66 lines
4 KiB
HTML
66 lines
4 KiB
HTML
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<TITLE>CLHS: Function NULL</TITLE>
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<A NAME="null"><I>Function</I> <B>NULL</B></A> <P>
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<P>
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<P><B>Syntax:</B><P>
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<P>
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<B>null</B> <I>object</I> => <I>boolean</I><P>
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<P>
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<P><B>Arguments and Values:</B><P>
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<P>
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<I>object</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. <P>
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<I>boolean</I>---a <A REL=DEFINITION HREF="26_glo_b.htm#boolean"><I>boolean</I></A>. <P>
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<P><B>Description:</B><P>
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<P>
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Returns <A REL=DEFINITION HREF="a_t.htm#t"><B>t</B></A> if <I>object</I> is the <A REL=DEFINITION HREF="26_glo_e.htm#empty_list"><I>empty list</I></A>; otherwise, returns <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. <P>
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<P><B>Examples:</B><P>
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<P>
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<PRE>
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(null '()) => T
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(null nil) => T
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(null t) => NIL
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(null 1) => NIL
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</PRE>
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</TT> <P>
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<P>
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<P><B>Side Effects:</B> None.
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<P>
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<P><B>Affected By:</B> None.
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<P>
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<P><B>Exceptional Situations:</B> None.
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<P>
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<P><B>See Also:</B><P>
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<P>
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<A REL=DEFINITION HREF="f_not.htm#not"><B>not</B></A> <P>
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<P><B>Notes:</B><P>
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<P>
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<A REL=DEFINITION HREF="#null"><B>null</B></A> is intended to be used to test for the <A REL=DEFINITION HREF="26_glo_e.htm#empty_list"><I>empty list</I></A> whereas <A REL=DEFINITION HREF="f_not.htm#not"><B>not</B></A> is intended to be used to invert a <A REL=DEFINITION HREF="26_glo_b.htm#boolean"><I>boolean</I></A> (or <A REL=DEFINITION HREF="26_glo_g.htm#generalized_boolean"><I>generalized boolean</I></A>). Operationally, <A REL=DEFINITION HREF="#null"><B>null</B></A> and <A REL=DEFINITION HREF="f_not.htm#not"><B>not</B></A> compute the same result; which to use is a matter of style. <P>
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<PRE>
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(null object) == (typep object 'null) == (eq object '<TT>()</TT>)
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</PRE>
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</TT> <P>
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<P><HR>The following <A REL=META HREF="../Front/X3J13Iss.htm">X3J13 cleanup issue</A>, <I>not part of the specification</I>, applies to this section:<P><UL><LI> <A REL=CHILD HREF="../Issues/iss249.htm">NOT-AND-NULL-RETURN-VALUE:X3J13-MAR-93</A><P></UL><HR>
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<A REL=COPYRIGHT HREF="../Front/Help.htm#Legal"><I>Copyright 1996-2005, LispWorks Ltd. All rights reserved.</I></A><P>
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