православный манкипатчинг

monkey-patching.rb

module MyStringThing
  refine String do
    def at(num)
      self[num..num]
    end
  end
end
 
class A
  using MyStringThing
  def fifth(str)
    str.at(4)
  end
end
 
class B
  def fifth(str)
    str.at(4)
  end
end
 
A.new.fifth("qwerty")
# => "t" 
B.new.fifth("qwerty")
# NoMethodError: undefined method `at' for "qwerty":String