start chapter 3

2 files changed, 71 insertions, 0 deletions

diff --git a/coding-exercises/3/1.rkt b/coding-exercises/3/1.rkt
new file mode 100644
index 0000000..4299e9d
--- /dev/null
+++ b/coding-exercises/3/1.rkt
@@ -0,0 +1,71 @@
+#lang racket
+;; 1
+(define (make-accumulator amount)
+  (lambda (add-this)
+    (set! amount (+ amount add-this))
+    amount))
+(define A (make-accumulator 5))
+(A 10)
+(A 15)
+
+(define B (make-accumulator 5))
+(B 10)
+(B 15)
+
+;; 2
+(define (make-monitored fn)
+  (define calls (make-accumulator 0))
+  (lambda (arg-or-message)
+    (cond ((equal? arg-or-message 'how-many-calls?) (calls 0))
+          ((equal? arg-or-message 'reset-count) (set! calls (make-accumulator 0)))
+          (else (begin
+                  (calls 1)
+                  (fn arg-or-message))))))
+(define s (make-monitored sqrt))
+(s 100)
+(s 'how-many-calls?)
+
+;; 3
+(define (make-account balance secret)
+  (define (withdraw amount)
+    (if (>= balance amount)
+      (begin (set! balance (- balance amount)) balance)
+      "Insufficient funds"))
+  (define (deposit amount)
+    (set! balance (+ balance amount))
+    balance)
+  (define (dispatch m)
+    (cond ((eq? m 'withdraw) withdraw)
+          ((eq? m 'deposit) deposit)
+          (else (error "Unknown request -- MAKE-ACCOUNT" m))))
+
+  (define (args->dispatch fn)
+    (lambda (arg-list)
+      (let ((password (car arg-list))
+            (m (cadr arg-list)))
+        (if (eq? password secret)
+          (fn m)
+          (lambda (a . n)
+            "Wrong password")))))
+
+  (define monitored-dispatch (make-monitored (args->dispatch dispatch)))
+  (define (call-the-cops)
+    (lambda (a . n)
+      "Calling the cops"))
+
+  (define (safe-dispatch . args)
+    (if (> 7 (monitored-dispatch 'how-many-calls?))
+      (monitored-dispatch args)
+      (call-the-cops)))
+  safe-dispatch)
+
+(define acc (make-account 100 'secret-password))
+((acc 'secret-password 'withdraw) 10)
+((acc 'some-other-password 'deposit) 50)
+((acc 'some-other-password 'deposit) 50)
+((acc 'some-other-password 'deposit) 50)
+((acc 'some-other-password 'deposit) 50)
+((acc 'some-other-password 'deposit) 50)
+((acc 'some-other-password 'deposit) 50)
+((acc 'some-other-password 'deposit) 50)
+((acc 'some-other-password 'deposit) 50)
diff --git a/coding-exercises/3/5.rkt b/coding-exercises/3/5.rkt
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/coding-exercises/3/5.rkt