not
x ⇒ boolean
x | a generalized boolean (i.e., any object). |
boolean | a boolean. |
(not nil) ⇒ T
(not '()) ⇒ T
(not (integerp 'sss)) ⇒ T
(not (integerp 1)) ⇒ NIL
(not 3.7) ⇒ NIL
(not 'apple) ⇒ NIL
not is intended to be used to invert the ‘truth value’ of a boolean (or generalized boolean) whereas null is intended to be used to test for the empty list. Operationally, not and null compute the same result; which to use is a matter of style.