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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>29.3.4. Object-Oriented Basis of Condition Handling</TITLE>
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<meta name="description" value=" Object-Oriented Basis of Condition Handling">
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html5683 HREF="node320.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5681 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5675 HREF="node318.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5685 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5686 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5684 HREF="node320.html"> Restarts</A>
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<B>Up:</B> <A NAME=tex2html5682 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5676 HREF="node318.html"> Handling Conditions</A>
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<HR> <P>
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<H2><A NAME=SECTION003334000000000000000>29.3.4. Object-Oriented Basis of Condition Handling</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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<A NAME=OBJECT0RIENTEDBASIS>Of</A>
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course, the ability of the handler to usefully handle an exceptional
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situation is related to the quality of the information it is provided. For
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example, if all errors were signaled by
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<P><pre>
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(error "<i>some format string</i>")
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</pre><P>
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then the only piece of information that would be accessible to the handler
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would be an object of type <tt>simple-error</tt> that had a slot containing the
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format string.
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<P>
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If this were done, <tt>string-equal</tt> would be the preferred way to tell one error
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from another, and it would be very hard to allow flexibility in the
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presentation of error messages because existing handlers would tend to be
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broken by even tiny variations in the wording of an error message. This
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phenomenon has been the major failing of most error systems previously
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available in Lisp. It is fundamentally important to decouple the error
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message string (the human interface) from the objects that formally
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represent the error state (the program interface). We therefore have the
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notion of typed conditions, and of formal operations on those conditions
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that make them inspectable in a structured way.
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<P>
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This object-oriented approach to condition handling has the following
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important advantages over a text-based approach:
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<UL><LI>
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Conditions are classified according to subtype relationships, making
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it easy to test for categories of conditions.
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<P>
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<LI>
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Conditions have named slot values through which parameters are conveyed
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from the program that signals the condition to the program that handles it.
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<P>
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<LI>
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Inheritance of methods and slots reduces the amount of explicit
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specification necessary to achieve various interesting effects.
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</UL>
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<P>
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Some condition types are defined by this document, but the set of
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condition types is extensible using <tt>define-condition</tt>.
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Common Lisp condition types are in fact CLOS classes, and condition objects
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are ordinary CLOS objects; <tt>define-condition</tt> merely
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provides an abstract interface that is a bit more convenient than
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<tt>defclass</tt> for defining conditions.
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<P>
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Here, as an example,
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we define a two-argument function called <tt>divide</tt> that is patterned after
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the <tt>/</tt> function but does some stylized error checking:
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<P><pre>
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(defun divide (numerator denominator)
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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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</pre><P>
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Note that in the first clause we have used <tt>error</tt> with a string argument
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and in the second clause we have named a particular condition type,
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<tt>division-by-zero</tt>. In the case of a string argument, the condition type that
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will be signaled is <tt>simple-error</tt>.
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<P>
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The particular kind of error that is signaled may be important
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in cases where handlers are active. For example, <tt>simple-error</tt> inherits
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from type <tt>error</tt>, which in turn inherits from type <tt>condition</tt>. On the
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other hand, <tt>division-by-zero</tt> inherits from <tt>arithmetic-error</tt>, which
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inherits from <tt>error</tt>, which inherits from <tt>condition</tt>. So if a handler
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existed for <tt>arithmetic-error</tt> while a <tt>division-by-zero</tt> condition was
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signaled, that handler would be tried; however, if a <tt>simple-error</tt>
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condition were signaled in the same context, the handler for type
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<tt>arithmetic-error</tt> would not be tried.
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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=tex2html5683 HREF="node320.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5681 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5675 HREF="node318.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5685 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5686 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5684 HREF="node320.html"> Restarts</A>
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<B>Up:</B> <A NAME=tex2html5682 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5676 HREF="node318.html"> Handling Conditions</A>
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<HR> <P>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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