Implement till 3.4

ex-3_01-xx.scm

@@ -0,0 +1,82 @@
+(load "util.scm")
+
+(display "\nex-3.1\n")
+
+(define (make-accumulator value)
+  (define (accumulate new-value)
+    (set! value (+ value new-value))
+    value)
+  accumulate)
+
+(define A (make-accumulator 5))
+(assert (A 10) 15)
+(assert (A 10) 25)
+
+(display "\nex-3.2\n")
+
+(define (make-monitored proc)
+  (define count 0)
+  (define (dispatch m)
+    (cond ((eq? m 'how-many-calls?) count)
+          ((eq? m 'reset-count) (set! count 0) count)
+          (else (set! count (inc count))
+                (proc m))))
+  dispatch)
+
+(define s (make-monitored sqrt))
+
+(assert (s 100) 10)
+(assert (s 9) 3)
+(assert (s 'how-many-calls?) 2)
+(s 'reset-count)
+(assert (s 'how-many-calls?) 0)
+
+
+(display "\nex-3.3\n")
+
+(define (make-account balance password)
+  (define invalid-pw-attempts 0)
+  (define (withdraw amount)
+    (if (>= balance amount)
+        (begin (set! balance (- balance amount))
+               balance)
+        "Insufficient funds"))
+  (define (deposit amount)
+    (set! balance (+ balance amount))
+    balance)
+  (define (incorrect-password value)
+    "Incorrect password")
+  (define (call-the-cops value)
+    "Call the cops!")
+  (define (dispatch pw m)
+    (if (eq? pw password)
+      (begin
+        (set! invalid-pw-attempts 0)
+        (cond
+          ((eq? m 'withdraw) withdraw)
+          ((eq? m 'deposit) deposit)
+          (else (error "Unknown request -- MAKE-ACCOUNT" m))))
+      (cond
+         ((= invalid-pw-attempts 3) call-the-cops)
+         (else
+           (set! invalid-pw-attempts (inc invalid-pw-attempts))
+           incorrect-password))))
+  dispatch)
+
+(define acc (make-account 100 'secret-password))
+
+(assert ((acc 'secret-password 'withdraw) 40) 60)
+(assert ((acc 'some-other-password 'deposit) 50) "Incorrect password")
+
+(display "\nex-3.4\n")
+
+((acc 'some-other-password 'deposit) 50)
+(assert ((acc 'secret-password 'deposit) 50) 110)
+((acc 'some-other-password 'deposit) 50)
+((acc 'some-other-password 'deposit) 50)
+((acc 'some-other-password 'deposit) 50)
+(assert ((acc 'some-other-password 'deposit) 50) "Call the cops!")
+
+
+(display "\nex-3.5\n")
+