Solution to Ara T. Howard's metakoans.rb Ruby Quiz: http://rubyquiz.com/quiz67.html

metakoans.rb

class Module
  def attribute(attribute, &block)
    case attribute
    when Hash
      name    = attribute.keys.first
      default = attribute[name]
    when String, Symbol
      name    = attribute
      default = nil
    end

    sym = name.to_sym
    instance_variable = :"@#{name}"

    define_method(:"#{name}?") do
      !!send(sym)
    end

    define_method(:"#{name}=") do |value|
      instance_variable_set(:"@#{name}", value)
    end

    define_method(sym) do
      if instance_variable_defined?(instance_variable)
        instance_variable_get(instance_variable)
      else
        block ? instance_eval(&block) : default
      end
    end
  end
end