how to make HTML from a list in scheme, racket

问题

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:

An HTML(H) is one of

Str
Tag

A Tag is

(cons Sym (listof H))

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"))

Turns into:

"<html><head><title>Hi</title></head><body><h1>Welcome</h1>Text</body></html>"

This should work for any list or string. There are rules for turning it into HTML:

A string does not need to turn into HTML.
Each Tag (this is from the definitions) begin with <tag> and end with </tag>. So <html> and </html>.

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))))]))

If I enter something like:

(list 'html (list 'head (list 'title "Hi")) (list 'body (list 'h1 "Welcome") "Text"))

Then it gives me:

(list "<" 'html ">" "<" (list 'head (list 'title "Hi")) ">" "</" (list 'head (list 'title "Hi")) ">" "<" (list  (list 'body  (list 'h1 "Welcome").....etc etc

It is a very long output and it does not work. How to be able to fix this?

回答1:

You have the right idea, but still - there are three main issues with your proposed solution:

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?

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)))]))

It works as expected:

(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>"

来源：https://stackoverflow.com/questions/64652970/how-to-make-html-from-a-list-in-scheme-racket

标签

recursion

scheme

racket

mutual-recursion