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<center><h2>cs150: Notes 22</h2></center>

<h4>Assignments Due</h4>

<ul>
<li> <b>Now</b>: <a href="/cs150/ps/ps5/">Problem Set 5</a>
<li> Read before 17 March: <A href="/cs150/book/ch10-objects.pdf">Chapter 10</a>
<li> <b>Before Monday, 19 March</b>: Read rest of GEB part I (Chapters 2-4 and 6-9, in
addition to Chapters 1 and 5 you have already read).  (There will be a
short quiz in class on Monday, 19 March.)
<li> <b>Friday, 23 March</b>: <a href="/cs150/ps/ps6/">Problem Set 6</a>
</ul>

<h4>Notes</h4>

<pre>
(define (make-counter)
   ((lambda (count)
      (lambda ()
        (set! count (+ 1 count))
        count))
   0))
</pre>

Draw the environment after: 
<pre>
   (define counter (make-counter))
   (counter)
</pre>
<p><br><p><br><p><br><p><br><p><br><p><br><p><br>
<p><br>

<pre>
(define (make-counter)
   (let ((count 0))
     (lambda (message)
       (cond ((eq? message 'reset!)  (set! count 0))
	     ((eq? message 'next!)   (set! count (+ 1 count)))
	     ((eq? message 'current) count)
	     (else (error "Unrecognized message"))))))
</pre>



What is an <em>object</em>?
<p>
<p><br><p><br><p><br><p><br>

Why is it useful to package state and procedures together?
<p>
<p><br><p><br><p><br>


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