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<title>avram - a virtual machine code interpreter: 3.1 Lists</title>

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<a name="Lists-1"></a>
<h2 class="section">3.1 Lists</h2>

<p>The basic data structure used for representing virtual code and data in
the <code>avram</code> library is declared as a <code>list</code>.
<a name="index-lists-7"></a>
<a name="index-head-field"></a>
<a name="index-tail-field"></a>
The <code>list</code> type is a pointer to a structure having a <code>head</code>
field and a <code>tail</code> field, which are also lists. The empty tree,
<code>nil</code>, is represented by the C constant <code>NULL</code>. A tree of the
form <code>cons(<var>a</var>,<var>b</var>)</code> is represented in C as a list whose
<code>head</code> is the representation of <code><var>a</var></code> and whose
<code>tail</code> is the representation of <code><var>b</var></code>.
</p>
<p>A number of other fields in the structure are maintained automatically
and should not be touched. For that matter, even the <code>head</code> and
<code>tail</code> fields should be considered read-only. Because of sharing,
it is almost never valid to modify a list &ldquo;in place&rdquo;, except for cases
that are already covered by library functions.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="Simple-Operations.html#Simple-Operations">3.1.1 Simple Operations</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">           
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<tr><td align="left" valign="top"><a href="Recoverable-Operations.html#Recoverable-Operations">3.1.2 Recoverable Operations</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">      
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<tr><td align="left" valign="top"><a href="List-Transformations.html#List-Transformations">3.1.3 List Transformations</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">        
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<tr><td align="left" valign="top"><a href="Type-Conversions.html#Type-Conversions">3.1.4 Type Conversions</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">            
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<tr><td align="left" valign="top"><a href="Comparison.html#Comparison">3.1.5 Comparison</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">      
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<tr><td align="left" valign="top"><a href="Deconstruction-Functions.html#Deconstruction-Functions">3.1.6 Deconstruction Functions</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">    
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<tr><td align="left" valign="top"><a href="Indirection.html#Indirection">3.1.7 Indirection</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">                 
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<tr><td align="left" valign="top"><a href="The-Universal-Function.html#The-Universal-Function">3.1.8 The Universal Function</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">      
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