> If a language makes it easy to conflate nil and absence, to write code that does not reliably distinguish between success and failure, then bugs are sure to follow.

This is why after working on large apps in Ruby and Python, I strongly prefer Python. Accessing a non-existent key in a Ruby dictionary returns nil, whereas in Python a KeyError exception is thrown.

If you store nil in a Ruby dictionary, then you can't test for membership unless you use hash.fetch.