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79 lines
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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>29.3.7. Named Restarts</TITLE>
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<BODY>
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<meta name="description" value=" Named Restarts">
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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=tex2html5719 HREF="node323.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5717 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5711 HREF="node321.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5721 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5722 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5720 HREF="node323.html"> Restart Functions</A>
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<B>Up:</B> <A NAME=tex2html5718 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5712 HREF="node321.html"> Anonymous Restarts</A>
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<HR> <P>
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<H2><A NAME=SECTION003337000000000000000>29.3.7. Named 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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In addition to anonymous restarts, one can have named restarts, which can be invoked
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by name from within code. As a trivial example, one could write
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<P><pre>
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(restart-case (invoke-restart 'foo 3)
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(foo (x) (+ x 1)))
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</pre><P>
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to add <tt>3</tt> to <tt>1</tt>, returning <tt>4</tt>. This trivial example is conceptually analogous to
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writing:
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<P><pre>
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(+ (catch 'something (throw 'something 3)) 1)
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</pre><P>
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<P>
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For a more realistic example, the code for the function <tt>symbol-value</tt> might signal an
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unbound variable error as follows:
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<P><pre>
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(restart-case (error "The variable ~S is unbound." variable)
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(continue ()
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:report
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(lambda (s) ;Argument <tt>s</tt> is a stream
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(format s "Retry getting the value of ~S." variable))
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(symbol-value variable))
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(use-value (value)
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:report
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(lambda (s) ;Argument <tt>s</tt> is a stream
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(format s "Specify a value of ~S to use this time."
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variable))
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value)
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(store-value (value)
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:report
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(lambda (s) ;Argument <tt>s</tt> is a stream
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(format s "Specify a value of ~S to store and use."
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variable))
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(setf (symbol-value variable) value)
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value))
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</pre><P>
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If this were part of the implementation of <tt>symbol-value</tt>, then it would be possible
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for users to write a variety of automatic handlers for unbound variable
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errors. For example, to make unbound variables evaluate to themselves, one
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might write
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<P><pre>
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(handler-bind ((unbound-variable
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#'(lambda (c) ;Argument <tt>c</tt> is a condition
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(when (find-restart 'use-value)
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(invoke-restart 'use-value
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(cell-error-name c))))))
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<i>body</i>)
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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=tex2html5719 HREF="node323.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5717 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5711 HREF="node321.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5721 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5722 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5720 HREF="node323.html"> Restart Functions</A>
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<B>Up:</B> <A NAME=tex2html5718 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5712 HREF="node321.html"> Anonymous Restarts</A>
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<HR> <P>
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AI.Repository@cs.cmu.edu
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