NULL is a hack

In C, the type “int” is for integers, and “int*” is for pointers to integers, right? Almost. A value of type “int*” is either a pointer to an integer or a special value called “null”, which is slightly different. If you want to declare an variable that is just a pointer to an integer, you’re out of luck. The same goes for Java and C#. There is no type that means “reference to a string”. The String type means “either a reference to a string or a special value called null”.

Some languages differentiate between references and optional references (i.e. a reference that might be null). C, Java, and C# only have the latter. I don’t know how null managed to sneak its way into every pointer type. This mess has been around for so long and had been so ingrained in my thinking that I never even noticed it until I was shown otherwise. Imagine if every int and boolean variable you used could optionally be null. Yeah, I know.

NullPointerException is a basically a runtime type error. You thought you were dealing with a pointer to something, but you weren’t. If Java hadn’t muddied every reference type with null, the NullPointerException would never have existed. That’s a million webpages that are crowding the internet for no good reason.

Haskell and ML (and statically typed functional languages in general) don’t let you stick null anywhere you want. There’s a generic type that adds optionality to an existing type. In Haskell, it’s just the extremely simple “Maybe” type; the type “Maybe String” is either a string or “Nothing”, but you can’t treat it like a string until you have checked.

The Nice programming language is an example of a C/Java-like language that distinguishes between references and optional references. If you want a reference to a string, the type is “String”. If you want something that is either a reference to a string or null, the type is “?String”. Unfortunately, due to the desire to interoperate well with Java, Nice’s implementation isn’t completely general. The wrinkles show in the treatment of tyep parameters and multiple levels of options (“??String”).

Some of Microsoft’s C#-derived research languages use C# annotations to say whether parameters and fields are null or not. The relevant paper discusses the details. One of the more interesting complications arises because that the compiler can no longer just initialize a reference to null when it doesn’t have anything else to assign.