gistfile1.txt

$ ruby hoist_wtf.rb
hoist_wtf.rb:8:in `do_thing': undefined method `+' for nil:NilClass (NoMethodError)
	from hoist_wtf.rb:13:in `<main>'
$

hoist_wtf.rb

class FoobarDoesThings
  def initialize(foo)
    @foo=foo
  end

  def do_thing
    foo
    foo += 1
  end
  attr_reader :foo
end

FoobarDoesThings.new(1).foo

$ irb
irb(main):001:0> class FoobarDoesThings
irb(main):002:1>   def initialize(foo)
irb(main):003:2>     @foo=foo
irb(main):004:2>   end
irb(main):005:1> 
irb(main):006:1*   def do_thing
irb(main):007:2>     foo
irb(main):008:2>     foo += 1
irb(main):009:2>   end
irb(main):010:1>   attr_reader :foo
irb(main):011:1> end
=> nil
irb(main):012:0> 
irb(main):013:0* FoobarDoesThings.new(1).foo
=> 1

$ cat foo.rb 
class FoobarDoesThings
  def initialize(foo)
    @foo=foo
  end

  def do_thing
    foo
    foo += 1
  end
  attr_reader :foo
end

$ ruby foo.rb

^ produces nothing, not an exception either.