Before you go too much deeper into LISP subrs and functions, you need to understand the types of data you may be manipulating. LISP data is either an atom or a list. Making this definition even simpler is the notion that an atom is anything that does not have parentheses, except the NIL symbol or value. NIL is an empty list and is usually not shown with parentheses. NIL is the atomic value nothing, or empty. You use NIL also to represent the value false when working with true-false logic in a LISP program. Anything that is not NIL is true.

Visual LISP contains several atomic data types, such as number and text atoms. You perform arithmetic-type operations on numbers, but not with text. The atomic types help keep your variables in order. Plus, you can test the types during program execution, giving you tremendous control over the environment your program runs in.

Table 3.1 describes the specific atomic types in Visual LISP. Because Visual LISP is intended for use in the AutoCAD environment, some elements are specific to AutoCAD and do not exist in other LISP environments.