Update step_2+3a.rkt : handling inappropriate messages for multiple message in...

Update step_2+3a.rkt : handling inappropriate messages for multiple message in send (send p 'bar 2) + sety func

step_2+3a.rkt

@@ -7,14 +7,16 @@
 ; It will allow us to improve the syntax of the code, by allowing to
 ; send messages to objects in a clearer manner.
 
-; Defined below, the send operater first looks if the receiver object is indeed
-; an appropriate receiver object, else it returns a specific error.
+; Defined further below, the send operater first looks if the receiver object is 
+; indeed an appropriate receiver object, else it returns a specific error.
 ; Then, it will try to apply the message.s to the object.
 
 ; A point object, constituted of two coordinates and returning a self function.
 (define (point x y)
   (define (setx value)
     (set! x value))
+  (define (sety value) ;added a sety function for consistency
+    (set! y value))
   (define (add p)
     (point (+ x (p 'getx)) (+ y (p 'gety))))
   (define (self m)
@@ -22,7 +24,8 @@
           ((eq? m 'gety) y)
           ((eq? m 'type) 'point)
           ((eq? m 'info) (list (self 'type) (self 'getx) (self 'gety)) )
-          ((eq? m 'setx!) setx )
+          ((eq? m 'setx!) setx)
+          ((eq? m 'sety!) sety)
           ((eq? m 'add) add)
           (else (display "Message not understood"))))
   self)
@@ -44,15 +47,20 @@
       
       (cond
         ((null? args) (p m))
-        (else ((p m) (car args))) ; We have to take the "car" of the args list
+        (else
+         (if (procedure? (p m))
+             ((p m) (car args)) ; We have to take the "car" of the args list
+             ; If (p m) is not a procedure, we do nothing and let the error be printed
+             )
+         ) 
         )
-      ; If the first parameter, p, is not a procedure, then we return a special error
+      ; If the first parameter, p, is not a procedure, then we return a special error,
+      ; because p is not an appropriate receiver object for the messages.
       (display "Inappropriate receiver object") )
   )
 
 
 ; Sample code illustrating the execution of the step
-
 ; We start by creating two points, p1 and p2
 
 (define p1 (point 1 2))
@@ -90,6 +98,6 @@
 (display (send p 'foo)) ; should display : Message not understood
 (newline)
 
-(display (send p 'bar 2)) ; should display : Message not understood, then crash
+(display (send p 'bar 2)) ; should display : Message not understood
 (newline)