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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>29.3.10. Generalized Restarts</TITLE>
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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=tex2html5755 HREF="node326.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5753 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5747 HREF="node324.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5757 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5758 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5756 HREF="node326.html"> Interactive Condition Handling</A>
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<B>Up:</B> <A NAME=tex2html5754 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5748 HREF="node324.html"> Comparison of Restarts </A>
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<HR> <P>
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<H2><A NAME=SECTION0033310000000000000000>29.3.10. Generalized 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=LASTRESTARTSSECTION><tt>restart-case</tt></A>
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is a mechanism that allows only imperative transfer of control
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for its associated restarts. <tt>restart-case</tt> is built on a lower-level mechanism
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called <tt>restart-bind</tt>, which does not force transfer of control.
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<P>
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<tt>restart-bind</tt> is to <tt>restart-case</tt> as <tt>handler-bind</tt> is to
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<tt>handler-case</tt>.
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The syntax is
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<P><pre>
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(restart-bind ((<i>name</i> <i>function</i> . <i>options</i>)) . <i>body</i>)
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</pre><P>
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The <i>body</i> is executed in a dynamic context within which the <i>function</i>
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will be called whenever
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<tt>(invoke-restart '<i>name</i>)</tt> is executed. The <i>options</i> are keyword-style and are
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used to pass information such as that provided with the
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<tt>:report</tt> keyword in <tt>restart-case</tt>.
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<P>
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A <tt>restart-case</tt> expands into a call to <tt>restart-bind</tt> where the function
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simply does an unconditional transfer of control to a particular body
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of code, passing along ``argument'' information in a structured way.
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<P>
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It is also possible to write restarts that do not transfer control. Such
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restarts may be useful in implementing various special commands for the
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debugger that are of interest only in certain situations. For example,
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one might imagine a situation where file space was exhausted and the
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following was done in an attempt to free space in directory <tt>dir</tt>:
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<P><pre>
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(restart-bind ((nil #'(lambda () (expunge-directory dir))
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:report-function
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#'(lambda (stream)
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(format stream "Expunge ~A."
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(directory-namestring dir)))))
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(cerror "Try this file operation again."
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'directory-full :directory dir))
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</pre><P>
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In this case, the debugger might be entered and the user could first
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perform the expunge (which would not transfer control from the debugger
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context) and then retry the file operation:
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<P><pre>
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Lisp> (open "FOO" :direction :output)
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Error: The directory PS:<JDOE> is full.
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To continue, type :CONTINUE followed by an option number:
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1: Try this file operation again.
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2: Expunge PS:<JDOE>.
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3: Return to Lisp Toplevel.
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Debug> :continue 2
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Expunging PS:<JDOE> ... 3 records freed.
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Debug> :continue 1
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=> #<OUTPUT-STREAM "PS:<JDOE>FOO.LSP" 2323473>
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</pre><P>
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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=tex2html5755 HREF="node326.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5753 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5747 HREF="node324.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5757 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5758 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5756 HREF="node326.html"> Interactive Condition Handling</A>
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<B>Up:</B> <A NAME=tex2html5754 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5748 HREF="node324.html"> Comparison of Restarts </A>
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<HR> <P>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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