Dr Racket problems with SICP

lisp scheme racket sicp
hraesvelgr picture hraesvelgr · Aug 30, 2010 · Viewed 10.6k times · Source

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: +

I have the language currently set to R5RS, which I was under the impression would take care of the compatibility issues with SICP.

Answer

Eli Barzilay picture Eli Barzilay · Aug 30, 2010

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.

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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