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schemerackethtdp

How to ignore side-effect in Racket from using `set!`?

In Exercise 35.4.2 from HtDP, I've implemented the GUI and have a button called "Remove" which invokes a callback function. Here it is:

(define (cb-remove x)
  ((lambda (name result)
     (cond
       [(number? result) (remove-name name address-book)]
       [else (draw-message msg "Not found")]))
   (string->symbol (text-contents label-name))
   (lookup (string->symbol (text-contents label-name)) address-book)))

When I run this, I get the following message: button-callback: result of type <Boolean> expected, your function produced #<set!-result>. The problem is that I have to call set! in order to change the address book. However, the result of set! is (void), which cannot cannot be combined with a Boolean type. How can I avert this problem? Thanks for any insight.

Solution

  • Simple:

    (begin (set! foo bar) #t)