46 lines
2.3 KiB
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46 lines
2.3 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>2.14. Unreadable Data Objects</TITLE>
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<BODY>
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<meta name="description" value=" Unreadable Data Objects">
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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=tex2html2011 HREF="node42.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2009 HREF="node15.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2003 HREF="node40.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2013 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2014 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2012 HREF="node42.html"> OverlapInclusion, and </A>
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<B>Up:</B> <A NAME=tex2html2010 HREF="node15.html"> Data Types</A>
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<B> Previous:</B> <A NAME=tex2html2004 HREF="node40.html"> Functions</A>
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<HR> <P>
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<H1><A NAME=SECTION006140000000000000000>2.14. Unreadable Data Objects</A></H1>
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<P>
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Some objects may print in implementation-dependent ways.
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Such objects cannot necessarily be reliably reconstructed from
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a printed representation, and so they are usually printed in
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a format informative to the user but not acceptable to the <tt>read</tt> function:
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<tt>#<<i>useful information</i>></tt>.
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The Lisp reader will signal an error on encountering <tt>#<</tt>.
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<P>
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As a hypothetical example, an implementation might print
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<P><pre>
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#<stack-pointer si:rename-within-new-definition-maybe #o311037552>
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</pre><P>
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for an implementation-specific ``internal stack pointer'' data type
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whose printed representation includes the name of the type,
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some information about the stack slot pointed to, and the machine address
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(in octal) of the stack slot.
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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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See <tt>print-unreadable-object</tt>, a macro that prints an object using <tt>#<</tt>
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syntax.
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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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<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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