gistfile1.rb

module UserAbilities
  def can?(verb, target)
    Ability[verb].call(self, verb, target)
  end

  Ability = {
    :use=>lambda { |user, verb, target|
      # This depends primarily on what the target is.
      Target[target.class.to_s].call(user, verb, target)
    }
  }
  Target = {
    "Folder"=>lambda {|user, verb, target_folder|
      user.folders.exists? target_folder
    },
    "User"=>lambda {|user, verb, target_user|
      # "Power users" could set advanced permissions
      # through a domain-specific language we come up with
      # that gets injected in this space, so to speak
    }
  }

end

class User
  include UserAbilities
  ...
end