The Null type is the counterpart of Void and is used for function input parameters, to tell a function that expects a parameter being set that this parameter is empty. Nil. Nada. Nothing. (You might contemplate the important difference between 0 as a value, followed by 1, 2… - and the total absence of a value at all). Not every parameter may be null - in fact, most parameters are required for functions to work. See The Syntax of Functions: