Constants

One other kind of variable I haven’t mentioned at all is the oxymoronic “constant variable.” All constants are global and are defined with **DEFCONSTANT**. The basic form of **DEFCONSTANT** is like **DEFPARAMETER**.

(defconstant name initial-value-form [ documentation-string ])

As with **DEFVAR** and **DEFPARAMETER**, **DEFCONSTANT** has a global effect on the name used—thereafter the name can be used only to refer to the constant; it can’t be used as a function parameter or rebound with any other binding form. Thus, many Lisp programmers follow a naming convention of using names starting and ending with + for constants. This convention is somewhat less universally followed than the *-naming convention for globally special names but is a good idea for the same reason.14

Another thing to note about **DEFCONSTANT** is that while the language allows you to redefine a constant by reevaluating a **DEFCONSTANT** with a different initial-value-form, what exactly happens after the redefinition isn’t defined. In practice, most implementations will require you to reevaluate any code that refers to the constant in order to see the new value since the old value may well have been inlined. Consequently, it’s a good idea to use **DEFCONSTANT** only to define things that are really constant, such as the value of NIL. For things you might ever want to change, you should use **DEFPARAMETER** instead.