50 lines
2.7 KiB
HTML
50 lines
2.7 KiB
HTML
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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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<TITLE>29.3.15. Signaling Conditions</TITLE>
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<meta name="description" value=" Signaling Conditions">
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<meta name="keywords" value="clm">
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<meta name="resource-type" value="document">
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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=tex2html5815 HREF="node331.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5813 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5807 HREF="node329.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5817 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5818 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5816 HREF="node331.html"> Resignaling Conditions</A>
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<B>Up:</B> <A NAME=tex2html5814 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5808 HREF="node329.html"> Condition Types</A>
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<HR> <P>
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<H2><A NAME=SECTION0033315000000000000000>29.3.15. Signaling 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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When a condition is signaled, the system tries to locate the most appropriate
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handler for the condition and to invoke that handler.
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<P>
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Handlers are established dynamically using <tt>handler-bind</tt> or abstractions built
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on <tt>handler-bind</tt>.
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<P>
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If an appropriate handler is found, it is called. In some circumstances,
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the handler may <i>decline</i> simply by returning without performing a
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non-local transfer of control. In such cases, the search for an
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appropriate handler is picked up where it left off, as if the called
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handler had never been present.
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<P>
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If no handler is found, or if all handlers that were found decline,
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<tt>signal</tt> returns <tt>nil</tt>.
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<P>
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Although it follows from the description above, it is perhaps worth noting
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explicitly that the lookup procedure described here will prefer a general
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but more (dynamically) local handler over a specific but less (dynamically)
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local handler. Experience with existing condition systems suggests that
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this is a reasonable approach and works adequately in most situations.
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Some care should be taken when binding handlers for very general kinds of
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conditions, such as is done in <tt>ignore-errors</tt>. Often, binding for a more
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specific condition type than <tt>error</tt> is more appropriate.
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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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<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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