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109 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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<TITLE>29.3.6. Anonymous Restarts</TITLE>
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<meta name="description" value=" Anonymous Restarts">
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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=tex2html5707 HREF="node322.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5705 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5699 HREF="node320.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5709 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5710 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5708 HREF="node322.html"> Named Restarts</A>
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<B>Up:</B> <A NAME=tex2html5706 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5700 HREF="node320.html"> Restarts</A>
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<HR> <P>
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<H2><A NAME=SECTION003336000000000000000>29.3.6. Anonymous 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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The simplest kind of restart involves structured transfer of control using
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a macro called <tt>restart-case</tt>. The <tt>restart-case</tt> form allows execution of
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a piece of code in a context where zero or more restarts are active, and
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where if one of those restarts is ``invoked,'' control will be transferred
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to the corresponding clause in the <tt>restart-case</tt> form. For example, we could
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rewrite the previous <tt>divide</tt> example as follows.
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<P><pre>
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(defun divide (numerator denominator)
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(loop
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(restart-case
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(return
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(cond ((or (not (numberp numerator))
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(not (numberp denominator)))
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(error "(DIVIDE '~S '~S) - Bad arguments."
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numerator denominator))
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((zerop denominator)
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(error 'division-by-zero
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:operator 'divide
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:operands (list numerator denominator)))
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(t ...)))
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(nil (arg1 arg2)
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:report "Provide new arguments for use by DIVIDE."
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:interactive
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(lambda ()
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(list (prompt-for 'number "Numerator: ")
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(prompt-for 'number "Denominator: ")))
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(setq numerator arg1 denominator arg2))
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(nil (result)
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:report "Provide a value to return from DIVIDE."
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:interactive
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(lambda () (list (prompt-for 'number "Result: ")))
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(return result)))))
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</pre><P>
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<P>
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<hr>
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<b>Remark:</b> The function <tt>prompt-for</tt> used in this chapter in a number of places is
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not a part of Common Lisp. It is used in the examples in this chapter only to keep
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the presentation simple. It is assumed to accept a type specifier
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and optionally a format string and associated arguments. It uses the
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format string and associated arguments as part of an interactive prompt,
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and uses <tt>read</tt> to read a Lisp object; however, only an object of the
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type indicated by the type specifier is accepted.
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<P>
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The question of whether or not <tt>prompt-for</tt> (or something like it) would be a
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useful addition to Common Lisp is under consideration by X3J13, but as of
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January 1989 no action has been taken. In spite of its use in a number of examples,
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nothing in the Common Lisp Condition System depends on this function.
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<hr>
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<P>
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In the example, the <tt>nil</tt> at the head of each clause
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means that it is an ``anonymous'' restart.
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Anonymous restarts are typically invoked only from within the
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debugger. As we shall see later, it is possible to have ``named restarts''
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that may be invoked from code without the need for user intervention.
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<P>
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If the arguments to anonymous restarts are not optional, then special
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information must be provided about what the debugger should use as arguments.
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Here the <tt>:interactive</tt> keyword is used to specify that information.
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<P>
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The <tt>:report</tt> keyword introduces information to be used when presenting the
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restart option to the user (by the debugger, for example).
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<P>
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Here is a sample interaction that takes advantage of the restarts provided
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by the revised definition of <tt>divide</tt>:
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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: Provide new arguments for use by the DIVIDE function.
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2: Provide a value to return from the DIVIDE function.
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3: Return to Lisp Toplevel.
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Debug> :continue 1
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1
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Numerator: 4
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Denominator: 2
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=> 9
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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=tex2html5707 HREF="node322.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5705 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5699 HREF="node320.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5709 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5710 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5708 HREF="node322.html"> Named Restarts</A>
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<B>Up:</B> <A NAME=tex2html5706 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5700 HREF="node320.html"> Restarts</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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