exercises and notes from "Gentle introduction"

optional-arguments.lisp

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+;;; &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)