interface.
COMPATIBILITY WITH OLDER PERL VERSIONS
Moo is compatible with perl versions back to 5.6. When running on
older versions, additional prerequisites will be required. If you are
packaging a script with its dependencies, such as with App::FatPacker,
you will need to be certain that the extra prerequisites are included.
MRO::Compat
Required on perl versions prior to 5.10.0.
Devel::GlobalDestruction
Required on perl versions prior to 5.14.0.
SUPPORT
IRC: #moose on irc.perl.org
Bugtracker: <https://rt.cpan.org/Public/Dist/Display.html?Name=Moo>
Git repository: <git://github.com/moose/Moo.git>
Git browser: <https://github.com/moose/Moo>
AUTHOR
mst - Matt S. Trout (cpan:MSTROUT) <mst@shadowcat.co.uk>
CONTRIBUTORS
dg - David Leadbeater (cpan:DGL) <dgl@dgl.cx>
frew - Arthur Axel "fREW" Schmidt (cpan:FREW) <frioux@gmail.com>
hobbs - Andrew Rodland (cpan:ARODLAND) <arodland@cpan.org>
jnap - John Napiorkowski (cpan:JJNAPIORK) <jjn1056@yahoo.com>
ribasushi - Peter Rabbitson (cpan:RIBASUSHI) <ribasushi@cpan.org>
chip - Chip Salzenberg (cpan:CHIPS) <chip@pobox.com>
ajgb - Alex J. G. Burzyński (cpan:AJGB) <ajgb@cpan.org>
doy - Jesse Luehrs (cpan:DOY) <doy at tozt dot net>
perigrin - Chris Prather (cpan:PERIGRIN) <chris@prather.org>
Mithaldu - Christian Walde (cpan:MITHALDU)
<walde.christian@googlemail.com>
ilmari - Dagfinn Ilmari Mannsåker (cpan:ILMARI) <ilmari@ilmari.org>
tobyink - Toby Inkster (cpan:TOBYINK) <tobyink@cpan.org>
haarg - Graham Knop (cpan:HAARG) <haarg@cpan.org>
mattp - Matt Phillips (cpan:MATTP) <mattp@cpan.org>
bluefeet - Aran Deltac (cpan:BLUEFEET) <bluefeet@gmail.com>
bubaflub - Bob Kuo (cpan:BUBAFLUB) <bubaflub@cpan.org>
ether = Karen Etheridge (cpan:ETHER) <ether@cpan.org>
COPYRIGHT
Copyright (c) 2010-2015 the Moo "AUTHOR" and "CONTRIBUTORS" as listed
above.
LICENSE
This library is free software and may be distributed under the same
terms as perl itself. See <https://dev.perl.org/licenses/>.
perl v5.36.0 2023-01-07 Moo(3pm)
Generated by dwww version 1.15 on Wed Jun 26 06:35:08 CEST 2024.