dynvar

;;; dynvar.lisp
;;; S. Edward Dolan
;;; Wednesday, May  1 2019
;;;
;;; If you call FN1 the output will be:
;;;
;;; global *foo* value: 42
;;;    fn1 *foo* value: 99
;;;    fn2 *foo* value: 99
;;;    fn3 *foo* value: 99
;;;    fn1 *foo* value: 0.0333
;;; global *foo* value: 42

;;; The thing to note here is the program isn't explicitly saving and
;;; restoring the values. Lisp handles it behind the scenes.

(defvar *foo* 42)                       ;globally bind *foo* to 42

(defun fn1 ()
  (format t " global *foo* value: ~S~%" *foo*)
  (let ((*foo* 99))                     ;dynamically bind *foo* to 99
    (format t "    fn1 *foo* value: ~S~%" *foo*)
    (fn2)                               ;call fn2
    (format t "    fn1 *foo* value: ~S~%" *foo*))
  ;; The global binding is reinstated when the LET form completes
  (format t " global *foo* value: ~S~%" *foo*))

(defun fn2 ()
  ;; Here, *foo* retains the value bound in fn1
  (format t "    fn2 *foo* value: ~S~%" *foo*)
  (fn3)                                 ;call fn3
  ;; You can reset the var at-will and only the current binding will be
  ;; affected. Now fn1 will see this value.
  (setf *foo* 33.3e-3))

(defun fn3 ()
  ;; Again, *foo* retains the value bound in fn1
  (format t "    fn3 *foo* value: ~S~%" *foo*)
  ;; You could rebind it again here in a LET form and any function called
  ;; within the body of the LET will see that value.
  )