Moose::Manual::ConceptUser Contributed Perl DocumentMoose::Manual::Concepts(3)NAMEMoose::Manual::Concepts - Moose OO concepts
VERSION
version 2.0402
MOOSE CONCEPTS (VS "OLD SCHOOL" Perl)
In the past, you may not have thought too much about the difference
between packages and classes, attributes and methods, constructors and
methods, etc. With Moose, these are all conceptually separate, though
under the hood they're implemented with plain old Perl.
Our meta-object protocol (aka MOP) provides well-defined introspection
features for each of those concepts, and Moose in turn provides
distinct sugar for each of them. Moose also introduces additional
concepts such as roles, method modifiers, and declarative delegation.
Knowing what these concepts mean in Moose-speak, and how they used to
be done in old school Perl 5 OO is a good way to start learning to use
Moose.
Class
When you say "use Moose" in a package, you are making your package a
class. At its simplest, a class will consist simply of attributes
and/or methods. It can also include roles, method modifiers, and more.
A class has zero or more attributes.
A class has zero or more methods.
A class has zero or more superclasses (aka parent classes). A class
inherits from its superclass(es).
A class has zero or more method modifiers. These modifiers can apply to
its own methods or methods that are inherited from its ancestors.
A class does (and consumes) zero or more roles.
A class has a constructor and a destructor. These are provided for you
"for free" by Moose.
The constructor accepts named parameters corresponding to the class's
attributes and uses them to initialize an object instance.
A class has a metaclass, which in turn has meta-attributes, meta-
methods, and meta-roles. This metaclass describes the class.
A class is usually analogous to a category of nouns, like "People" or
"Users".
package Person;
use Moose;
# now it's a Moose class!
Attribute
An attribute is a property of the class that defines it. It always has
a name, and it may have a number of other properties.
These properties can include a read/write flag, a type, accessor method
names, delegations, a default value, and more.
Attributes are not methods, but defining them causes various accessor
methods to be created. At a minimum, a normal attribute will have a
reader accessor method. Many attributes have other methods, such as a
writer method, a clearer method, or a predicate method ("has it been
set?").
An attribute may also define delegations, which will create additional
methods based on the delegation mapping.
By default, Moose stores attributes in the object instance, which is a
hashref, but this is invisible to the author of a Moose-based class!
It is best to think of Moose attributes as "properties" of the opaque
object instance. These properties are accessed through well-defined
accessor methods.
An attribute is something that the class's members have. For example,
People have first and last names. Users have passwords and last login
datetimes.
has 'first_name' => (
is => 'rw',
isa => 'Str',
);
Method
A method is very straightforward. Any subroutine you define in your
class is a method.
Methods correspond to verbs, and are what your objects can do. For
example, a User can login.
sub login { ... }
Role
A role is something that a class does. We also say that classes consume
roles. For example, a Machine class might do the Breakable role, and so
could a Bone class. A role is used to define some concept that cuts
across multiple unrelated classes, like "breakability", or "has a
color".
A role has zero or more attributes.
A role has zero or more methods.
A role has zero or more method modifiers.
A role has zero or more required methods.
A required method is not implemented by the role. Required methods are
a way for the role to declare "to use this role you must implement this
method".
A role has zero or more excluded roles.
An excluded role is a role that the role doing the excluding says it
cannot be combined with.
Roles are composed into classes (or other roles). When a role is
composed into a class, its attributes and methods are "flattened" into
the class. Roles do not show up in the inheritance hierarchy. When a
role is composed, its attributes and methods appear as if they were
defined in the consuming class.
Role are somewhat like mixins or interfaces in other OO languages.
package Breakable;
use Moose::Role;
requires 'break';
has 'is_broken' => (
is => 'rw',
isa => 'Bool',
);
after 'break' => sub {
my $self = shift;
$self->is_broken(1);
};
Method modifiers
A method modifier is a hook that is called when a named method is
called. For example, you could say "before calling "login()", call this
modifier first". Modifiers come in different flavors like "before",
"after", "around", and "augment", and you can apply more than one
modifier to a single method.
Method modifiers are often used as an alternative to overriding a
method in a parent class. They are also used in roles as a way of
modifying methods in the consuming class.
Under the hood, a method modifier is just a plain old Perl subroutine
that gets called before or after (or around, etc.) some named method.
before 'login' => sub {
my $self = shift;
my $pw = shift;
warn "Called login() with $pw\n";
};
Type
Moose also comes with a (miniature) type system. This allows you to
define types for attributes. Moose has a set of built-in types based on
the types Perl provides in its core, such as "Str", "Num", "Bool",
"HashRef", etc.
In addition, every class name in your application can also be used as a
type name.
Finally, you can define your own types with their own constraints. For
example, you could define a "PosInt" type, a subtype of "Int" which
only allows positive numbers.
Delegation
Moose attributes provide declarative syntax for defining delegations. A
delegation is a method which in turn calls some method on an attribute
to do its real work.
Constructor
A constructor creates an object instance for the class. In old school
Perl, this was usually done by defining a method called "new()" which
in turn called "bless" on a reference.
With Moose, this "new()" method is created for you, and it simply does
the right thing. You should never need to define your own constructor!
Sometimes you want to do something whenever an object is created. In
those cases, you can provide a "BUILD()" method in your class. Moose
will call this for you after creating a new object.
Destructor
This is a special method called when an object instance goes out of
scope. You can specialize what your class does in this method if you
need to, but you usually don't.
With old school Perl 5, this is the "DESTROY()" method, but with Moose
it is the "DEMOLISH()" method.
Object instance
An object instance is a specific noun in the class's "category". For
example, one specific Person or User. An instance is created by the
class's constructor.
An instance has values for its attributes. For example, a specific
person has a first and last name.
In old school Perl 5, this is often a blessed hash reference. With
Moose, you should never need to know what your object instance actually
is. (Okay, it's usually a blessed hashref with Moose, too.)
Moose vs old school summary
· Class
A package with no introspection other than mucking about in the
symbol table.
With Moose, you get well-defined declaration and introspection.
· Attributes
Hand-written accessor methods, symbol table hackery, or a helper
module like "Class::Accessor".
With Moose, these are declaratively defined, and distinct from
methods.
· Method
These are pretty much the same in Moose as in old school Perl.
· Roles
"Class::Trait" or "Class::Role", or maybe "mixin.pm".
With Moose, they're part of the core feature set, and are
introspectable like everything else.
· Method Modifiers
Could only be done through serious symbol table wizardry, and you
probably never saw this before (at least in Perl 5).
· Type
Hand-written parameter checking in your "new()" method and
accessors.
With Moose, you define types declaratively, and then use them by
name with your attributes.
· Delegation
"Class::Delegation" or "Class::Delegator", but probably even more
hand-written code.
With Moose, this is also declarative.
· Constructor
A "new()" method which calls "bless" on a reference.
Comes for free when you define a class with Moose.
· Destructor
A "DESTROY()" method.
With Moose, this is called "DEMOLISH()".
· Object Instance
A blessed reference, usually a hash reference.
With Moose, this is an opaque thing which has a bunch of attributes
and methods, as defined by its class.
· Immutabilization
Moose comes with a feature called "immutabilization". When you make
your class immutable, it means you're done adding methods,
attributes, roles, etc. This lets Moose optimize your class with a
bunch of extremely dirty in-place code generation tricks that speed
up things like object construction and so on.
META WHAT?
A metaclass is a class that describes classes. With Moose, every class
you define gets a "meta()" method. The "meta()" method returns a
Moose::Meta::Class object, which has an introspection API that can tell
you about the class it represents.
my $meta = User->meta();
for my $attribute ( $meta->get_all_attributes ) {
print $attribute->name(), "\n";
if ( $attribute->has_type_constraint ) {
print " type: ", $attribute->type_constraint->name, "\n";
}
}
for my $method ( $meta->get_all_methods ) {
print $method->name, "\n";
}
Almost every concept we defined earlier has a meta class, so we have
Moose::Meta::Class, Moose::Meta::Attribute, Moose::Meta::Method,
Moose::Meta::Role, Moose::Meta::TypeConstraint, Moose::Meta::Instance,
and so on.
BUT I NEED TO DO IT MY WAY!
One of the great things about Moose is that if you dig down and find
that it does something the "wrong way", you can change it by extending
a metaclass. For example, you can have arrayref based objects, you can
make your constructors strict (no unknown parameters allowed!), you can
define a naming scheme for attribute accessors, you can make a class a
Singleton, and much, much more.
Many of these extensions require surprisingly small amounts of code,
and once you've done it once, you'll never have to hand-code "your way
of doing things" again. Instead you'll just load your favorite
extensions.
package MyWay::User;
use Moose;
use MooseX::StrictConstructor
use MooseX::MyWay;
has ...;
WHAT NEXT?
So you're sold on Moose. Time to learn how to really use it.
If you want to see how Moose would translate directly into old school
Perl 5 OO code, check out Moose::Manual::Unsweetened. This might be
helpful for quickly wrapping your brain around some aspects of "the
Moose way".
Or you can skip that and jump straight to Moose::Manual::Classes and
the rest of the Moose::Manual.
After that we recommend that you start with the Moose::Cookbook. If you
work your way through all the recipes under the basics section, you
should have a pretty good sense of how Moose works, and all of its
basic OO features.
After that, check out the Role recipes. If you're really curious, go on
and read the Meta and Extending recipes, but those are mostly there for
people who want to be Moose wizards and extend Moose itself.
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 v5.14.2 2012-02-04 Moose::Manual::Concepts(3)