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6.4 KiB
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114 lines
6.4 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>29.3.18. Printing Conditions</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Printing Conditions">
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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=tex2html5849 HREF="node334.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5847 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5843 HREF="node332.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5851 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5852 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5850 HREF="node334.html"> Program Interface to </A>
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<B>Up:</B> <A NAME=tex2html5848 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5844 HREF="node332.html"> Condition Handlers</A>
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<HR> <P>
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<H2><A NAME=SECTION0033318000000000000000>29.3.18. Printing Conditions</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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When <tt>*print-escape*</tt> is <tt>nil</tt> (for example,
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when the <tt>princ</tt> function or the <tt>~A</tt>
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directive is used with <tt>format</tt>), the report method for the condition will be invoked. This will
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be done automatically by functions such as <tt>invoke-debugger</tt>, <tt>break</tt>, and <tt>warn</tt>,
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but there may still be situations in which it is desirable to have a
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condition report under explicit user control. For example,
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<P><pre>
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(let ((form '(open "nosuchfile")))
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(handler-case (eval form)
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(serious-condition (c)
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(format t "~&Evaluation of ~S failed:~%~A" form c))))
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</pre><P>
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might print something like
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<P><pre>
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Evaluation of (OPEN "nosuchfile") failed:
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The file "nosuchfile" was not found.
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</pre><P>
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Some suggestions about the form of text typed by report methods:
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<UL> <LI>
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The message should generally be a complete sentence, beginning with a
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capital letter and ending with appropriate punctuation (usually a period).
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<P>
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<LI>
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The message should <i>not</i> include any introductory text such as ``<tt>Error:</tt>''
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or ``<tt>Warning:</tt>'' and should not be followed by a trailing newline. Such
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text will be added as may be appropriate to context by the routine invoking
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the report method.
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<P>
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<LI>
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Except where unavoidable, the tab character (which is only semi-standard anyway)
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should not be used in
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error messages. Its effect may vary from one implementation to another and may
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cause problems even within an implementation because it may do different
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things depending on the column at which the error report begins.
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<P>
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<LI>
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Single-line messages are preferred, but newlines in the middle of long
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messages are acceptable.
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<P>
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<LI>
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If any program (for example, the debugger) displays messages indented from the
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prevailing left margin (for example, indented seven spaces because they
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are prefixed by the seven-character herald ``<tt>Error: </tt>''), then that program
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will take care of inserting the appropriate indentation into the extra
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lines of a multi-line error message. Similarly, a program that prefixes
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error messages with semicolons so that they appear to be comments should
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take care of inserting a semicolon at the beginning of each line in a
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multi-line error message. (These rules are important because, even within
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a single implementation, there may be more than one program that presents
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error messages to the user, and they may use different styles of
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presentation. The caller of <tt>error</tt> cannot anticipate all such possible
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styles, and so it is incumbent upon the presenter of the message to make
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any necessary adjustments.)
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</UL>
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[Note: These recommendations expand upon those in section <A HREF="node220.html#ERRORSIGNALLINGFUNCTIONS">24.1</A>.-GLS]
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<P>
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When <tt>*print-escape*</tt> is not <tt>nil</tt>, the object should print in some useful (but
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usually fairly abbreviated) fashion according to the style of the
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implementation. It is not expected that a condition will be printed in a form
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suitable for <tt>read</tt>. Something like <tt>#<ARITHMETIC-ERROR 1734></tt>
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is fine.
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<P>
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X3J13 voted in March 1989 (ZLOS-CONDITIONS) <A NAME=36660> </A> to integrate the
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Condition System and the Object System.
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In the original Condition System proposal,
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no function was provided for directly accessing or setting the printer for
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a condition type, or for invoking it; the techniques described above were
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the sole interface to reporting. The vote specified that, in CLOS terms,
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condition reporting is mediated through the <tt>print-object</tt>
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method for the condition type (that is, class) in question, with <tt>*print-escape*</tt>
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bound to <tt>nil</tt>. Specifying <tt>(:report <i>fn</i>)</tt> to
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<tt>define-condition</tt> when defining
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condition type <i>C</i> is equivalent to a separate method definition:
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<P><pre>
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(defmethod print-object ((x <i>C</i>) stream)
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(if *print-escape*
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(call-next-method)
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(funcall #'<i>fn</i> x stream)))
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</pre><P>
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Note that the method uses <i>fn</i> to print the condition
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only when <tt>*print-escape*</tt> has the value <tt>nil</tt>.
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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=tex2html5849 HREF="node334.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5847 HREF="node315.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5843 HREF="node332.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5851 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5852 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5850 HREF="node334.html"> Program Interface to </A>
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<B>Up:</B> <A NAME=tex2html5848 HREF="node315.html"> Survey of Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5844 HREF="node332.html"> Condition Handlers</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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