83 lines
4.6 KiB
HTML
83 lines
4.6 KiB
HTML
|
<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
|
||
|
|
<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
|
||
|
|
<HEAD>
|
||
|
|
<TITLE>29.3.10. Generalized Restarts</TITLE>
|
||
|
|
</HEAD>
|
||
|
|
<BODY>
|
||
|
|
<meta name="description" value=" Generalized Restarts">
|
||
|
|
<meta name="keywords" value="clm">
|
||
|
|
<meta name="resource-type" value="document">
|
||
|
|
<meta name="distribution" value="global">
|
||
|
|
<P>
|
||
|
|
<b>Common Lisp the Language, 2nd Edition</b>
|
||
|
|
<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>
|
||
|
|
<B> Next:</B> <A NAME=tex2html5756 HREF="node326.html"> Interactive Condition Handling</A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html5754 HREF="node315.html"> Survey of Concepts</A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html5748 HREF="node324.html"> Comparison of Restarts </A>
|
||
|
|
<HR> <P>
|
||
|
|
<H2><A NAME=SECTION0033310000000000000000>29.3.10. Generalized Restarts</A></H2>
|
||
|
|
<P>
|
||
|
|
<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
|
||
|
|
<A NAME=LASTRESTARTSSECTION><tt>restart-case</tt></A>
|
||
|
|
is a mechanism that allows only imperative transfer of control
|
||
|
|
for its associated restarts. <tt>restart-case</tt> is built on a lower-level mechanism
|
||
|
|
called <tt>restart-bind</tt>, which does not force transfer of control.
|
||
|
|
<P>
|
||
|
|
<tt>restart-bind</tt> is to <tt>restart-case</tt> as <tt>handler-bind</tt> is to
|
||
|
|
<tt>handler-case</tt>.
|
||
|
|
The syntax is
|
||
|
|
<P><pre>
|
||
|
|
(restart-bind ((<i>name</i> <i>function</i> . <i>options</i>)) . <i>body</i>)
|
||
|
|
</pre><P>
|
||
|
|
The <i>body</i> is executed in a dynamic context within which the <i>function</i>
|
||
|
|
will be called whenever
|
||
|
|
<tt>(invoke-restart '<i>name</i>)</tt> is executed. The <i>options</i> are keyword-style and are
|
||
|
|
used to pass information such as that provided with the
|
||
|
|
<tt>:report</tt> keyword in <tt>restart-case</tt>.
|
||
|
|
<P>
|
||
|
|
A <tt>restart-case</tt> expands into a call to <tt>restart-bind</tt> where the function
|
||
|
|
simply does an unconditional transfer of control to a particular body
|
||
|
|
of code, passing along ``argument'' information in a structured way.
|
||
|
|
<P>
|
||
|
|
It is also possible to write restarts that do not transfer control. Such
|
||
|
|
restarts may be useful in implementing various special commands for the
|
||
|
|
debugger that are of interest only in certain situations. For example,
|
||
|
|
one might imagine a situation where file space was exhausted and the
|
||
|
|
following was done in an attempt to free space in directory <tt>dir</tt>:
|
||
|
|
<P><pre>
|
||
|
|
(restart-bind ((nil #'(lambda () (expunge-directory dir))
|
||
|
|
:report-function
|
||
|
|
#'(lambda (stream)
|
||
|
|
(format stream "Expunge ~A."
|
||
|
|
(directory-namestring dir)))))
|
||
|
|
(cerror "Try this file operation again."
|
||
|
|
'directory-full :directory dir))
|
||
|
|
</pre><P>
|
||
|
|
In this case, the debugger might be entered and the user could first
|
||
|
|
perform the expunge (which would not transfer control from the debugger
|
||
|
|
context) and then retry the file operation:
|
||
|
|
<P><pre>
|
||
|
|
Lisp> (open "FOO" :direction :output)
|
||
|
|
Error: The directory PS:<JDOE> is full.
|
||
|
|
To continue, type :CONTINUE followed by an option number:
|
||
|
|
1: Try this file operation again.
|
||
|
|
2: Expunge PS:<JDOE>.
|
||
|
|
3: Return to Lisp Toplevel.
|
||
|
|
Debug> :continue 2
|
||
|
|
Expunging PS:<JDOE> ... 3 records freed.
|
||
|
|
Debug> :continue 1
|
||
|
|
=> #<OUTPUT-STREAM "PS:<JDOE>FOO.LSP" 2323473>
|
||
|
|
</pre><P>
|
||
|
|
<br><img align=bottom alt="change_end" src="gif/change_end.gif">
|
||
|
|
<P>
|
||
|
|
<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>
|
||
|
|
<B> Next:</B> <A NAME=tex2html5756 HREF="node326.html"> Interactive Condition Handling</A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html5754 HREF="node315.html"> Survey of Concepts</A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html5748 HREF="node324.html"> Comparison of Restarts </A>
|
||
|
|
<HR> <P>
|
||
|
|
<HR>
|
||
|
|
<P><ADDRESS>
|
||
|
|
AI.Repository@cs.cmu.edu
|
||
|
|
</ADDRESS>
|
||
|
|
</BODY>
|