Add exercise 3-7

Author: ramsayleung

chapter3/exercise3-3.rkt

@@ -35,3 +35,5 @@
      ))
 
   (run-tests module-test))
+
+(provide make-account)

chapter3/exercise3-7.rkt

@@ -0,0 +1,25 @@
+#lang racket
+(require "exercise3-3.rkt")
+(define (make-joint account old-password new-password)
+  (lambda (pwd m)
+    (if (eq? pwd new-password)
+        (account old-password m)
+        (error "Incorrect joint account password"))))
+
+(module+ test
+  (require rackunit)
+  (require rackunit/text-ui)
+  (define peter-acc (make-account 100 'open-sesame))
+  (define paul-acc (make-joint peter-acc 'open-sesame 'rosebud))
+
+  (define module-test
+    (test-suite
+     "Tests for make-joint"
+     (check-equal? ((peter-acc 'open-sesame 'withdraw) 40) 60 "Test original account with old password")
+     (check-equal? ((paul-acc 'rosebud 'withdraw) 40) 20 "Test joint account with new password")
+     (check-exn (regexp "Incorrect joint account password")
+                (lambda () ((paul-acc 'wrong-password 'deposit) 50)))
+     (check-equal? ((peter-acc 'open-sesame 'deposit) 30) 50 "Test original account with old password still works")
+     ))
+
+  (run-tests module-test))