Imitating Moose-like functionality for Ruby: as seen here http://www.perlmonks.org/?node_id=656019

ruby moose.rb

### Defining some Moose-like functionality
class Module
  def has(options)
      options.each_pair do |k, v|
        singleton_class.send(:attr_accessor, k)
        self.send("#{k}=", v)
    end
  end
  
  def after(meth, &after_block)
    old_meth = instance_method(meth)
    define_method(meth) do |*args, &block|
      old_meth.bind(self).(*args, &block)
      after_block.(self.class)
    end
  end
end

def metaclass(name, &block)
  Object.const_set(name,
                   Module.new do
                     define_singleton_method(:extended) do |klass|
                       klass.class_eval &block
                     end
                   end)
end
### End of moose-like functionality

metaclass :CountingModule do
  has :count => 0
  
  after(:initialize) do |klass|
    puts "construction finished"
    klass.count += 1
  end
end

class Foo
  extend CountingModule
end

Foo.new #=> "construction finished"
Foo.new #=> "construction finished"

puts Foo.count #=> 2