Paul Rosania

Hi! I'm Paul. I work at Slack.
You should follow me at @ptr.
I can also be reached at paul@rosania.org.

Don't overload #nil?

November 09, 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.

DO NOT DO THIS.

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?
    true
  end
end

o = NullObject.new

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.