;;; &optional (with and without default value)
;;; &rest
;;; &key (with or without default value)
;;; &aux - similar to LET* - additional variables, that can be calculated from other inputs, for example &rest
;;;
;; cool

;; default argument
(defun lala-1 (x &optional y)
  (format t "~&x value is ~S" x)
  (format t "~&y value is ~S" y)
  (list x y))

(lala-1 2 5)
(lala-1 7)

(defun lala-2 (x &optional (y 2))
    (format t "~&x value is ~S" x)
  (format t "~&y value is ~S" y)
  (list x y))

(lala-2 1 5)
(lala-2 7)

;; rest arguments - accessed as list, unlimited number on call
(defun lala-3 (x y &rest args)
  (append (list x y) args))

(lala-3 7 5 1 2 34 5 6)

;;; keyword arguments
(defun make-sundae (name &key (ice-cream `vanilla)
                           (syrup `chocolate)
                           nuts
                           cherries
                           whipped-cream)
  (list 'sundae
        (list 'for name)
        (list ice-cream 'with syrup 'sypup)
        (list 'toppings '=
              (remove nil
                      (list (and nuts 'nuts)
                            (and cherries 'cherries)
                            (and whipped-cream 'whipped-cream))))))

(make-sundae 'yo)
(make-sundae 'yo
             :syrup 'maple
             :whipped-cream t)

;;; auxillary variables
;; &aux - additional local variables?
;; "a matter of taste, whether to use LET* around body or &aux attributes"

(defun my-average (&rest args
                   &aux (len (length args)))
  (/ (reduce #'+ args) len))

(my-average 1 2 3 4 5 6)