102 lines
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102 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>24. Errors</TITLE>
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<meta name="description" value=" Errors">
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<meta name="resource-type" value="document">
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html4286 HREF="node220.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4284 HREF="clm.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4278 HREF="node218.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4288 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4289 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4287 HREF="node220.html"> General Error-Signaling Functions</A>
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<B>Up:</B> <A NAME=tex2html4285 HREF="clm.html">Common Lisp the Language</A>
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<B> Previous:</B> <A NAME=tex2html4279 HREF="node218.html"> Accessing Directories</A>
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<HR> <P>
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<H1><A NAME=SECTION002800000000000000000>24. Errors</A></H1>
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<P>
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<A NAME=XERROR>Errors</A>
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may be signaled for a variety of reasons.
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Many built-in Common Lisp functions may signal an error when given incorrect
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arguments. Other functions, described in this chapter,
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may be called by user programs for the purpose of signaling
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an error.
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<P>
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When an error is signaled, it is
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handled in an implementation-dependent way. It is expected
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that each implementation of Common Lisp will provide an interactive debugger that
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prints the error message along with suitable contextual information
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such as which function detected the error. The user may interact with
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the debugger to examine or modify the state of the program in various
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ways, including abandoning the current computation (``aborting to top
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level'') and continuing from the error. What ``continuing'' means
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depends on how the error is signaled; the details of this are specified below
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for each error-signaling function.
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<P>
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<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
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An implementation may also choose to provide means (such as the
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<tt>errset</tt> special form in MacLisp) for a program to trap
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all errors and prevent the debugger from stepping in for
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certain errors.
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<P>
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<hr>
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<b>Rationale:</b> Error handling of adequate
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flexibility and power for all systems written in Common Lisp appears to
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require a complex error classification system.
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Experience with several error-handling systems
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in such dialects as MacLisp and Lisp Machine Lisp indicates that
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further experimentation is needed in this area;
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it is too early to define a standard error-handling mechanism.
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Therefore Common Lisp provides standard ways to <i>signal</i> errors,
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but no standard ways to <i>handle</i> errors.
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Of course a
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complete Lisp system requires error-handling mechanisms, but many useful
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portable programs do not require them. It is expected that a future
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revision of Common Lisp will address the problem of portable error-handling
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mechanisms.
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<hr>
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<br><img align=bottom alt="old_change_end" src="gif/old_change_end.gif">
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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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X3J13 voted in June 1988
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(CONDITION-SYSTEM) <A NAME=27856> </A>
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to adopt a proposal for a Common Lisp Condition System.
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This was the result of the research and experimentation
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alluded to in the preceding paragraph.
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Conditions subsume and generalize the notion of errors.
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The condition system also provides means for handling
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conditions (of which errors are a special case) and
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for restarting a computation after a condition has been signaled.
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See chapter <A HREF="node312.html#CONDITION">29</A>.
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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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<hr>
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<b>Compatibility note:</b> What is here called ``continuing,''
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Lisp Machine Lisp calls ``proceeding'' from an error.
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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 the new terminology introduced in chapter <A HREF="node312.html#CONDITION">29</A>,
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what Lisp Machine Lisp called ``proceeding'' would be called
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``restarting,'' and ``continuing'' refers to the particular
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restart named <tt>continue</tt>.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<hr>
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<P>
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<HR>
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<UL>
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<LI> <A NAME=tex2html4290 HREF="node220.html#SECTION002810000000000000000"> General Error-Signaling Functions</A>
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<LI> <A NAME=tex2html4291 HREF="node221.html#SECTION002820000000000000000"> Specialized Error-Signaling Forms and Macros</A>
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<LI> <A NAME=tex2html4292 HREF="node222.html#SECTION002830000000000000000"> Special Forms for Exhaustive Case Analysis</A>
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</UL>
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<BR> <HR><A NAME=tex2html4286 HREF="node220.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4284 HREF="clm.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4278 HREF="node218.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4288 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4289 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4287 HREF="node220.html"> General Error-Signaling Functions</A>
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<B>Up:</B> <A NAME=tex2html4285 HREF="clm.html">Common Lisp the Language</A>
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<B> Previous:</B> <A NAME=tex2html4279 HREF="node218.html"> Accessing Directories</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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