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92 lines
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<TITLE>29.1. Introduction</TITLE>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html5593 HREF="node314.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5591 HREF="node312.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5585 HREF="node312.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5595 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5596 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5594 HREF="node314.html"> Changes in Terminology</A>
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<B>Up:</B> <A NAME=tex2html5592 HREF="node312.html"> Conditions</A>
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<B> Previous:</B> <A NAME=tex2html5586 HREF="node312.html"> Conditions</A>
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<HR> <P>
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<H1><A NAME=SECTION003310000000000000000>29.1. Introduction</A></H1>
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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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Often we find it useful to describe a function in terms of its behavior in
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``normal situations.'' For example, we may say informally that the function
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<tt>+</tt> returns the sum of its arguments or that the function
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<tt>read-char</tt> returns the next available character on a given input
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stream.
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<P>
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Sometimes, however, an ``exceptional situation'' will arise that does not fit
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neatly into such descriptions. For example, <tt>+</tt> might receive an argument
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that is not a number, or <tt>read-char</tt> might receive as a single argument
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a stream that has no more available characters. This distinction between normal
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and exceptional situations is in some sense arbitrary but is often very
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useful in practice.
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<P>
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For example, suppose a function <tt>f</tt> were defined to allow only
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integer arguments but also guaranteed to
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detect and signal an error for non-integer arguments.
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Such a description is in fact internally inconsistent (that is,
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paradoxical) because the function's behavior is well-defined for non-integers.
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Yet we would not want this annoying paradox to force description of <tt>f</tt>
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as a function that accepts any kind of argument (just in case <tt>f</tt>
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is being called only as a quick way to signal an error, for example).
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Using the normal/exceptional distinction, we can say clearly that <tt>f</tt> accepts integers
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in the normal situation and signals an error in exceptional situations.
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Moreover, we can say that when we refer to the definition of a
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function informally, it is acceptable to speak only of its normal behavior.
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For example, we can speak informally about <tt>f</tt> as a function that accepts only
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integers without feeling that we are committing some awful fraud.
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<P>
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Not all exceptional situations are errors. For example, a program that is
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directing the typing of a long line of text may come to an end-of-line.
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It is possible that no real harm will result from failing to signal end-of-line
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to its caller because the operating system will simply force a carriage
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return on the output device, which will continue typing on the next line. However, it
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may still be interesting to establish a protocol whereby the printing program can
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inform its caller of end-of-line exceptions. The caller could
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then opt to deal with these situations in interesting ways at certain times.
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For example, a caller might choose to terminate printing, obtaining an end-of-line
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truncation. The important thing, however, is that the failure of the
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caller to provide advice about the situation need not prevent
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the printer program from operating correctly.
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<P>
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Mechanisms for dealing with exceptional situations vary widely. When an
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exceptional situation is encountered, a program may attempt to handle
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it by returning a distinguished value, returning an additional value,
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setting a variable, calling a function, performing a special transfer of
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control, or stopping the program altogether and entering the debugger.
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<P>
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For the most part, the facilities described in this chapter do not introduce
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any fundamentally new way of dealing with exceptional situations. Rather, they
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encapsulate and formalize useful patterns of data and control flow that have
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been seen to be useful in dealing with exceptional situations.
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<P>
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A proper conceptual approach to errors should perhaps begin from first
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principles, with a discussion of <i>conditions</i> in general, and eventually work
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up to the concept of an <i>error</i> as just one of the many kinds of
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conditions. However, given the primitive state of error-handling
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technology, a proper buildup may be as inappropriate as requiring that a
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beggar learn to cook a gourmet meal before being allowed to eat. Thus,
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we deal first with the essentials-error handling-and then
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go back later to fill in the missing details.
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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=tex2html5593 HREF="node314.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5591 HREF="node312.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5585 HREF="node312.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5595 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5596 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5594 HREF="node314.html"> Changes in Terminology</A>
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<B>Up:</B> <A NAME=tex2html5592 HREF="node312.html"> Conditions</A>
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<B> Previous:</B> <A NAME=tex2html5586 HREF="node312.html"> Conditions</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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