Implement till 2.83

util.scm

@@ -35,16 +35,21 @@
       nil
       (cons low (enumerate-interval (+ low 1) high))))
 
-; Put and get functions. We could have implemented this via a list of
-; three-tuples, but I don't know how to create global variables yet so we just
-; use this code from SO. Doesn't look too complicated.
-; https://stackoverflow.com/questions/5499005/how-do-i-get-the-functions-put-and-get-in-sicp-scheme-exercise-2-78-and-on
-(define *op-table* (make-hash-table))
-(define (put op type proc)
-  (hash-table/put! *op-table* (list op type) proc))
-(define (get op type)
-  (let ((e (hash-table/get *op-table* (list op type) #f)))
-    (if (eq? e #f)
-      (error "Unknown op type -- GET" (list op type))
-      e)))
+; Returns #t if there is no #f in xs, otherwise returns #f.
+(define (all? xs)
+  (cond ((null? xs) #t)
+        ((eq? (car xs) #f) #f)
+        (else (all? (cdr xs)))))
+
+(define (all-eq? xs)
+  (cond ((null? xs) #t)
+        ((null? (cdr xs)) #t)
+        ((eq? (car xs) (cadr xs)) (all-eq? (cdr xs)))
+        (else #f)))
+
+(define (fold-right op initial sequence) ; same as accumulate
+  (if (null? sequence)
+      initial
+      (op (car sequence)
+          (fold-right op initial (cdr sequence)))))