92 lines
4.4 KiB
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92 lines
4.4 KiB
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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>29.3.9. Comparison of Restarts and Catch/Throw</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Comparison of Restarts and Catch/Throw">
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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=tex2html5743 HREF="node325.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5741 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5735 HREF="node323.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5745 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5746 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5744 HREF="node325.html"> Generalized Restarts</A>
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<B>Up:</B> <A NAME=tex2html5742 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5736 HREF="node323.html"> Restart Functions</A>
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<HR> <P>
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<H2><A NAME=SECTION003339000000000000000>29.3.9. Comparison of Restarts and Catch/Throw</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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One important feature that <tt>restart-case</tt> (or <tt>restart-bind</tt>) offers that
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<tt>catch</tt> does not is the ability to reason about the available points to
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which control might be transferred without actually attempting the
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transfer. One could, for example, write
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<P><pre>
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(ignore-errors (throw ...))
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</pre><P>
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which is a sort of poor man's variation of
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<P><pre>
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(when (find-restart 'something)
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(invoke-restart 'something))
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</pre><P>
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but there is no way to use <tt>ignore-errors</tt> and <tt>throw</tt> to simulate something
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like
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<P><pre>
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(when (and (find-restart 'something)
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(find-restart 'something-else))
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(invoke-restart 'something))
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</pre><P>
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or even just
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<P><pre>
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(when (and (find-restart 'something)
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(yes-or-no-p "Do something? "))
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(invoke-restart 'something))
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</pre><P>
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because the degree of inspectability that comes with simply writing
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<P><pre>
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(ignore-errors (throw ...))
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</pre><P>
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is too primitive-getting the desired information also forces
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transfer of control, perhaps at a time when it is not desirable.
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<P>
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Many programmers have previously evolved strategies like the following
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on a case-by-case basis:
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<P><pre>
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(defvar *foo-tag-is-available* nil)
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(defun fn-1 ()
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(catch 'foo
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(let ((*foo-tag-is-available* t))
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... (fn-2) ...)))
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(defun fn-2 ()
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...
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(if *foo-tag-is-available* (throw 'foo t))
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...)
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</pre><P>
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The facility provided by <tt>restart-case</tt> and <tt>find-restart</tt> is intended to
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provide a standardized protocol for this sort of information to be
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communicated between programs that were developed independently so that
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individual variations from program to program
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do not thwart the overall modularity and debuggability of programs.
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<P>
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Another difference between the restart facility and the <tt>catch</tt>/<tt>throw</tt>
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facility is that a <tt>catch</tt> with any given tag completely shadows any
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outer pending <tt>catch</tt> that uses the same tag. Because of the presence
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of <tt>compute-restarts</tt>, however, it is possible to see shadowed restarts,
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which may be very useful in some situations (particularly in an
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interactive debugger).
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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=tex2html5743 HREF="node325.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5741 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5735 HREF="node323.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5745 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5746 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5744 HREF="node325.html"> Generalized Restarts</A>
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<B>Up:</B> <A NAME=tex2html5742 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5736 HREF="node323.html"> Restart Functions</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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