63 lines
3.9 KiB
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63 lines
3.9 KiB
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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> 1.2.2. Nil, False, and the Empty List</TITLE>
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<meta name="description" value=" Nil, False, and the Empty List">
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<meta name="resource-type" value="document">
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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=tex2html1596 HREF="node10.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html1594 HREF="node7.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html1588 HREF="node8.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html1598 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html1599 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html1597 HREF="node10.html"> EvaluationExpansion, and </A>
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<B>Up:</B> <A NAME=tex2html1595 HREF="node7.html"> Notational Conventions</A>
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<B> Previous:</B> <A NAME=tex2html1589 HREF="node8.html"> Decimal Numbers</A>
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<HR> <P>
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<H2><A NAME=SECTION00522000000000000000> 1.2.2. Nil, False, and the Empty List</A></H2>
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<P>
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In Common Lisp, as in most Lisp dialects, the symbol <tt>nil</tt>
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is used to represent both the empty list and the ``false'' value
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for Boolean tests. An empty list may, of course, also be written
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<tt>()</tt>; this normally denotes the same object as <tt>nil</tt>.
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(It is possible, by extremely perverse manipulation of the package system,
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to cause the sequence of letters <tt>nil</tt> to be recognized
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not as the symbol that represents the empty list but as another
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symbol with the same name. This obscure possibility will be ignored
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in this book.)
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These two notations may be used interchangeably as far as the Lisp
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system is concerned. However, as a matter of style,
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this book
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uses the notation <tt>()</tt> when it is desirable to emphasize
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the use of an empty list, and uses the notation <tt>nil</tt>
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when it is desirable to emphasize the use of the Boolean ``false''.
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The notation <tt>'nil</tt> (note the explicit quotation mark) is used to emphasize
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the use of a symbol.
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For example:
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<P><pre>
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(defun three () 3) ;Emphasize empty parameter list <BR>
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(append '() '()) => () ;Emphasize use of empty lists <BR>
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(not nil) => t ;Emphasize use as Boolean ``false'' <BR>
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(get 'nil 'color) ;Emphasize use as a symbol
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</pre><P>
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<P>
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Any data object other than <tt>nil</tt> is construed to be Boolean
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``not false'', that is, ``true''. The symbol <tt>t</tt> is conventionally
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used to mean ``true'' when no other value is more appropriate.
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When a function is said to ``return <i>false</i>'' or to ``be <i>false</i>''
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in some circumstance, this means that it returns <tt>nil</tt>.
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However, when a function is said to ``return <i>true</i>'' or to ``be <i>true</i>''
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in some circumstance, this means that it returns some value other
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than <tt>nil</tt>, but not necessarily <tt>t</tt>.
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<P>
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<BR> <HR><A NAME=tex2html1596 HREF="node10.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html1594 HREF="node7.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html1588 HREF="node8.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html1598 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html1599 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html1597 HREF="node10.html"> EvaluationExpansion, and </A>
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<B>Up:</B> <A NAME=tex2html1595 HREF="node7.html"> Notational Conventions</A>
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<B> Previous:</B> <A NAME=tex2html1589 HREF="node8.html"> Decimal Numbers</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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