Booleans and Generalized Booleans

True and False

The special symbol T represents the value true in Common Lisp, while the special symbol NIL represents false:

CL-USER> (= 3 3)
T
CL-USER> (= 3 4)
NIL

They are called “Constant Variables” (sic!) in the standard, since they are variables whose value cannot be modified. As a consequence, you cannot use their names for normal variables, like in the following, incorrect, example:

CL-USER> (defun my-fun(t)
           (+ t 1))
While compiling MY-FUN :
Can't bind or assign to constant T.

Actually, one can consider them simply as constants, or as self-evaluated symbols. T and NIL are specials in other senses, too. For instance, T is also a type (the supertype of any other type), while NIL is also the empty list:

CL-USER> (eql NIL '())
T
CL-USER> (cons 'a (cons 'b nil))
(A B)

Generalized Booleans

Actually any value different from NIL is considered a true value in Common Lisp. For instance:

CL-USER> (let ((a (+ 2 2)))
           (if a
               a
               "Oh my! 2 + 2 is equal to NIL!"))
4

This fact can be combined with the boolean operators to make programs more concise. For instance, the above example is equivalent to:

CL-USER> (or (+ 2 2) "Oh my! 2 + 2 is equal to NIL!")
4

The macro OR evaluates its arguments in order from left to right and stops as soon as it finds a non-NIL value, returning it. If all of them are NIL, the value returned is NIL:

CL-USER> (or (= 1 2) (= 3 4) (= 5 6))
NIL

Analogously, the macro AND evaluates its arguments from left to right and returns the value of the last, if all of them are evaluated to non-NIL, otherwise stops the evaluation as soon as it finds NIL, returning it:

CL-USER> (let ((a 2)
               (b 3))
           (and (/= b 0) (/ a b)))
2/3
CL-USER> (let ((a 2)
               (b 0))
           (and (/= b 0) (/ a b)))
NIL

For these reasons, AND and OR can be considered more similar to control structures of other languages, rather than to boolean operators.