Function Parameter Lists

There’s not a lot more to say about function names or documentation strings, and it will take a good portion of the rest of this book to describe all the things you can do in the body of a function, which leaves us with the parameter list.

The basic purpose of a parameter list is, of course, to declare the variables that will receive the arguments passed to the function. When a parameter list is a simple list of variable names—as in verbose-sum--the parameters are called required parameters. When a function is called, it must be supplied with one argument for every required parameter. Each parameter is bound to the corresponding argument. If a function is called with too few or too many arguments, Lisp will signal an error.

However, Common Lisp’s parameter lists also give you more flexible ways of mapping the arguments in a function call to the function’s parameters. In addition to required parameters, a function can have optional parameters. Or a function can have a single parameter that’s bound to a list containing any extra arguments. And, finally, arguments can be mapped to parameters using keywords rather than position. Thus, Common Lisp’s parameter lists provide a convenient solution to several common coding problems.