adjoin item list &key key test test-not → new-list

item — an object.

list — a proper list.

test — a designator for a function of two arguments that returns a generalized boolean.

test-not — a designator for a function of two arguments that returns a generalized boolean.

key — a designator for a function of one argument, or nil.

new-list — a list.

Tests whether item is the same as an existing element of list. If the item is not an existing element, adjoin adds it to list (as if by cons) and returns the resulting list; otherwise, nothing is added and the original list is returned.

The test, test-not, and key affect how it is determined whether item is the same as an element of list. For details, see Section 17.2.1 (Satisfying a Two-Argument Test).

(setq slist '()) → NIL 
(adjoin 'a slist) → (A) 
slist → NIL 
(setq slist (adjoin '(test-item 1) slist)) → ((TEST-ITEM 1)) 
(adjoin '(test-item 1) slist) → ((TEST-ITEM 1) (TEST-ITEM 1)) 
(adjoin '(test-item 1) slist :test 'equal) → ((TEST-ITEM 1)) 
(adjoin '(new-test-item 1) slist :key #'cadr) → ((TEST-ITEM 1)) 
(adjoin '(new-test-item 1) slist) → ((NEW-TEST-ITEM 1) (TEST-ITEM 1))

Should be prepared to signal an error of type type-error if list is not a proper list.

pushnew, Section 3.6 (Traversal Rules and Side Effects)

The :test-not parameter is deprecated.

(adjoin item list :key fn) 
  ≡ (if (member (fn item) list :key fn) list (cons item list))