问题描述
我正在通过SICP工作.目前，在第一章中，让Racket重新定义基本体"时遇到了问题.例如，我的印象是我应该能够任意执行(define + 5)，这很好，或者重新定义了sqrt过程.相反，我得到了:
I'm working through SICP. Currently, in the first chapter, I'm having problems getting Racket to let me redefine "primitives". For instance, I was under the impression that I should be able to arbitrarily do (define + 5) and that would be fine, or redefine the sqrt procedure. Instead, I get this:
define-values: cannot change constant variable: +
我目前将语言设置为R5RS，在我看来，该语言将解决与SICP的兼容性问题.
I have the language currently set to R5RS, which I was under the impression would take care of the compatibility issues with SICP.
推荐答案
即使在可能的情况下，在没有真正了解系统如何对此做出响应的情况下，也不应执行此类重新定义.例如，如果您重新定义+，是否还会有其他代码中断?在Racket的情况下，答案是否"-但这是因为您并没有真正重新定义+:而是定义了一个 new +，该代码只包含您的代码可以使用.
Even if possible, such redefinitions are not something that you should do without really understanding how the system will react to this. For example, if you redefine +, will any other code break? The answer to that in Racket's case is "no" -- but this is because you don't really get to redefine +: instead, you define a new +, which only your code can use.
关于语言选择-球拍R5RS模式是一种非常严格的模式，它不是您通常想要使用的模式.要获得更适合SICP的 环境，请参见Neil Van Dyke的 SICP支持页面，它将为您提供专门针对该书的语言. (IIRC，甚至还具有书籍展示的图形语言.)
As for the language choice -- Rackets R5RS mode is a very strict one, and it's not something that you'd usually want to use. For a much more SICP-friendly environment, see Neil Van Dyke's SICP Support page which will provide you with a language specifically made for the book. (IIRC, it even has the graphical language that the books shows off.)
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