qquark

  irb(main):001:0> a = 1
  => 1
  irb(main):002:0> class Integer
  irb(main):003:1>   def hello
  irb(main):004:2>     puts "world"
  irb(main):005:2>   end
  irb(main):006:1> end
  => :hello
  irb(main):007:0> a.hello
  world
  => nil

  >>> x = 1
  >>> f"{x}"
  '1'

  >>> "{x}".format(x=x)
  '1'

  >>> "{}".format(x)
  '1'