lisp - pushnew without place support. Is it considered a destructive macro?

Question

One function and one macro

;; I am sorry for the confusing function name.
;; As one answer suggests, cons-if-not-member is the better name.
(defun cons-if-member (element list)
  (if (member element list)
      list
    (cons element list)))

(defmacro pushnew-no-bells (element list)
  "pushnew without place support"
  `(setq ,list (cons-if-member ,element ,list)))

(let ((xx (list 1 2)))
  (pushnew-no-bells 0 xx)
  xx)

I do not know which of the following is correct:

1. cons-if-member is a non-destructive function and pushnew-no-bells is a destructive macro.

2. Both are non-destructive.

3. cons-if-member is a non-destructive function and the adjectives "destructive" and "non-destructive" do not apply to macros.

4. none of the above

I do not have any idea on whether pushnew is considered destructive or not either, but I wanted to make things simple by dropping place support first.

Answer 1

pushnew changes the value of it's place-form (2nd argument), so it is destructive: it changes something "in place" instead of just creating a new object (which may share structure with an existing one). Your cons-if-member (which would better be called cons-unless-member or cons-if-not-member) does not modify anything "in place", so it's actually non-destructive.

Note, BTW., that you cannot really exclude "general place" support, due to the presence of symbol macros. Observe:

(defclass foo ()
  ((x :initform nil)))

(let ((instance (make-instance 'foo)))
  (with-slots (x) instance
    (pushnew-no-bells 1 x))
  (format t "~&Now: ~S~%" (slot-value instance 'x)))