Most of the time there just isn't any need to make a difference between the three. There are basically just two common situations where you need to be able to t

Most of the time there just isn't any need to make a difference between the three. There are basically just two common situations where you need to be able to tell the difference: 1. Telling apart optional arguments to a function that weren't given from arguments that were given and were NIL. Typically giving a default value for the argument is sufficient to avoid the problem, but you can also add an extra parameter that tells whether the argument was given. (defun foo (&optional (x :default-value xp)) ;; X holds the argument given by the caller, or the keyword ;; :DEFAULT-VALUE if nothing was given. ;; XP is T if the argument was given, or NIL if it was not given. ) 2. Finding an item that may be NIL. Functions such as GETHASH (which looks up a key in a hash table) usually allow you to specify a default value, and they will return a second value to indicate whether something was found. ;; Increment the value of KEY in TABLE, or if the key doesn't ;; exist, use 10 as a default, increment that and set the ;; result to the table. (incf (gethash key table 10)) (multiple-value-bind (value existsp) (gethash key table) ;; VALUE holds the value if it exists, or NIL if it doesn't. ;; EXISTSP is T if the key exists, or NIL if it doesn't. ) Notice that in CL additional return values can be ignored if you're not interested in them, so you only need to use MULTIPLE-VALUE-BIND when you actually need it. You could also wrap it in a trivial macro if you're using it a lot. (defmacro if-some-let ((var form) &body (then &optional else)) (let ((somep (gensym "SOMEP"))) `(multiple-value-bind (,var ,somep) ,form (if ,somep ,then ,else)))) (if-some-let (val (gethash key table)) (format t "Found: ~a~%" val) (format t "Didn't find.~%"))