scheme - 有人可以解释一下下面的方案代码吗？

Question

我一直在听斯坦福的programming paradigm lecture series ，但我对下面的代码(来自第 20 讲)感到困惑。有人会逐行解释一下这是在做什么吗？

谢谢。

(define (flatten sequence)
  (cond ((null? sequence) '())
        ((list? (car sequence)) (append (flatten (car sequence))
                                        (flatten (cdr sequence))))
        (else (cons (car sequence)
                    (flatten (cdr sequence))))))

Answer 1

# define a procedure 'flatten' that takes a list 'sequence'
(define (flatten sequence)
  # if the 'sequence' is empty, return an empty list
  (cond ((null? sequence) (list))
        # if the first element of 'sequence' is itself a list, return a new list
        # made by appending the flattened first element of 'sequence' with the
        # flattened rest of the 'sequence'
        ((list? (car sequence))
         (append (flatten (car sequence))
                 (flatten (cdr sequence))))
        # if the first element of 'sequence' is not a list, return a new list
        # made by cons-ing that element with the flattened rest of the 'sequence'
        (else
         (cons (car sequence)
               (flatten (cdr sequence))))))

我漂亮地打印了它(插入了一些换行符并缩进代码以显示其结构)并替换了 '()与 (list) (具有相同的值)只是为了防止代码被错误地突出显示。

+1 what is cons-ing? I'd appreciate if you explain other keywords as well. thanks

当我说缺点时，我只是指 cons程序。当您看到(cons <expression> <list>)时其中 <expression> 是任何Scheme 表达式或值， <list> 是任何计算结果为列表的Scheme 表达式，cons将返回 <list> 的值为 <expression> 钉在它的前面。例如，(cons 1 (list 2 3 4))返回列表 (list 1 2 3 4) 。事实上，(list 1 2 3 4)在Scheme中只是简写(cons 1 (cons 2 (cons 3 (cons 4 '() )))) .

您可能遇到麻烦的另外两个词是 car和cdr 。你可以想到(car <list>)表示 <list> 的 first 元素或 head 和 (cdr <list>)表示元素的 rest 或 <list> 的 tail 。例如，(car (list 1 2 3 4))返回值 1 ，和(cdr (list 1 2 3 4))返回列表 (list 2 3 4) .

如果您需要有关其他关键字的任何帮助，请告诉我。