NAME¶
Moo - Minimalist Object Orientation (with Moose compatibility)
SYNOPSIS¶
package Cat::Food;
use Moo;
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¶
This module is an extremely light-weight subset of Moose optimised for rapid
startup and "pay only for what you use".
It also avoids depending on any XS modules to allow simple deployments. The name
"Moo" is based on the idea that it provides almost -- but not quite
-- two thirds of Moose.
Unlike Mouse this module does not aim at full compatibility with Moose's surface
syntax, preferring instead of 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.
However, sometimes you're writing a command line script or a CGI script where
fast startup is essential, or code designed to be deployed as a single file
via App::FatPacker, or you're writing a CPAN module and you want it to be
usable by people with those constraints.
I've tried several times to use Mouse but it's 3x the size of Moo and takes
longer to load than most of my Moo based CGI scripts take to run.
If you don't want Moose, you don't want "less metaprotocol" like
Mouse, you want "as little as possible" -- which means "no
metaprotocol", which is what Moo provides.
Better still, if you install and load Moose, we set up metaclasses for your Moo
classes and Moo::Role roles, so you can use them in Moose code without ever
noticing that some of your codebase is using Moo.
Hence, Moo exists as its name -- Minimal Object Orientation -- with a pledge to
make it smooth to upgrade to Moose when you need more than minimal features.
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 anybody ever noticing you aren't using Moose everywhere.
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.
So will extending a Mouse class or consuming a Mouse::Role - 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 want types to be upgraded to the Moose types, use MooX::Types::MooseLike
and install the MooseX::Types library to match the MooX::Types::MooseLike
library you're using - Moo will load the MooseX::Types library and use that
type for the newly created metaclass.
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
currently 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 a speed boost. Simple
accessors are those without lazy defaults, type checks/coercions, or triggers.
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
<
http://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.
IMPORTED METHODS¶
new¶
Foo::Bar->new( attr1 => 3 );
or
Foo::Bar->new({ attr1 => 3 });
BUILDARGS¶
sub BUILDARGS {
my ( $class, @args ) = @_;
unshift @args, "attr1" if @args % 2 == 1;
return { @args };
};
Foo::Bar->new( 3 );
The default implementation of this method accepts a hash or hash reference of
named parameters. If it receives a single argument that isn't a hash reference
it throws an error.
You can override this method in your class to handle other types of options
passed to the constructor.
This method should always return a hash reference of named options.
FOREIGNBUILDARGS¶
If you are inheriting from a non-Moo class, the arguments passed to the parent
class constructor can be manipulated by defining a
"FOREIGNBUILDARGS" method. It will receive the same arguments as
"BUILDARGS", and should return a list of arguments to pass to the
parent class constructor.
BUILD¶
Define a "BUILD" method on your class and the constructor will
automatically call the "BUILD" method from parent down to child
after the object has been instantiated. Typically this is used for object
validation or possibly logging.
DEMOLISH¶
If you have a "DEMOLISH" method anywhere in your inheritance
hierarchy, a "DESTROY" method is created on first object
construction which will call
"$instance->DEMOLISH($in_global_destruction)" for each
"DEMOLISH" method from child upwards to parents.
Note that the "DESTROY" method is created on first construction of an
object of your class in order to not add overhead to classes without
"DEMOLISH" methods; this may prove slightly surprising if you try
and define your own.
does¶
if ($foo->does('Some::Role1')) {
...
}
Returns true if the object composes in the passed role.
IMPORTED SUBROUTINES¶
extends¶
extends 'Parent::Class';
Declares base class. Multiple superclasses can be passed for multiple
inheritance (but please use roles instead). The class will be loaded, however
no errors will be triggered if it 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 have conflicting methods. The roles will
be loaded using the same mechansim 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.
The options for "has" are as follows:
- •
- "is"
required, may be "ro", "lazy", "rwp"
or "rw".
"ro" 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_${attribute_name}" to allow on-demand generated
attributes. This feature was my attempt to fix my incompetence when
originally designing "lazy_build", and is also implemented by
MooseX::AttributeShortcuts. There is, however, nothing to stop you using
"lazy" and "builder" yourself with "rwp" or
"rw" - it's just that this isn't generally a good idea so we
don't provide a shortcut for it.
"rwp" generates a reader like "ro", but also sets
"writer" to "_set_${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" generates a normal getter/setter by defaulting
"accessor" to the name of the attribute.
- •
- "isa"
Takes a coderef which is meant to validate the attribute. Unlike Moose, Moo
does not include a basic type system, so instead of doing "isa =>
'Num'", one should do
isa => sub {
die "$_[0] is not a number!" unless looks_like_number $_[0]
},
Note that the return value is ignored, only whether the sub lives or dies
matters.
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 MooseX::Types style named types, look at MooX::Types::MooseLike.
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] : $_[0] + 1
},
Note that Moo will always fire your coercion: this is to permit
"isa" entries to be used purely for bug trapping, whereas
coercions are always structural to your code. We do, however, apply any
supplied "isa" check after the coercion has run to ensure that
it returned a valid value.
Sub::Quote aware
If the "isa" option is a blessed object providing a
"coerce" or "coercion" method, then the
"coerce" option may be set to just 1.
- •
- "handles"
Takes a string
handles => 'RobotRole'
Where "RobotRole" is a role (Moo::Role) that defines an interface
which becomes the list of methods to handle.
Takes a list of methods
handles => [ qw( one two ) ]
Takes a hashref
handles => {
un => 'one',
}
- •
- "trigger"
Takes a coderef which will get called any time the attribute is set. This
includes the constructor, but not default or built values. Coderef will be
invoked against the object with the new value as an argument.
If you set this to just 1, it generates a trigger which calls the
"_trigger_${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 $self as its only argument to
populate an attribute if no value is supplied to the constructor - or if
the attribute is lazy, when the attribute is first retrieved if no value
has yet been provided.
If a simple scalar is provided, it will be inlined as a string. Any non-code
reference (hash, array) will result in an error - for that case instead
use a code reference that returns the desired value.
Note that if your default is fired during new() there is no guarantee
that other attributes have been populated yet so you should not rely on
their existence.
Sub::Quote aware
- •
- "predicate"
Takes a method name which will return true if an attribute has a value.
If you set this to just 1, the predicate is automatically named
"has_${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.
- •
- "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
instantiation.
- •
- "reader"
The value of this attribute will be the name of the method to get 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 are possible,
which will cause leaks.
- •
- "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($self->name);
}
1;
If you were to import "md5_hex" after namespace::clean you would be
able to call "->md5_hex()" on your "Record" instances
(and it probably wouldn't do what you expect!).
Moo::Roles behave slightly differently. Since their methods are composed into
the consuming class, they can do a little more for you automatically. As long
as you declare your imports before calling "use Moo::Role", those
imports and the ones Moo::Role itself provides will not be composed into
consuming classes, so there's usually no need to use namespace::clean.
On namespace::autoclean: If you're coming to Moo from the Moose world,
you may be accustomed to using namespace::autoclean in all your packages. This
is not recommended for Moo packages, because namespace::autoclean will inflate
your class to a full Moose class. It'll work, but you will lose the benefits
of Moo. Instead you are recommended to just use namespace::clean.
INCOMPATIBILITIES WITH MOOSE¶
There is no built-in type system. "isa" is verified with a coderef; if
you need complex types, just make a library of coderefs, or better yet,
functions that return quoted subs. MooX::Types::MooseLike provides a similar
API to MooseX::Types::Moose so that you can write
has days_to_live => (is => 'ro', isa => Int);
and have it work with both; it is hoped that providing only subrefs as an API
will encourage the use of other type systems as well, since it's probably the
weakest part of Moose design-wise.
"initializer" is not supported in core since the author considers it
to be a bad idea and Moose best practices recommend avoiding it. Meanwhile
"trigger" or "coerce" are more likely to be able to
fulfill your needs.
There is no meta object. If you need this level of complexity you wanted Moose -
Moo succeeds at being small because it explicitly does not provide a
metaprotocol. However, if you load Moose, then
Class::MOP::class_of($moo_class_or_role)
will return an appropriate metaclass pre-populated by Moo.
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.
"BUILDARGS" is not triggered if your class does not have any
attributes. Without attributes, "BUILDARGS" return value would be
ignored, so we just skip calling the method instead.
Handling of warnings: when you "use Moo" we enable FATAL warnings, and
some several extra pragmas when used in development: indirect,
multidimensional, and bareword::filehandles. See the strictures documentation
for the details on this.
A similar invocation for Moose would be:
use Moose;
use warnings FATAL => "all";
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;
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;
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.
SUPPORT¶
Users' IRC: #moose on irc.perl.org
Development and contribution IRC: #web-simple 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. BurzyXski (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 Mannsaaker (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>
COPYRIGHT¶
Copyright (c) 2010-2011 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 <
http://dev.perl.org/licenses/>.