scheme - 方案 - 如何在不由更多列表组成的列表中获取每个列表

Question

   (define (walk-list lst fun)                       ;;walk-list(list, fun)
    (if (not(null? lst))                             ;;IF the list isn't NULL
      (begin
        (if (list? lst)                              ;;&& the list is actually a list ,       THEN{
          (begin  
            (if (equal? (car lst) '())              ;;IF the first element in the list is empty
              (fun lst)                              ;;THEN call the function on the list (funct is supose to get each word)
            (if (not (null? lst))                    ;;ELSE IF the first item isn't a list 
              (begin                                 ;;{         
                (walk-list (car lst) fun)            ;;walk-list((car lst),fun) 
                (walk-list (cdr lst) fun)))))))))    ;;walk-list((cdr lst),fun)
(walk-list test-document display)                    ;;walk through the list with the given document 

看起来像这样：

(define test-document '(
                       ((h e l l o));;paragraph1
                       ((t h i s)(i s)(t e s t));;paragraph2
                       ))

我试图让文档中的每个单词都应用一个函数。在哪里说（有趣的列表）。但是该函数永远不会被调用。

Answer 1

First off. begin is if you need to do more than one expression. The first expression then needs to have side effects or else it's just a waste of processing power.

Ie.

(begin 
  (display "hello") ; display is a side effect
  (something-else))

When you don't have more than one expression begin isn't needed. if has 3 parts. They are:

(if predicate-expression    ; turnas into something true or #f (the only false value)
    consequent-expression   ; when predicate-expression evalautes to anything but #f
    alternative-expression) ; when predicate-expression evaluates to #f this is done

You should ident your code properly. Here is the code idented with DrRacket IDE, with reduncant begin removed and missing alternative-expressions added so you see where they return:

(define (walk-list lst fun)                       ;;walk-list(list, fun)
  (if (not (null? lst))                           ;;IF the list isn't NULL
      (if (list? lst)                             ;; && the list is actually a list ,       THEN{
          (if (equal? (car lst) '())              ;; IF the first element in the list is empty
              (fun lst)                           ;; THEN call the function on the list (funct is supose to get each word)
              (if (not (null? lst))               ;; ELSE IF the first item isn't a list 
                  (begin                          ;; Here begin is needed        
                    (walk-list (car lst) fun)     ;; walk-list((car lst),fun) 
                    (walk-list (cdr lst) fun))    ;; walk-list((cdr lst),fun)
                  'undfined-return-1))            ;; stop recursion, return undefined value
          'undefined-return-2)                    ;; stop recursion, return undefined value
      'undefined-return-3))                       ;; stop recursion, return undefined value

So when does (fun lst) get called? Never! There is no () in any car in (((h e l l o))((t h i s) (i s) (t e s t))) and (equal? (car lst) '()) which is (null? (car lst)) will always be #f. Since we know (not (null? lst)) is #t so it will walk car and cdr where either 'undefined-return-2 or 'undefined-return-3 will be evaluated and the procedure stops when everything is visited and nothing processed.

You haven't shown what (walk-list test-document display) should have displayed but I make a wild guess that you want it for every element except pairs and null, thus I would have written this like this:

(accumulate-tree test-document display (lambda (a d) 'return) '())

accumulate-tree you'll find in this SICP handout. It demonstrates many uses for it as well. For completeness I'll supply it here:

(define (accumulate-tree tree term combiner null-value)
  (cond ((null? tree) null-value)
        ((not (pair? tree)) (term tree))
        (else (combiner 
               (accumulate-tree (car tree) 
                                term
                                combiner
                                null-value)
               (accumulate-tree (cdr tree)
                                term
                                combiner
                                null-value)))))

Judging from you code you are an Algol programmer learning your first Lisp. I advice you to look at the SICP videoes and book.