gistfile1.rb

# environment.rb
VERSION_DEFAULT = Version1


# user.rb
class User
  include Builder::UserMarkup
end


# builder/base.rb
module Builder

  class Base

    def to_markup(markup, options = {})
      case options[:version]
      when 'v3'
        Version3.to_markup(self, markup, options)
      when 'v2'
        Version2.to_markup(self, markup, options)
      when 'v1'
        Version1.to_markup(self, markup, options)
      else # Default
        DefaultVersion.to_markup(self, markup, options)
      end
    end

    def to_xml
      self.to_markup(Builder::XmlBuilder.new, options)
    end

    def to_json
      self.to_markup(Builder::JsonBuilder.new, options)
    end

  end

end


# builder/user_markup.rb
module Builder

  class UserMarkup < Base

    class Version1
      def to_markup(user, markup, options = {})
      end
    end

    class Version2
      def to_markup(user, markup, options = {})
      end
    end

    class Version3
      def to_markup(user, markup, options = {})
      end
    end

    class DefaultVersion < VERSION_DEFAULT
    end

  end

end