"Optional" parameters - p. 83

--If the symbol &optional occurs in the parameter list of a function, then all

the arguments after it are optional, and default to nil.

1.

(defun philosoph (thing &optional property) ;; property defaults to nil

(list thing 'is property))

>(philosoph 'existence)

(EXISTENCE IS NIL)

(defun philosoph (thing &optional (property 'fun)) ;; property defaults to 'fun

(list thing 'is property))

>(philosoph 'existence)

(EXISTENCE IS FUN)

2. Using &optional for tail recursion

;;Vers. 1 - no tail recursion, (uses backtracking)

(defun my-length (lst)

(if (null lst)

0

(+ 1 my-length(rest (lst)))))

;;Vers. 2 - tail recursion (no backtracking) with an auxiliary function

(defun my-length (lst)

(my-length-aux lst 0))

(defun my-length-aux (lst count)

(if (null lst)

count

(my-length-aux (rest lst) (+ 1 count))))

;;Vers. 3 - tail recursion (no backtracking) with &optional

(defun my-length (lst &optional (count 0)) ;;count defaults to 0

(if (null lst)

count

(my-length (rest lst) (+ 1 count))))

>(my-length '(a (b c) (d e) f))

4