Moose is great! At its very basic, it simplifies the boilerplate required to create Perl objects immensely, providing attributes with type constraints, method modifiers for semantic enhancement, and role-based class composition for better code re-use.

Moose is built on top of Class::MOP. MOP stands for Meta-Object Protocol. A meta-object is an object that describes an object. So, each attribute and method in your class has a corresponding entry in the meta-object describing it. The meta-object is where you can find out what type constraints are on an attribute, or what methods a class has available.

Since the meta-object is a Plain Old Perl Object, we can call methods on it at runtime. Using those meta-object methods to add an attribute would modify our object, adding that attribute to the object. Using Class::MOP, we can compose classes at runtime!

I have recently used this to great effect in a custom dependency injection and configuration framework we have at Bank of America. By adding a "with" key to the configuration YAML file, the DI will create a new, anonymous class that composes the roles specified.

{
    name: "Repository",
    class: "Bank::HistoricalData::DailyRepository",
    with: [ "Bank::Role::FlattenIntraday", "Bank::Role::CalculateHighLow" ],
    constructor_args: { }
}

So, when I ask the DI for the "Repository" object, it will get the meta-object for Bank::HistoricalData::DailyRepository, create an anonymous class that extends Bank::HistoricalData::DailyRepository, and then compose the two roles into the new class.

The code to do all this is extremely short:

my $class = $conf->{class};
my $meta = Moose::Meta::Class->create_anon_class( 
    superclasses => [ $class ],
    roles => $conf->{with},
);
$meta->make_immutable; # for performance
my $object = $meta->name->new( %{ $conf->{constructor_args} } );
return $object;

If a lot of objects end up composing the same roles, I can create a concrete class to get a bit of a performance boost. Since I create a new, anonymous meta-class, I don't have to worry about the class I'm extending being modified, or being made mutable again ($class->meta->make_immutable speeds up a lot of things, but doesn't allow us to add attributes or methods).

With this, I can be a lot more flexible about my dependencies, adding whatever role I want to change their behavior whenever I need.