SICP_3.1-3.22

 1 ;;;;;;;;;;;3.1
 2 
 3 (define (make-accumulator base)
 4   (lambda (num)
 5     (set! base (+ base num))
 6     base))
 7 
 8 (define A (make-accumulator 5))
 9 
10 (A 10)
11 (A 10)
12 
13 ;;;;;;;;;;3.2
14 
15 (define (make-monitored f)
16   (let ((count 0))
17     (define (mf x)
18       (cond ((eq? x ‘how-many-calls?) count)
19             ((eq? x ‘reset-count) (set! count 0))
20             (else (begin (set! count (+ count 1))
21                     (f x)))))
22     mf))
23 
24 (define square (lambda (x) (* x x)))
25 (define s (make-monitored square))
26 
27 (s 100)
28 (s 4)
29 (s ‘how-many-calls?)
30 (s ‘reset-count)
31 (s ‘how-many-calls?)