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.
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!
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',
);
Methods correspond to verbs, and are what your objects can do. For example, a User can login.
sub login { ... }
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 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";
};
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.
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.
With old school Perl 5, this is the "DESTROY()" method, but with Moose it is the "DEMOLISH()" method.
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.)
A package with no introspection other than mucking about in the symbol table.
With Moose, you get well-defined declaration and introspection.
Hand-written accessor methods, symbol table hackery, or a helper module like "Class::Accessor".
With Moose, these are declaratively defined, and distinct from methods.
These are pretty much the same in Moose as in old school Perl.
"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.
Could only be done through serious symbol table wizardry, and you probably never saw this before (at least in Perl 5).
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.
"Class::Delegation" or "Class::Delegator", but probably even more hand-written code.
With Moose, this is also declarative.
A "new()" method which calls "bless" on a reference.
Comes for free when you define a class with Moose.
A "DESTROY()" method.
With Moose, this is called "DEMOLISH()".
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.
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.
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.
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 ...;
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.
This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.