configuration.rb

class Configuration
  def method_missing(name)
    self.class.instance_variable_get("@#{name}")
  end

  def self.set(name, value)
    instance_variable_set("@#{name}", value)
  end

  def self.default(name, value)
    instance_variable_set("@#{name}", value)
  end
end

class Foo < Configuration
  default :bar, "baz"
  default :a, 9

end

f = Foo.new
f.bar
f.a

Foo.set(:bar, 12)

f.bar