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65 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>29.3.5. Restarts</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Restarts">
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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=tex2html5695 HREF="node321.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5693 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5687 HREF="node319.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5697 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5698 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5696 HREF="node321.html"> Anonymous Restarts</A>
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<B>Up:</B> <A NAME=tex2html5694 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5688 HREF="node319.html"> Object-Oriented Basis of </A>
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<HR> <P>
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<H2><A NAME=SECTION003335000000000000000>29.3.5. Restarts</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=RESTARTS>In</A>
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older Lisp dialects (such as MacLisp), an attempt to signal an error of a
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given type often carried with it an implicit promise to support the standard
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recovery strategy for that type of error. If the signaler knew the type of
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error but for whatever reason was unable to deal with the standard recovery
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strategy for that kind of error, it was necessary to signal an untyped error
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(for which there was no defined recovery strategy). This sometimes led to
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confusion when people signaled typed errors without realizing the full
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implications of having done so, but more often than not it meant that users
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simply avoided typed errors altogether.
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<P>
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The Common Lisp Condition System, which is modeled after the Zetalisp condition system,
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corrects this troublesome aspect of previous Lisp dialects by creating a clear
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separation between the act of signaling an error of a particular type and the
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act of saying that a particular way of recovery is appropriate. In the <tt>divide</tt>
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example above, simply signaling an error does not imply a willingness on the
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part of the signaler to cooperate in any corrective action. For example, the
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following sample interaction illustrates that the only recovery action
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offered for this error is ``Return to Lisp Toplevel'':
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<P><pre>
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Lisp> (+ (divide 3 0) 7)
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Error: Attempt to divide 3 by 0.
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To continue, type :CONTINUE followed by an option number:
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1: Return to Lisp Toplevel.
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Debug> :continue 1
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Returned to Lisp Toplevel.
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Lisp>
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</pre><P>
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When an error is detected and the function <tt>error</tt> is called, execution cannot
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continue normally because <tt>error</tt> will not directly return. Control can be
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transferred to other points in the program, however, by means of specially
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established ``restarts.''
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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=tex2html5695 HREF="node321.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5693 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5687 HREF="node319.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5697 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5698 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5696 HREF="node321.html"> Anonymous Restarts</A>
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<B>Up:</B> <A NAME=tex2html5694 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5688 HREF="node319.html"> Object-Oriented Basis of </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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