与+和append一起使用的树型定义

问题描述

 code>定义，它可以同时处理 + 和追加？

However if I want to use (tree-fold append '(1 (2 2)(2 2))),
I have to modify the tree-fold with list around (car tree),
which breaks it for +.
Is there some mechanism that can be used in the tree-fold definition that would make it work with both + and append?
推荐答案
这应该会起作用，并添加一个参数来初始化结果：   $ b 

This should work, adding one parameter to initialise the result:
(define (tree-fold f n sxp)
  (let loop ((sxp sxp) (res n))
    (cond
      ((null? sxp) res)
      ((pair? sxp) (loop (car sxp) (loop (cdr sxp) res)))
      (else        (f sxp res)))))
测试： 
 
 
Testing:
> (tree-fold + 0 '(1 (2 2)(2 2)))
9
> (tree-fold cons '() '(1 (2 3)(4 5)))
'(1 2 3 4 5)
                        
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