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52 lines
5.2 KiB
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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 2.4.1</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="02_d.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="02_d.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="02_db.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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2.4.1 Left-Parenthesis</H2> <P>
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The <A REL=DEFINITION HREF="26_glo_l.htm#left-parenthesis"><I>left-parenthesis</I></A> initiates reading of a <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A>. <A REL=DEFINITION HREF="f_rd_rd.htm#read"><B>read</B></A> is called recursively to read successive <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>objects</I></A> until a right parenthesis is found in the input <A REL=DEFINITION HREF="26_glo_s.htm#stream"><I>stream</I></A>. A <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A> of the <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>objects</I></A> read is returned. Thus <P>
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<PRE>
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(a b c)
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</PRE>
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</TT> is read as a <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A> of three <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>objects</I></A> (the <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbols</I></A> <TT>a</TT>, <TT>b</TT>, and <TT>c</TT>). The right parenthesis need not immediately follow the printed representation of the last <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>; <A REL=DEFINITION HREF="26_glo_w.htm#whitespace"><I>whitespace</I></A>[2] characters and comments may precede it. <P>
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If no <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>objects</I></A> precede the right parenthesis, it reads as a <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A> of zero <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>objects</I></A> (the <A REL=DEFINITION HREF="26_glo_e.htm#empty_list"><I>empty list</I></A>). <P>
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If a <A REL=DEFINITION HREF="26_glo_t.htm#token"><I>token</I></A> that is just a dot not immediately preceded by an escape character is read after some <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> then exactly one more <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> must follow the dot, possibly preceded or followed by <A REL=DEFINITION HREF="26_glo_w.htm#whitespace"><I>whitespace</I></A>[2] or a comment, followed by the right parenthesis: <P>
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<PRE>
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(a b c . d)
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</PRE>
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</TT> This means that the <A REL=DEFINITION HREF="26_glo_c.htm#cdr"><I>cdr</I></A> of the last <A REL=DEFINITION HREF="26_glo_c.htm#cons"><I>cons</I></A> in the <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A> is not <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>, but rather the <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> whose representation followed the dot. The above example might have been the result of evaluating <P>
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<PRE>
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(cons 'a (cons 'b (cons 'c 'd)))
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</PRE>
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</TT> Similarly, <P>
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<PRE>
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(cons 'this-one 'that-one) => (this-one . that-one)
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</PRE>
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</TT> It is permissible for the <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> following the dot to be a <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>list</I></A>: <P>
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<PRE>
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(a b c d . (e f . (g))) == (a b c d e f g)
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</PRE>
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</TT> <P>
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For information on how the <A REL=DEFINITION HREF="26_glo_l.htm#lisp_printer"><I>Lisp printer</I></A> prints <A REL=DEFINITION HREF="26_glo_l.htm#list"><I>lists</I></A> and <A REL=DEFINITION HREF="26_glo_c.htm#cons"><I>conses</I></A>, see <A REL=CHILD HREF="22_ace.htm">Section 22.1.3.5 (Printing Lists and Conses)</A>. <P>
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