;; Exercise 3.2: In software-testing applications, it is useful to be
;; able to count the number of times a given procedure is called
;; during the course of a computation. Write a procedure
;; make-monitored that takes as input a procedure, f, that itself
;; takes one input. The result returned by make-monitored is a third
;; procedure, say mf, that keeps track of the number of times it has
;; been called by maintaining an internal counter. If the input to mf
;; is the special symbol how-many-calls?, then mf returns the value of
;; the counter. If the input is the special symbol reset-count, then
;; mf resets the counter to zero. For any other input, mf returns the
;; result of calling f on that input and increments the counter. For
;; instance, we could make a monitored version of the sqrt procedure:
;;    (define s (make-monitored sqrt)) (s 100)
;;    10
;;    (s ’how-many-calls?)
;;    1
;; -----------------------------------
;; We did this together in class on Oct 21.
;;
;; $ scheme 3-2.scm 
;; (msqrt 100) is 10.
;; (msqrt 16) is 4.
;; (msqrt 'how-many-calls?) is 2.

(define (make-monitored f)
  (let ((counter 0))
    (lambda (x)
      ;; monitored version of f
      (cond ((eq? x 'reset) (set! counter 0))
            ((eq? x 'how-many-calls?) counter)
            (else (begin
                    (set! counter (+ 1 counter))
                    (f x)))))))

(define msqrt (make-monitored sqrt))
(printf "(msqrt 100) is ~s.\n" (msqrt 100))
(printf "(msqrt 16) is ~s.\n" (msqrt 16))
(printf "(msqrt 'how-many-calls?) is ~s.\n"
        (msqrt 'how-many-calls?))