why_nil.rb

class Test
  def thing
    @thing ||= "Hi"
  end

  def thing=(new_thing)
    @thing = new_thing
  end

  def do_stuff
    puts thing

    if false
      thing = "Huh!"
    end

    puts thing || "Why nil?"
  end
end

t = Test.new
t.do_stuff

why_nil2.rb

class Test
  attr_accessor :thing

  def do_stuff
    puts thing

    if false
      thing = "Huh!"
    end

    puts thing || "Why nil?"
  end
end

t = Test.new
t.thing = "hi"
t.do_stuff