;;; Returns a list of members of LIST. Useful for dealing with circular lists.
;;; For a dotted list returns a secondary value of T -- in which case the
;;; primary return value does not include the dotted tail.
-(defun list-members (list)
+;;; If the maximum length is reached, return a secondary value of :MAYBE.
+(defun list-members (list &key max-length)
(when list
(do ((tail (cdr list) (cdr tail))
- (members (list (car list)) (cons (car tail) members)))
- ((or (not (consp tail)) (eq tail list))
- (values members (not (listp tail)))))))
+ (members (list (car list)) (cons (car tail) members))
+ (count 0 (1+ count)))
+ ((or (not (consp tail)) (eq tail list)
+ (and max-length (>= count max-length)))
+ (values members (or (not (listp tail))
+ (and (>= count max-length) :maybe)))))))
;;; Default evaluator mode (interpeter / compiler)