Run-time Class Composition With Moose

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.