Is there any pratical difference for atom and null in Common Lisp if they receive a list as an argument?

Question

I am using SBCL, Emacs, and Slime. According to the book Symbolic Computation: A gentle introduction to Common Lisp, the definition of atom is:

The ATOM predicate returns T if its input is anything other than a cons cell.

On the other hand:

The NULL predicate returns T if its input is NIL. Its behavior is the same as the NOT predicate.

Using the REPL and taking only lists as arguments, I can only think of examples in which they retrieve the same results:

CL-USER> (atom (cons 1 nil))
NIL
CL-USER> (null (cons 1 nil))
NIL
CL-USER> (atom (cons 1 (cons 2 nil))) 
NIL
CL-USER> (null (cons 1 (cons 2 nil))) 
NIL
CL-USER> (atom (cons 1 2))
NIL
CL-USER> (null (cons 1 2))
NIL
CL-USER> (atom '())
T
CL-USER> (null '())
T
CL-USER> (null nil) ;; just changing notation from previous
T
CL-USER> (atom nil) ;; just changing notation from previous
T
CL-USER> (atom '(1 . 2)) ;; just changing notation from previous
NIL
CL-USER> (null '(1 . 2)) ;; just changing notation from previous
NIL
CL-USER> (atom '(1 2))  ;; just changing notation from previous
NIL
CL-USER> (null '(1 2))  ;; just changing notation from previous
NIL 
CL-USER> (atom '(1))  ;; just changing notation from previous
NIL
CL-USER> (null '(1))  ;; just changing notation from previous
NIL

Based on the definitions, the examples above, and considering that non-empty lists are cons cells, I am concluding that:

if the argument is a list, null and atom do have the same behavior as predicates.

Is there any counter-example for this statement? Did I miss something? Is it correct?

Answer 1

A list is either:

• the empty list, () aka nil, which is not a cons;
• or a cons of any object and a list.

null returns true only for nil; atom returns true for any object which is not a cons.

Therefore, when restricted to lists, null and atom return true only for nil, being the only object which is both a list and not a cons, and are, for lists, equivalent predicates.