codecaller.rb

    class OnCodeCaller
      def self.call(code, needer)
        if needer.respond_to?("on_#{code}")
          needer.send("on_#{code}")
        elsif needer.respond_to?("on_#{code / 100}XX")
          needer.send("on_#{code / 100}XX")
        else
          needer.on_failure
        end
      end
    end

    class Foo
      def bar(code)
        CodeCaller.call(code, self)
      end

      def on_422
        raise 'specific error called if defined'
      end

      def on_4XX
        raise 'generic server error if no specific error code defined'
      end
      
      def on_failure
        {}
      end
    end