Question

I am new to scheme. This is code sample from SICP course of MIT.

 (define (+ x y)
  (if (= x 0)
      y
      (+ (-1+ x) (1+ y))))

How do I convert this to Racket code? I want to convert to Racket because I am using DrRacket for running codes and I like that. It worked until now but complained about increment operators of scheme.

The errors I get are:

1. define-values: cannot change constant variable: +
2. reference to undefined identifier: -1+

Answer 1

This will work fine in Racket:

(define (add x y)
  (if (= x 0)
      y
      (add (sub1 x) (add1 y))))

Some comments:

• The name + for the procedure will be troublesome, because it will clash with the primitive add operation in Scheme; it’s simpler if you use a different name, like add (this will fix the first error)
• -1+ is not a procedure in Racket, replace it with sub1 (this will fix the second error). Optionally, you could define an alias for this procedure, like this: (define -1+ sub1)
• 1+ is not a procedure in Racket, replace it with add1. Optionally, you could define an alias for this procedure, like this: (define 1+ add1)
• Optionally: instead of (= x 0) you can write (zero? x)