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r6rs-lib
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<a name="node_chap_17"></a>
<h1 class=chapter>
<div class=chapterheading><a href="r6rs-lib-Z-H-1.html#node_toc_node_chap_17">Chapter 17</a></div><br>
<a href="r6rs-lib-Z-H-1.html#node_toc_node_chap_17">Mutable pairs</a></h1>
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<p>
The procedures provided by the <tt>(rnrs mutable-pairs (6))</tt><a name="node_idx_1272"></a>library
allow new values to be assigned to the car and cdr fields of
previously allocated pairs. </p>
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<div align=left><tt>(<a name="node_idx_1274"></a>set-car!<i> pair obj</i>)</tt> procedure </div>
<p>
Stores <i>obj</i> in the car field of <i>pair</i>.
The <tt>set-car!</tt> procedure returns unspecified values.</p>
<p>
</p>
<tt>(define (f) (list ’not-a-constant-list))<br>
(define (g) ’(constant-list))<br>
(set-car! (f) 3) ⇒ <em>unspecified</em><br>
(set-car! (g) 3) ⇒ <i>unspecified</i><br>
; should raise <tt> &assertion</tt> <i>exception</i><br>
<p></tt></p>
<p>
If an immutable pair is passed to <tt>set-car!</tt>, an exception
with condition type <tt>&assertion</tt> should be raised.
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<div align=left><tt>(<a name="node_idx_1276"></a>set-cdr!<i> pair obj</i>)</tt> procedure </div>
<p>
Stores <i>obj</i> in the cdr field of <i>pair</i>.
The <tt>set-cdr!</tt> procedure returns unspecified values.</p>
<p>
If an immutable pair is passed to <tt>set-cdr!</tt>, an exception
with condition type <tt>&assertion</tt> should be raised.</p>
<p>
</p>
<tt>(let ((x (list ’a ’b ’c ’a))<br>
(y (list ’a ’b ’c ’a ’b ’c ’a)))<br>
(set-cdr! (list-tail x 2) x)<br>
(set-cdr! (list-tail y 5) y)<br>
(list<br>
(equal? x x)<br>
(equal? x y)<br>
(equal? (list x y ’a) (list y x ’b)))) <br> ⇒ (<tt>#t</tt> <tt>#t</tt> <tt>#f</tt>)<br>
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