not

not Function

Syntax:

not x → boolean

Arguments and Values:

x—a generalized boolean (i.e., any object).

boolean—a boolean.

Description:

Returns t if x is false; otherwise, returns nil.

Examples:

(not nil) → T 
(not ’()) → T 
(not (integerp ’sss)) → T 
(not (integerp 1)) → NIL 
(not 3.7) → NIL 
(not ’apple) → NIL 

See Also:

null

Notes:

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.

Expanded Reference: not

Basic boolean negation

not returns T if its argument is NIL, and NIL otherwise.

(not nil) → T
(not t) → NIL

Any non-NIL value is truthy

In Common Lisp, everything except NIL is considered true. not reflects this.

(not 42) → NIL
(not "hello") → NIL
(not 'apple) → NIL
(not '(1 2 3)) → NIL

Using not with predicates

not is commonly used to invert the result of a predicate call.

(not (integerp 5)) → NIL
(not (integerp "hello")) → T
(not (> 3 5)) → T
(not (member 'x '(a b c))) → T

not vs null

not and null compute the same result but express different intent. Use not for boolean logic and null for testing empty lists.

;; Boolean logic intent
(not (evenp 3)) → T

;; Empty list testing intent
(null '()) → T
(null '(a b)) → NIL

;; They are functionally identical
(eq (not '()) (null '())) → T

Chaining with not

not can be used to coerce a generalized boolean to a strict boolean (T or NIL).

(not (not 42)) → T
(not (not nil)) → NIL
(not (not (member 'b '(a b c)))) → T