AND, OR, and NOT

When writing the conditions in **IF**, **WHEN**, **UNLESS**, and **COND** forms, three operators that will come in handy are the boolean logic operators, **AND**, **OR**, and **NOT**.

**NOT** is a function so strictly speaking doesn’t belong in this chapter, but it’s closely tied to **AND** and **OR**. It takes a single argument and inverts its truth value, returning **T** if the argument is **NIL** and **NIL** otherwise.

**AND** and **OR**, however, are macros. They implement logical conjunction and disjunction of any number of subforms and are defined as macros so they can short-circuit. That is, they evaluate only as many of their subforms—in left-to-right order—as necessary to determine the overall truth value. Thus, **AND** stops and returns **NIL** as soon as one of its subforms evaluates to **NIL**. If all the subforms evaluate to non-**NIL**, it returns the value of the last subform. **OR**, on the other hand, stops as soon as one of its subforms evaluates to non-**NIL** and returns the resulting value. If none of the subforms evaluate to true, **OR** returns **NIL**. Here are some examples:

(not nil)             ==> T
(not (= 1 1))         ==> NIL
(and (= 1 2) (= 3 3)) ==> NIL
(or (= 1 2) (= 3 3))  ==> T