ZelphirKaltstahl
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<body>
<div id="content">
<h1 class="title">Common Lisp to Scheme translation
<br />
<span class="subtitle">Help for translating the book's code to Scheme</span>
</h1>
<div id="table-of-contents">
<h2>Table of Contents</h2>
<div id="text-table-of-contents">
<ul>
<li><a href="#org036c9f9">1. Common Lisp to Scheme translation</a>
<ul>
<li><a href="#org4eb5e32">1.1. Straight forward translations</a></li>
<li><a href="#org724aca9">1.2. Others</a></li>
</ul>
</li>
</ul>
</div>
</div>

<div id="outline-container-org036c9f9" class="outline-2">
<h2 id="org036c9f9"><span class="section-number-2">1</span> Common Lisp to Scheme translation</h2>
<div class="outline-text-2" id="text-1">
</div>
<div id="outline-container-org4eb5e32" class="outline-3">
<h3 id="org4eb5e32"><span class="section-number-3">1.1</span> Straight forward translations</h3>
<div class="outline-text-3" id="text-1-1">
<ul class="org-ul">
<li><code>defun</code> -&gt; <code>define</code></li>
<li><code>defvar</code> -&gt; <code>define</code> or <code>let</code></li>
<li><code>setf</code> -&gt; <code>define</code> or <code>let</code></li>
<li><code>function</code> or <code>#'</code> -&gt; nothing, no difference is made in referring to values or procedures, Scheme is a Lisp 1.</li>
<li><code>mapcar</code> -&gt; <code>map</code></li>
<li><code>funcall</code> -&gt; simply call the procedure, s-expr with procedure in the first place</li>
</ul>
</div>
</div>

<div id="outline-container-org724aca9" class="outline-3">
<h3 id="org724aca9"><span class="section-number-3">1.2</span> Others</h3>
<div class="outline-text-3" id="text-1-2">
</div>
<div id="outline-container-orgbd8e713" class="outline-4">
<h4 id="orgbd8e713"><span class="section-number-4">1.2.1</span> NIL</h4>
<div class="outline-text-4" id="text-1-2-1">
<p>
In Common Lisp <code>NIL</code> is a defined symbol:
</p>

<div class="org-src-container">
<pre class="src src-scheme" id="org1e99618">(<span style="color: #9FCA56;">define</span> <span style="color: #55B5DB;">NIL</span> '())
</pre>
</div>
</div>
</div>

<div id="outline-container-org80d76c3" class="outline-4">
<h4 id="org80d76c3"><span class="section-number-4">1.2.2</span> car</h4>
<div class="outline-text-4" id="text-1-2-2">
<p>
<code>car</code> of an empty list in Common Lisp returns NIL, the empty list again. <code>car</code> in Scheme does not return the empty list, but raises an error. This means, that, for compatibility reasons, we have to write a little procedure, which acts like <code>car</code> from Common Lisp:
</p>

<div class="org-src-container">
<pre class="src src-scheme">(<span style="color: #9FCA56;">define</span> <span style="color: #55B5DB;">NIL</span> '())

(<span style="color: #9FCA56;">define</span> <span style="color: #55B5DB;">car/nil</span>
  (<span style="color: #9FCA56;">lambda</span> (possibly-empty-list)
    <span style="color: #55B5DB;">"In Common Lisp car of an empty list returns the empty list called NIL. In</span>
<span style="color: #55B5DB;">Scheme it would be an error to call car on an empty list. We write a wrapper, to</span>
<span style="color: #55B5DB;">avoid an error."</span>
    (<span style="color: #9FCA56;">cond</span>
     [(null? possibly-empty-list) NIL]
     [<span style="color: #9FCA56;">else</span> (car possibly-empty-list)])))
</pre>
</div>
</div>
</div>

<div id="outline-container-org610dbc1" class="outline-4">
<h4 id="org610dbc1"><span class="section-number-4">1.2.3</span> rest</h4>
<div class="outline-text-4" id="text-1-2-3">
<p>
<code>rest</code> in Common Lisp also returns <code>NIL</code>, when called with the empty list as argument, so a wrapper is needed:
</p>

<div class="org-src-container">
<pre class="src src-scheme">(<span style="color: #9FCA56;">define</span> <span style="color: #55B5DB;">NIL</span> '())

(<span style="color: #9FCA56;">define</span> <span style="color: #55B5DB;">rest</span>
  (<span style="color: #9FCA56;">lambda</span> (lst)
    (<span style="color: #9FCA56;">cond</span>
     [(null? lst) NIL]
     [<span style="color: #9FCA56;">else</span> (cdr lst)])))
</pre>
</div>
</div>
</div>

<div id="outline-container-orgf608477" class="outline-4">
<h4 id="orgf608477"><span class="section-number-4">1.2.4</span> defparameter</h4>
<div class="outline-text-4" id="text-1-2-4">
<p>
A parameter is explained in the book as a binding, that usually does not chance, a constant. A change to it is explained as a change <i>to</i> the program, not a change <i>by</i> the program, in contrast to using <code>defvar</code>.
</p>

<p>
Since in Scheme we usually try to avoid using assignment or <code>set!</code> anyway, we can get away with following a convention of naming constants with enclosing asterisks: <code>*constant*</code> and using <code>define</code>.
</p>
</div>
</div>

<div id="outline-container-orgfed180c" class="outline-4">
<h4 id="orgfed180c"><span class="section-number-4">1.2.5</span> case</h4>
<div class="outline-text-4" id="text-1-2-5">
<p>
???
</p>
</div>
</div>
</div>
</div>
</div>
<div id="postamble" class="status">
<p class="author">Author: Zelphir Kaltstahl</p>
<p class="date">Created: 2020-05-03 So 14:06</p>
<p class="validation"><a href="http://validator.w3.org/check?uri=referer">Validate</a></p>
</div>
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