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

I don't think it's helpful to talk about null outside the context of the whole language design. First point of confusion: is the null type empty, or does it include a single, distinguished value (often called "nil")? A completely empty type is not very useful---although C uses the empty return type void to mark a procedure that is executed only for side effect, many other languages use a singleton type (usually the empty tuple) for this purpose.

I find that a nil value is used most effectively in dynamically typed languages. In Smalltalk it is the value used when you need a value but you don't have any information. In Lua it is used even more effectively: the nil value is the only value that cannot be a key or a value in a Lua table. In Lua, nil is also used as the value of missing parameters or results.

Overall I would say that a nil value can be useful in a dynamically typed setting, but in a statically typed setting, a null type is useful only for talking about functions (or procedures or methods) that are executed for side effect.

At all costs, avoid the NULL pointer used in C and Java. These are artifacts inherent in the implementations of pointers and objects, and in a well designed lanugage they should not be allowed. By all means give your users a way to extend an existing type with a null value, but make them do it explicitly, on purpose---don't force every type to have one by accident. (As an example of explicit use, I recently implemented Bentley and Sedgewick's ternary search trees in Haskell, and I needed to extend the character type with one additional value meaning 'not a character'. For this purpose Haskell provides the Maybe type.)

Finally, if you are writing a compiler, it is good to remember that the easiest parts of the language to compile, and the parts that cause the fewest bugs, are the parts that aren't there :-)