Completed 2.38

2_38.sch

@@ -0,0 +1,21 @@
+(define (fold-right op initial sequence)
+  (if (null? sequence)
+      initial
+      (op (car sequence)
+          (fold-right op initial (cdr sequence)))))
+
+(define (fold-left op initial sequence)
+  (define (iter result rest)
+    (if (null? rest)
+        result
+        (iter (op result (car rest))
+              (cdr rest))))
+  (iter initial sequence))
+
+; fold-left and fold-right equal if op is commutative
+
+(define (reverse-r sequence)
+  (fold-right (lambda (x y) (append y (list x))) '() sequence))
+
+(define (reverse-l sequence)
+  (fold-left (lambda (x y) (append (list y) x)) '() sequence))