My first solution for the Metakoans (https://rubyquiz.com/quiz67.html)

knowledge.rb

require 'set'

class Module

  def attribute(a, &block)
    (attr_name, attr_default) = if a.is_a? Hash
                                  name, value = a.first
                                  [name, -> { value }]
                                elsif block_given?
                                  [a.to_s, block]
                                else
                                  [a.to_s, -> { nil }]
                                end

    class_eval do
      #-----------------------------------------------------------------------
      # Getter
      #-----------------------------------------------------------------------
      define_method(attr_name) do
        iv_get = instance_variable_get(:"@#{attr_name}")
        flag = instance_variable_get(:"@__#{attr_name}_defined")
        (iv_get.nil? && flag.nil?) ? instance_exec(&attr_default) : iv_get
      end

      #-----------------------------------------------------------------------
      # Setter (=)
      #-----------------------------------------------------------------------
      define_method(:"#{attr_name}=") do |v|
        instance_variable_set(:"@#{attr_name}", v)
        instance_variable_set(:"@__#{attr_name}_defined", !v.nil?)
      end

      #-----------------------------------------------------------------------
      # Query (?)
      #-----------------------------------------------------------------------
      define_method(:"#{attr_name}?") do
        flag = instance_variable_get(:"@__#{attr_name}_defined")
        (flag.nil?) ? (!instance_exec(&attr_default).nil?) : flag
      end

    end
  end
 end