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3.8 KiB
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65 lines
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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>7.8.1. Indefinite Iteration</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Indefinite Iteration">
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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=tex2html2613 HREF="node88.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2611 HREF="node86.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2605 HREF="node86.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2615 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2616 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2614 HREF="node88.html"> General Iteration</A>
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<B>Up:</B> <A NAME=tex2html2612 HREF="node86.html"> Iteration</A>
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<B> Previous:</B> <A NAME=tex2html2606 HREF="node86.html"> Iteration</A>
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<HR> <P>
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<H2><A NAME=SECTION001181000000000000000>7.8.1. Indefinite Iteration</A></H2>
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<P>
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The <tt>loop</tt> construct is the simplest iteration facility.
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It controls no variables, and simply executes its body repeatedly.
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>loop</tt> <tt>{<i>form</i>}*</tt><P>Each <i>form</i> is evaluated in turn from left to right.
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When the last <i>form</i> has been evaluated, then the first <i>form</i>
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is evaluated again, and so on, in a never-ending cycle.
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The <tt>loop</tt> construct never returns a value. Its execution must be terminated
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explicitly, using <tt>return</tt> or <tt>throw</tt>, for example.
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<P>
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<tt>loop</tt>, like most iteration constructs,
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establishes an implicit block named <tt>nil</tt>.
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Thus <tt>return</tt> may be used to exit from a <tt>loop</tt> with specified results.
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<P>
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<img align=bottom alt="old_change_begin" src="gif/old_change_begin.gif"><br>
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A <tt>loop</tt> construct has this meaning only if every <i>form</i> is
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non-atomic (a list). The case where some <i>form</i> is
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atomic is reserved for future extensions.
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<P>
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<hr>
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<b>Implementation note:</b>
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There have been several proposals for a powerful iteration
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mechanism to be called <tt>loop</tt>. One version is provided in Lisp Machine Lisp.
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Implementors are encouraged to experiment with extensions to the <tt>loop</tt>
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syntax, but users should be advised that in all likelihood some specific
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set of extensions to <tt>loop</tt> will be adopted in a future revision of Common Lisp.
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<hr>
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<br><img align=bottom alt="old_change_end" src="gif/old_change_end.gif">
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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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X3J13 voted in January 1989
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(LOOP-FACILITY) <A NAME=7041> </A>
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to include just such an extension of <tt>loop</tt>. See chapter <A HREF="node235.html#LOOP">26</A>.
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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=tex2html2613 HREF="node88.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2611 HREF="node86.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2605 HREF="node86.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2615 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2616 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2614 HREF="node88.html"> General Iteration</A>
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<B>Up:</B> <A NAME=tex2html2612 HREF="node86.html"> Iteration</A>
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<B> Previous:</B> <A NAME=tex2html2606 HREF="node86.html"> Iteration</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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