What is the purpose of null?

Question

I am in a compilers class and we are tasked with creating our own language, from scratch. Currently our dilemma is whether to include a \'null\' type or not. What purpose do

Answer 1

Null provides an easy way out for programmers who haven't completely thought through the logic and domains needed by their program, or the future maintenance implications of using a value with essentially no clear and agreed upon definition.

It may seem obvious at first that it must mean "no value", but what that ACTUALLY means depends on context. If, for instance LastName === null, does that mean that person doesn't have a last name, or that we don't know what their last name is, or that it hasn't be entered into the system yet? Does null equal itself, or doesn't it? In SQL it does not. In many languages it does. But if we don't know the value of personA.lastName, or personB.lastName, how can we know that personA.lastName === personB.lastName, eh? Should the result be false, or .. . null?

It depends on what you're doing, which is why it's dangerous and silly to have some kind of system wide value that can be used for any kind of situation that kind of looks like "nothing", since how other parts of your program and external libraries or modules can't really be depended upon to correctly interpret what you meant by "null".

You're much better off clearly defining the DOMAIN of possible values of lastName, and exactly what every possible value actually means, rather than depending on some vague systemwide notion of null, which may or may not have any relevance to what you're doing, depending on which language you're using, and what you're trying to do. A value, which may in fact, behave in exactly the wrong way when you begin to operate on your data.