null object → boolean

object — an object.

boolean — a boolean.

Returns t if object is the empty list; otherwise, returns nil.

(null '()) → T 
(null nil) → T 
(null t) → NIL 
(null 1) → NIL

not

null is intended to be used to test for the empty list whereas not is intended to be used to invert a boolean (or generalized boolean). Operationally, null and not compute the same result; which to use is a matter of style.

(null object) ≡ (typep object 'null) ≡ (eq object '())