Ruby: def def

Sometimes some Ruby programmers that want to give me a heart attack write code like this:

class Thing
  def run
    def loop_thing(i)
      puts i
    end
    10.times { loop_thing(i) }
  end
end

That’s a really cool nested function you made there! Only one problem before you go — RUBY DOESN’T HAVE NESTED FUNCTIONS.

thing = Thing.new
thing.run # 0..9
thing.loop_thing(31337) # 31337, no <NameError>

As we see here — in addition to being able to use define_method, you can actually run def inside of a method body.

Now, cover your eyes if you don’t want to see the scary thing

class Array
  def stop_the_mapping!
    def map
      raise 'But you told me to stop ;('
    end
  end
end

arr = [1, 2, 3]
arr.map { |x| x + 1 } # [2, 3, 4]
arr.stop_the_mapping!
arr.map { |x| x + 1 } # <RuntimeError>