string - Lisp Scheme 使用 (string-set!) 遇到错误

Question

我是 (Lisp) 方案编程的新手，一直在学习教程。在我使用 string-set 之前一切都很顺利!

字符串设置!用于替换索引处的字符。下面是我所做的示例:

Welcome to Racket v5.1.3.
> (define greeting "Hello; Hello!")
> (string-set! greeting 1 #\a)
string-set!: expects type <mutable string> as 1st argument, given: "Hello; Hello!"; other arguments were: 1 #\a

 === context ===
/usr/share/racket/collects/racket/private/misc.rkt:85:7

> greeting
"Hello; Hello!"
> 

当 (string-set! greeting 1 #\a)执行后，它应该根据教程将第一个 'e' 替换为 'a'。输出应该是:

> greeting
"Hallo; Hello!"

但是我收到了错误 string-set!: expects type <mutable string> as 1st argument .我做错了吗？

Answer 1

根据 Racket documentation :

A string can be mutable or immutable. When an immutable string is provided to a procedure like string-set!, the exn:fail:contract exception is raised. String constants generated by the default reader (see Reading Strings) are immutable, and they are interned in read-syntax mode.

您可以使用 string-copy 复制一个字符串，这总是返回一个可变字符串。

(define greeting (string-copy "Hello; Hello!"))