Any symbol can be used as a function name.2 Usually function names contain only alphabetic characters and hyphens, but other characters are allowed and are used in certain naming conventions. For instance, functions that convert one kind of value to another sometimes use -> in the name. For example, a function to convert strings to widgets might be called string->widget. The most important naming convention is the one mentioned in Chapter 2, which is that you construct compound names with hyphens rather than underscores or inner caps. Thus, frob-widget is better Lisp style than either frob_widget or .

A function’s parameter list defines the variables that will be used to hold the arguments passed to the function when it’s called.3 If the function takes no arguments, the list is empty, written as (). Different flavors of parameters handle required, optional, multiple, and keyword arguments. I’ll discuss the details in the next section.

Finally, the body of a **DEFUN** consists of any number of Lisp expressions. They will be evaluated in order when the function is called and the value of the last expression is returned as the value of the function. Or the **RETURN-FROM** special operator can be used to return immediately from anywhere in a function, as I’ll discuss in a moment.

In Chapter 2 we wrote a hello-world function, which looked like this:

The following is a slightly more complex function:

This function is named verbose-sum, takes two arguments that will be bound to the parameters x and y, has a documentation string, and has a body consisting of two expressions. The value returned by the call to **+** becomes the return value of verbose-sum.