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55 lines
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<!-- Common Lisp HyperSpec (TM), version 7.0 generated by Kent M. Pitman on Mon, 11-Apr-2005 2:31am EDT -->
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<TITLE>CLHS: Section 1.4.1.4.4</TITLE>
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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="01_dadc.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="01_dad.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="01_dae.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>
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1.4.1.4.4 NIL</H2> <P>
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<A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A> has a variety of meanings. It is a <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A> in the <TT>COMMON-LISP</TT> package with the <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>name</I></A> <TT>"NIL"</TT>, it is <A REL=DEFINITION HREF="26_glo_b.htm#boolean"><I>boolean</I></A> (and <A REL=DEFINITION HREF="26_glo_g.htm#generalized_boolean"><I>generalized boolean</I></A>) <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A>, it is the <A REL=DEFINITION HREF="26_glo_e.htm#empty_list"><I>empty list</I></A>, and it is the <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>name</I></A> of the <A REL=DEFINITION HREF="26_glo_e.htm#empty_type"><I>empty type</I></A> (a <A REL=DEFINITION HREF="26_glo_s.htm#subtype"><I>subtype</I></A> of all <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>types</I></A>). <P>
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Within Common Lisp, <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A> can be notated interchangeably as either <TT>NIL</TT> or <TT>()</TT>. By convention, the choice of notation offers a hint as to which of its many roles it is playing. <P>
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<PRE>
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For Evaluation? Notation Typically Implied Role
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----------<P>
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Yes nil use as a <A REL=DEFINITION HREF="26_glo_b.htm#boolean">boolean</A>.
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Yes 'nil use as a <A REL=DEFINITION HREF="26_glo_s.htm#symbol">symbol</A>.
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Yes '() use as an <A REL=DEFINITION HREF="26_glo_e.htm#empty_list">empty list</A>
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No nil use as a <A REL=DEFINITION HREF="26_glo_s.htm#symbol">symbol</A> or <A REL=DEFINITION HREF="26_glo_b.htm#boolean">boolean</A>.
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No () use as an <A REL=DEFINITION HREF="26_glo_e.htm#empty_list">empty list</A>.
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</PRE>
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<P><B>Figure 1-1. Notations for NIL</B> <P>
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Within this document only, <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A> is also sometimes notated as <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A> to emphasize its role as a <A REL=DEFINITION HREF="26_glo_b.htm#boolean"><I>boolean</I></A>. <P>
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For example: <P>
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<PRE>
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(print ()) ;avoided
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(defun three nil 3) ;avoided
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'(nil nil) ;list of two symbols
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'(() ()) ;list of empty lists
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(defun three () 3) ;Emphasize empty parameter list.
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(append '() '()) => () ;Emphasize use of empty lists
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(not nil) => <A REL=DEFINITION HREF="26_glo_t.htm#true">true</A> ;Emphasize use as Boolean false
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(get 'nil 'color) ;Emphasize use as a symbol
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</PRE>
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</TT> <P>
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A <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> is sometimes said to ``be <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A>'' or ``be <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>'' in some circumstance. Since no <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> object can be the same as <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A> and all <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>objects</I></A> represent <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A> when viewed as <A REL=DEFINITION HREF="26_glo_b.htm#boolean"><I>booleans</I></A>, it would be meaningless to say that the <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> was literally <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A> and uninteresting to say that it was literally <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>. Instead, these phrases are just traditional alternative ways of saying that the <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> ``returns <A REL=DEFINITION HREF="26_glo_f.htm#false"><I>false</I></A>'' or ``returns <A REL=DEFINITION HREF="26_glo_t.htm#true"><I>true</I></A>,'' respectively. <P>
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