;; C311 Programming Languages
;; Dana Vrajitoru
;; A small example used in class

;; Examples of function indirection
(fset 'first (symbol-function 'car))
#<subr car>
(first '(1 2 3))
1
(fset 'xfirst 'car)
car
(xfirst '(1 2 3))
1
(fset 'yfirst 'xfirst)
xfirst
(yfirst '(1 2 3))
1
(fset 'first 'yfirst)
yfirst
(fset 'xfirst 'first)
first
(yfirst '(1 2 3))
(defun g (x) (* x x)) 
g
(fset 'g2 (symbol-function 'g))
(lambda (x) (* x x))


;; First version of the function that copies a list using a recursive
;; call.
(defun copy (L)
  (let ((C '()) (N nil))
  (setq C 
	(cond 
	 ((not L) nil)
	 ((not (cdr L))
	  (list (car L)))
	 (t (setq N (copy (cdr L)))
	    (push (car L) N))))
  ))

(setq M '(1 2 3))
(1 2 3)
(copy '())
nil
(copy M)
(1 2 3)

(eq M (copy M))
nil


;; Second implementation of the function copy-list using a dolist
;; loop.
(defun copy1 (L)
  (let ((C '()))
    (dolist (x L C)
      (setq C (append C (list x))))))

(copy1 '())
nil
(copy1 M)
(1 2 3)


;; Third implementation of the function copy-list using mapcar and a
;; simple lambda expression for the identity function.
(defun copy2 (L)
  (mapcar (lambda (x) x) L))
copy2

(copy2 M)
(1 2 3)
(copy2 '())
nil

(eq M (copy2 M))
nil

;; Another example where we use a recursive call and make a deep copy
;; of the list.
(defun copy3 (L)
  (mapcar (lambda (x) (if (listp x) (copy3 x) x)) L))

(setq M '(1 2 (3 4) 5))

(copy3 M)
(1 2 (3 4) 5)

; Note that copy3 make a hard copy of the list nested inside
(eq (car (cdr (cdr M))) (car (cdr (cdr (copy3 M)))))
nil

; while copy2 just makes a reference to the same list inside
(eq (car (cdr (cdr M))) (car (cdr (cdr (copy2 M)))))
t