如何从方案中的列表中制作 HTML，球拍 [英] how to make HTML from a list in scheme, racket
问题描述
这是一个很长的问题......我是新加入的，所以请不要攻击我.为我糟糕的英语交流道歉.我有一些定义:
This is a very long question ... I am new and joined, so please don't attack me. Apologies for my bad communications in English. I have some defintions:
HTML(H) 是其中之一
An HTML(H) is one of
Str标签
标签是
(cons Sym (listof H))
我想使用相互递归，把HTML变成真正的HTML代码.例如，
I want to use mutual recursion,make the HTML into real HTML code. For example,
(list 'html (list 'head (list 'title "Hi")) (list 'body (list 'h1 "Welcome") "Text"))
变成:
"<html><head><title>Hi</title></head><body><h1>Welcome</h1>Text</body></html>"
这应该适用于任何列表或字符串.有将其转换为 HTML 的规则:
This should work for any list or string. There are rules for turning it into HTML:
- 字符串不需要转成 HTML.
- 每个标签(来自定义)以
结尾.所以和.
如果他们输入字符串Hello"，则它不会转换任何内容.我在这里写了代码，但它不起作用......
If they enter a string, "Hello", then it does not convert anything. I wrote code here, but it does not work ...
(define (html->string html)
(cond [(string? html) html]
[else (append (list "<" (first html) ">") (change-tag (rest html)) (list "</" (first html) ">"))]))
(define (change-tag lst)
(cond [(empty? lst) empty]
[else (append (html->string (list (first lst)))
(html->string (list (rest lst))))]))
如果我输入以下内容:
(list 'html (list 'head (list 'title "Hi")) (list 'body (list 'h1 "Welcome") "Text"))
然后它给了我:
(list "<" 'html ">" "<" (list 'head (list 'title "Hi")) ">" "</" (list 'head (list 'title "Hi")) ">" "<" (list (list 'body (list 'h1 "Welcome").....etc etc
这是一个很长的输出，它不起作用.如何解决这个问题?
It is a very long output and it does not work. How to be able to fix this?
推荐答案
您的想法是正确的，但仍然 - 您提出的解决方案存在三个主要问题:
You have the right idea, but still - there are three main issues with your proposed solution:
- 输出将是一个字符串，而不是一个列表，因此我们必须在递归步骤中附加字符串并在基本情况下返回字符串.
- 如果我们想将符号与其他字符串连接起来，必须将它们显式转换为字符串.
- 最后但并非最不重要的一点:您遗漏了几个基本情况，这些对于编写正确的解决方案至关重要:如果给定的 html 是空列表，会发生什么情况?如果列表中的第一个元素不是符号而是另一个列表怎么办?
- The output will be a string, not a list, so we must append strings in the recursive steps and return strings in the base cases.
- Symbols must be explicitly converted to strings if we want to concatenate them with other strings.
- And last but not least: you're missing a couple of base cases, and these are essential for writing a correct solution: what should happen if the given html is an empty list? What if the first element in a list is not a symbol, but another list?
这行得通，仔细看看发生了什么变化:
This will work, take a careful look at the things that changed:
(define (html->string html)
(cond [(empty? html) ""]
[(string? html) html]
[(not (symbol? (first html)))
(html->string (first html))]
[else (string-append
"<" (symbol->string (first html)) ">"
(change-tag (rest html))
"</" (symbol->string (first html)) ">")]))
(define (change-tag lst)
(cond [(empty? lst) ""]
[else (string-append
(html->string (first lst))
(html->string (rest lst)))]))
它按预期工作:
(define html
(list 'html
(list 'head
(list 'title "Hi"))
(list 'body
(list 'h1 "Welcome")
"Text")))
(html->string html)
=> "<html><head><title>Hi</title></head><body><h1>Welcome</h1>Text</body></html>"
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