Sometimes Ruby is Weird

nil_what_are_you_doing_stahp.rb

class NilWhatAreYouDoingStahp
  attr_accessor :not_nil

  def initialize
    @not_nil = "I'm not nil! I'm a value!"
  end

  def wat
    if not_nil.nil?
      not_nil = "Ok, now I'm not nil."
    end
    not_nil
  end

  def omglol
    not_nil
  end
end

stahp = NilWhatAreYouDoingStahp.new
stahp.wat # => nil
stahp.omglol # => "I'm not nil! I'm a value!"

The puzzle: Why does calling wat return nil?

It's actually a very straightforward answer, just a bit surprising.

This comes to us from @Sparkboxx btw!

Thanks for turning it into a fun puzzle instead of a head-banging-against-keyboard piece of code :-)

from @olery instead of @Sparkboxx, since I somehow can't swap the Gist page back to @Sparkboxx instead of the organization account

Although confusing at first this makes sense once you become familiar with the MRI internals (which is a bit of a sad requirement). This is because the Ruby parser pre scans scopes for the assignments of variables and processes them up to a certain extend. It doesn't assign any values yet though, instead it sets them to their default values (NilClass in this case).

In this particular bit of code Ruby sees that there's a possible local variable assignment in Foo#wat and as such pre-creates it. You can test this by adding p local_variables between lines 8 and 9 and you'll see that it displays [:not_nil].

Normally this wouldn't be an issue but since your local variable and the getter method are named the same way you get a conflict. In Ruby local variables have precedence over methods in the same scope. As a result this method will return the value of the local variable and not the getter.

To work around this issue one would have to use the following code:

class NilWhatAreYouDoingStahp
  attr_accessor :not_nil

  def initialize
    @not_nil = "I'm not nil! I'm a value!"
  end

  def wat
    # note the use of "self.", it's important in this case.
    if self.not_nil.nil?
      self.not_nil = "Ok, now I'm not nil."
    end
    self.not_nil
  end

  def omglol
    not_nil
  end
end

stahp = NilWhatAreYouDoingStahp.new
p stahp.wat
stahp.omglol

An even better alternative would be to use local variables with a different name.

Spot on! Another fun way to present this puzzle is to say that adding five characters makes the code work as expected. That's almost a giveaway though.