67 lines
3.9 KiB
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67 lines
3.9 KiB
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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>29.3.17. Condition Handlers</TITLE>
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<BODY>
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<meta name="description" value=" Condition Handlers">
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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=tex2html5839 HREF="node333.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5837 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5831 HREF="node331.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5841 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5842 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5840 HREF="node333.html"> Printing Conditions</A>
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<B>Up:</B> <A NAME=tex2html5838 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5832 HREF="node331.html"> Resignaling Conditions</A>
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<HR> <P>
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<H2><A NAME=SECTION0033317000000000000000>29.3.17. Condition Handlers</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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<A NAME=CONDITIONHANDLERS>A</A>
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<i>handler</i> is a function of one argument, the condition to be handled. The
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handler may inspect the object
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to be sure it is ``interested'' in handling the condition.
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<P>
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A handler is executed in the dynamic context of the signaler, except that the
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set of available condition handlers will have been rebound to the value that
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was active at the time the condition handler was made active. The intent of
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this is to prevent infinite recursion because of errors in a condition handler.
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<P>
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After inspecting the condition, the handler should take one of the following
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actions:
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<UL> <LI>
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It might <i>decline</i> to handle the condition (by simply returning). When
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this happens, the returned values are ignored and the effect is the same
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as if the handler had been invisible to the mechanism seeking to find a
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handler. The next handler in line will be tried, or if no such handler
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exists, the condition will go unhandled.
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<P>
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<LI>
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It might <i>handle</i> the condition (by performing some non-local transfer
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of control). This may be done either primitively using <tt>go</tt>, <tt>return</tt>, or <tt>throw</tt>,
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or more abstractly using a function such as <tt>abort</tt> or <tt>invoke-restart</tt>.
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<P>
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<LI>
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It might signal another condition.
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<P>
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<LI>
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It might invoke the interactive debugger.
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</UL>
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In fact, the latter two actions (signaling another condition or entering the
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debugger) are really just ways of putting off the decision to either handle
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or decline, or trying to get someone else to make such a decision. Ultimately,
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all a handler can do is to handle or decline to handle.
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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=tex2html5839 HREF="node333.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5837 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5831 HREF="node331.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5841 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5842 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5840 HREF="node333.html"> Printing Conditions</A>
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<B>Up:</B> <A NAME=tex2html5838 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5832 HREF="node331.html"> Resignaling Conditions</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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