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71 lines
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>29.3.12. Serious Conditions</TITLE>
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<meta name="description" value=" Serious Conditions">
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<meta name="keywords" value="clm">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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<P>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html5779 HREF="node328.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5777 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5771 HREF="node326.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5781 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5782 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5780 HREF="node328.html"> Non-Serious Conditions</A>
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<B>Up:</B> <A NAME=tex2html5778 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5772 HREF="node326.html"> Interactive Condition Handling</A>
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<HR> <P>
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<H2><A NAME=SECTION0033312000000000000000>29.3.12. Serious Conditions</A></H2>
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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The <tt>ignore-errors</tt> macro will trap conditions of type <tt>error</tt>. There are,
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however, conditions that are not of type <tt>error</tt>.
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<P>
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Some conditions are not considered errors but are still very serious, so
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we call them <i>serious conditions</i> and we use the type <tt>serious-condition</tt> to
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represent them. Conditions such as those that might be signaled for
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``stack overflow'' or ``storage exhausted'' are in this category.
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<P>
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The type <tt>error</tt> is a subtype of <tt>serious-condition</tt>, and it would technically
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be correct to use the term ``serious condition'' to refer to all serious
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conditions whether errors or not. However, normally we use the term
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``serious condition'' to refer to things of type <tt>serious-condition</tt> but not
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of type <tt>error</tt>.
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<P>
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The point of the distinction between errors and other serious conditions
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is that some conditions are known to occur for reasons that are beyond the
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scope of Common Lisp to specify clearly. For example, we know that a stack
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will generally be used to implement function calling, and we know that stacks
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tend to be of finite size and are prone to overflow. Since the available
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stack size may vary from implementation to implementation, from session
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to session, or from function call to function call, it would be confusing
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to have expressions such as <tt>(ignore-errors (+ a b))</tt> return a number sometimes
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and <tt>nil</tt> other times if <tt>a</tt> and <tt>b</tt> were always bound to numbers and the stack
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just happened to overflow on a particular call. For this reason, only
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conditions of type <tt>error</tt> and not all conditions of type <tt>serious-condition</tt>
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are trapped by <tt>ignore-errors</tt>. To trap other conditions, a lower-level
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facility must be used (such as <tt>handler-bind</tt> or <tt>handler-case</tt>).
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<P>
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By convention, the function <tt>error</tt> is preferred over <tt>signal</tt> to signal conditions
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of type <tt>serious-condition</tt> (including those of type <tt>error</tt>). It is the use of
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the function <tt>error</tt>, and not the type of the condition being signaled, that
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actually causes the debugger to be entered.
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<P>
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<hr>
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<b>Compatibility note:</b> The Common Lisp Condition System differs from that of Zetalisp in this respect.
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In Zetalisp the debugger is entered for an unhandled signal if the <tt>error</tt>
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function is used <i>or</i> if the condition is of type <tt>error</tt>.
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<hr>
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR> <HR><A NAME=tex2html5779 HREF="node328.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5777 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5771 HREF="node326.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5781 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5782 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5780 HREF="node328.html"> Non-Serious Conditions</A>
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<B>Up:</B> <A NAME=tex2html5778 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5772 HREF="node326.html"> Interactive Condition Handling</A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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</ADDRESS>
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</BODY>
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