Another option in the language design space which this article doesn't mention is to have different kinds of null indicating what kind of object this null value is intended to be a stand-in for. NULL is a generic non-value, but it's also possible to have null-number, null-string, null-character, etc. which can be handy for detecting certain kinds of type errors. In particular, a constructor for a class C can return a null-C if it fails, which can be very useful for debugging. One of the problems with generic NULLs is that it can be very hard to track down where they came from.