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<title>avram - a virtual machine code interpreter: 2.7.15 Exception Handling</title>

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<a name="Exception-Handling"></a>
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<a name="Exception-Handling-1"></a>
<h3 class="subsection">2.7.15 Exception Handling</h3>

<a name="index-exceptions-2"></a>
<p>In quite a few cases, the properties given for the operator up to this
point do not imply any particular result. A good example would be an
expression such as [[<code>left</code>]] <code>nil</code>, which appears to
represent the left side of an empty pair. It can be argued that
expressions like this have no sensible interpretation and should never
be used, so it would be appropriate to leave them undefined.  On the
other hand, attempts to evaluate such expressions occur frequently by
mistake, and in any case, the virtual machine emulator should be
designed to do something reasonable about them if only for the sake of
reporting the error.  The chosen remedy for this situation addresses the
need for error reporting, and also turns out to be useful in other ways.
</p>
<table class="menu" border="0" cellspacing="0">
<tr><td align="left" valign="top"><a href="A-Hierarchy-of-Sets.html#A-Hierarchy-of-Sets">2.7.15.1 A Hierarchy of Sets</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">         
</td></tr>
<tr><td align="left" valign="top"><a href="Operator-Generalization.html#Operator-Generalization">2.7.15.2 Operator Generalization</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">     
</td></tr>
<tr><td align="left" valign="top"><a href="Error-Messages.html#Error-Messages">2.7.15.3 Error Messages</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">              
</td></tr>
<tr><td align="left" valign="top"><a href="Expedient-Error-Messages.html#Expedient-Error-Messages">2.7.15.4 Expedient Error Messages</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">    
</td></tr>
<tr><td align="left" valign="top"><a href="Computable-Error-Messages.html#Computable-Error-Messages">2.7.15.5 Computable Error Messages</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">   
</td></tr>
<tr><td align="left" valign="top"><a href="Exception-Handler-Usage.html#Exception-Handler-Usage">2.7.15.6 Exception Handler Usage</a></td><td>&nbsp;&nbsp;</td><td align="left" valign="top">     
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