Photo of Paul

Paul Rosania

Hi! I'm Paul. I work at Balsa.
You should follow me at @ptr.
I can also be reached at

Don't overload #nil?

November 9, 2011

There’s a popular post on Hacker News about writing confident code by, among other things, overloading Object#nil? and returning “null objects” instead of nil itself.


In Ruby, all objects (except nil itself) coerce to the boolean value true. Your object will be nil and true at the same time. Bad things will happen. Your coworkers will cry. Sad people from around the world will ask bewildering questions on your mailing list.

Here’s what happens:

class NullObject
  def nil?

o =

o.nil?             #=> true, great.
!!o                #=> true. not so great.
puts "exists" if o #=> "exists". ut oh.

Do you write all your guards using if o.nil? Neither do I.

If you overload #nil?, you will get burned. Please don’t.