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POHow to retrieve a button's label in Racket/PLT-Scheme?
 

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  1. POHow to retrieve a button's label in Racket/PLT-Scheme?
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    <p>I'm attempting <a href="http://htdp.org/2003-09-26/Book/curriculum-Z-H-28.html#node_chap_22" rel="nofollow">Exercise 22.3.3 from HtDP</a> but do not know how to retrieve the label of the button that was clicked. I get this message <code>draw-message: expected &lt;string&gt; as second argument, given: (instantiate (class ...) ...)</code> which seems to suggest that I need a string but I'm getting an instance of a class. Is the answer in the callback? If so, how do I de-structure it?</p> <hr> <p>This is what I have so far:</p> <pre><code>(define pad1 '((1 2 3) (4 5 6) (7 8 9) (\# 0 *))) (define pad2 '((1 2 3 +) (4 5 6 -) (7 8 9 *) (0 = \. /))) (define (title t) (make-message t)) (define display (make-message "")) (define (pad-&gt;gui p) (cond [(empty? p) empty] [else (cons (button-maker (first p)) (pad-&gt;gui (rest p)))])) ;; make buttons out of a list (define (button-maker a-list) (cond [(empty? a-list) empty] [(number? (first a-list))(cons (make-button (number-&gt;string (first a-list)) call-back) (button-maker (rest a-list)))] [(symbol? (first a-list))(cons (make-button (symbol-&gt;string (first a-list)) call-back) (button-maker (rest a-list)))])) (define (call-back b) (draw-message display ...)) (create-window (append (list (list (title "Virtual Phone"))) (list (list display)) (pad-&gt;gui pad1))) </code></pre> <p>If I understand things correctly, each button will call <code>call-back</code> when it is pressed. This in turn should call <code>display</code> which will update the text. However, I do not understand how to retrieve the caller's label. e.g. if the button "9" is pressed, it will call <code>call-back</code>. But how do I retrieve the value "9"? This is what I'm unsure about.</p>
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