Add exercise 3.8

3_8.rkt

@@ -0,0 +1,24 @@
+#lang sicp
+
+(define f
+  (let ((x 0))
+    (lambda (n)
+      (let ((old-x x))
+        (begin
+          (set! x n)
+          (cond ((= n 0)
+                 (if (= old-x 0) -1 0))
+                (else n)))))))
+
+(define left-to-right
+  (let ((left (f 0))
+        (right (f 1)))
+    (+ left right)))
+
+(define right-to-left
+  (let ((right (f 1))
+        (left (f 0)))
+    (+ left right)))
+
+;; (f 0) first: (+ (f 0) (f 1)) -> 0
+;; (f 1) first: (+ (f 0) (f 1)) -> 1