考虑如何实现 append(或者，更一般地说，考虑如何实现右折叠).现在，如果您将一个列表附加到包含要添加的元素的单例列表中，那么您基本上已经附加了您的元素.

Think about how you would implement append (or, more generally, think about how you would implement a right-fold). Now, if you append a list to a singleton list containing your element-to-add, you've basically appended your element.

(显然，这是 O(n)，所以不要这样单独添加元素.)

(Obviously, this is O(n), so don't add elements individually this way.)

这是使用右折叠的解决方案:

Here's a solution using a right-fold:

(define (append-element lst elem)
  (foldr cons (list elem) lst))

和使用 append 的解决方案:

and a solution using append:

(define (append-element lst elem)
  (append lst (list elem)))

因此，如果您可以自己实现 foldr 或 append，使用您列出的操作(很简单！尝试一下)，那么您就可以开始了.

So if you can implement either foldr or append yourself, using the operations you've listed (it's easy! try it), you're good to go.

附言实际上，您可以使用右折叠实现 append:

P.S. In fact, you can implement append using a right-fold:

(define (append lst1 lst2)
  (foldr cons lst2 lst1))

但这仍然让您自己实现 foldr.;-)(提示:这很容易.查看我对左折叠的实现以获取开始的想法.)

but that still leaves you to implement foldr yourself. ;-) (Hint: it's easy. Look at my implementation of left-fold for starting ideas.)