Complete 3.22

3_22.rkt

@@ -0,0 +1,46 @@
+#lang sicp
+
+(define (make-queue)
+  (let ((front-ptr '())
+        (rear-ptr '()))
+    (define (front)
+      (cond ((empty?)
+             (error "FRONT called with empty queue"))
+            (else
+             (car front-ptr))))
+    (define (insert! item)
+      (let ((new-pair (cons item '())))
+        (cond ((empty?) 
+               (set! front-ptr new-pair)
+               (set! rear-ptr new-pair))
+              (else
+               (set-cdr! rear-ptr new-pair)
+               (set! rear-ptr new-pair)))))
+    (define (delete!)
+      (cond ((empty?)
+             (error "DELETE! called with empty queue"))
+            (else
+             (set! front-ptr (cdr front-ptr)))))
+    (define (dispatch m)
+      (cond ((eq? m 'empty?)
+             (null? front-ptr))
+            ((eq? m 'front) (front))
+            ((eq? m 'insert!) insert!)
+            ((eq? m 'delete!) (delete!))
+            ((eq? m 'print)
+             (display front-ptr)
+             (newline))
+            (else
+             (error "Unknown method: " m))))
+    dispatch))
+    
+(define q1 (make-queue))
+(q1 'print)
+((q1 'insert!) 'a)
+(q1 'print)
+((q1 'insert!) 'b)
+(q1 'print)
+(q1 'front)
+(q1 'print)
+(q1 'delete!)
+(q1 'print)