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116 lines
6.1 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>29.3.1. Signaling Errors</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Signaling Errors">
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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=tex2html5647 HREF="node317.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5645 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5639 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5649 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5650 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5648 HREF="node317.html"> Trapping Errors</A>
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<B>Up:</B> <A NAME=tex2html5646 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5640 HREF="node315.html"> Survey of Concepts</A>
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<HR> <P>
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<H2><A NAME=SECTION003331000000000000000>29.3.1. Signaling Errors</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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Conceptually, signaling an error in a program is an admission by that program
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that it does not know how to continue and requires external intervention. Once
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an error is signaled, any decision about how to continue must come from the
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``outside.''
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<P>
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The simplest way to signal an error is to use the <tt>error</tt> function with
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<tt>format</tt>-style arguments describing the error for the sake of the user interface.
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If <tt>error</tt> is called and there are no active handlers (described
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in sections <A HREF="node317.html#TRAPPINGERRORS">29.3.2</A> and <A HREF="node318.html#HANDLINGCONDITIONS">29.3.3</A>), the
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debugger will be entered and the error message will be typed out. For example:
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<P><pre>
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Lisp> (defun factorial (x)
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(cond ((or (not (typep x 'integer)) (minusp x))
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(error "~S is not a valid argument to FACTORIAL."
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x))
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((zerop x) 1)
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(t (* x (factorial (- x 1))))))
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=> FACTORIAL
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Lisp> (factorial 20)
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=> 2432902008176640000
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Lisp> (factorial -1)
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Error: -1 is not a valid argument to FACTORIAL.
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To continue, type :CONTINUE followed by an option number:
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1: Return to Lisp Toplevel.
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Debug>
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</pre><P>
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In general, a call to <tt>error</tt> cannot directly return. Unless special work has
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been done to override this behavior, the debugger will be entered and there
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will be no option to simply continue.
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<P>
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The only exception may be that some implementations may provide debugger
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commands for interactively returning from individual stack frames; even then,
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however, such commands should never be used except by someone who has read the
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erring code and understands the consequences of continuing from that point. In
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particular, the programmer should feel confident
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about writing code like this:
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<P><pre>
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(defun wargames:no-win-scenario ()
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(when (true) (error "Pushing the button would be stupid."))
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(push-the-button))
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</pre><P>
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In this scenario, there should be no chance that the function <tt>error</tt> will return
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and the button will be pushed.
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<P>
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<hr>
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<b>Remark:</b> It should be noted that the notion of
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``no chance'' that the button will be pushed is relative only to the language
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model; it assumes that the language is accurately implemented. In practice,
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compilers have bugs, computers have glitches, and users have been known
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to interrupt at inopportune moments and use the debugger to return from
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arbitrary stack frames. Such violations of the language model are
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beyond the scope of the condition system but not necessarily beyond the
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scope of potential failures that the programmer should consider and defend against.
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The possibility of such unusual failures may of course also influence the design of
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code meant to handle less drastic situations,
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such as maintaining a database uncorrupted.-KMP and GLS
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<hr>
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<P>
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In some cases, the programmer may have a single, well-defined idea of a
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reasonable recovery strategy for this particular error. In that case, he can
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use the function <tt>cerror</tt>, which specifies information about what would happen
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if the user did simply continue from the call to <tt>cerror</tt>. For example:
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<P><pre>
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Lisp> (defun factorial (x)
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(cond ((not (typep x 'integer))
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(error "~S is not a valid argument to FACTORIAL."
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x))
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((minusp x)
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(let ((x-magnitude (- x)))
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(cerror "Compute -(~D!) instead."
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"(-~D)! is not defined." x-magnitude)
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(- (factorial x-magnitude))))
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((zerop x) 1)
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(t (* x (factorial (- x 1))))))
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=> FACTORIAL
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Lisp> (factorial -3)
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Error: (-3)! is not defined.
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To continue, type :CONTINUE followed by an option number:
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1: Compute -(3!) instead.
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2: Return to Lisp Toplevel.
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Debug> :continue 1
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=> -6
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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=tex2html5647 HREF="node317.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5645 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5639 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5649 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5650 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5648 HREF="node317.html"> Trapping Errors</A>
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<B>Up:</B> <A NAME=tex2html5646 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5640 HREF="node315.html"> Survey of Concepts</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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