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34 lines
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<TITLE>CLHS: Section 2.1.4.4</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="02_adc.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="02_ad.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="02_ade.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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2.1.4.4 Macro Characters</H2> <P>
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When the <A REL=DEFINITION HREF="26_glo_l.htm#lisp_reader"><I>Lisp reader</I></A> encounters a <A REL=DEFINITION HREF="26_glo_m.htm#macro_character"><I>macro character</I></A> on an <A REL=DEFINITION HREF="26_glo_i.htm#input"><I>input</I></A> <A REL=DEFINITION HREF="26_glo_s.htm#stream"><I>stream</I></A>, special parsing of subsequent <A REL=DEFINITION HREF="26_glo_c.htm#character"><I>characters</I></A> on the <A REL=DEFINITION HREF="26_glo_i.htm#input"><I>input</I></A> <A REL=DEFINITION HREF="26_glo_s.htm#stream"><I>stream</I></A> is performed. <P>
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A <A REL=DEFINITION HREF="26_glo_m.htm#macro_character"><I>macro character</I></A> has an associated <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> called a <A REL=DEFINITION HREF="26_glo_r.htm#reader_macro_function"><I>reader macro function</I></A> that implements its specialized parsing behavior. An association of this kind can be established or modified under control of a <A REL=DEFINITION HREF="26_glo_c.htm#conforming_program"><I>conforming program</I></A> by using the <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>functions</I></A> <A REL=DEFINITION HREF="f_set_ma.htm#set-macro-character"><B>set-macro-character</B></A> and <A REL=DEFINITION HREF="f_set__1.htm#set-dispatch-macro-character"><B>set-dispatch-macro-character</B></A>. <P>
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Upon encountering a <A REL=DEFINITION HREF="26_glo_m.htm#macro_character"><I>macro character</I></A>, the <A REL=DEFINITION HREF="26_glo_l.htm#lisp_reader"><I>Lisp reader</I></A> calls its <A REL=DEFINITION HREF="26_glo_r.htm#reader_macro_function"><I>reader macro function</I></A>, which parses one specially formatted object from the <A REL=DEFINITION HREF="26_glo_i.htm#input"><I>input</I></A> <A REL=DEFINITION HREF="26_glo_s.htm#stream"><I>stream</I></A>. The <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> either returns the parsed <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>, or else it returns no <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>values</I></A> to indicate that the characters scanned by the <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> are being ignored (e.g., in the case of a comment). Examples of <A REL=DEFINITION HREF="26_glo_m.htm#macro_character"><I>macro characters</I></A> are <A REL=DEFINITION HREF="26_glo_b.htm#backquote"><I>backquote</I></A>, <A REL=DEFINITION HREF="26_glo_s.htm#single-quote"><I>single-quote</I></A>, <A REL=DEFINITION HREF="26_glo_l.htm#left-parenthesis"><I>left-parenthesis</I></A>, and <A REL=DEFINITION HREF="26_glo_r.htm#right-parenthesis"><I>right-parenthesis</I></A>. <P>
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A <A REL=DEFINITION HREF="26_glo_m.htm#macro_character"><I>macro character</I></A> is either <A REL=DEFINITION HREF="26_glo_t.htm#terminating"><I>terminating</I></A> or <A REL=DEFINITION HREF="26_glo_n.htm#non-terminating"><I>non-terminating</I></A>. The difference between <A REL=DEFINITION HREF="26_glo_t.htm#terminating"><I>terminating</I></A> and <A REL=DEFINITION HREF="26_glo_n.htm#non-terminating"><I>non-terminating</I></A> <A REL=DEFINITION HREF="26_glo_m.htm#macro_character"><I>macro characters</I></A> lies in what happens when such characters occur in the middle of a <A REL=DEFINITION HREF="26_glo_t.htm#token"><I>token</I></A>. If a <A REL=DEFINITION HREF="26_glo_n.htm#non-terminating"><I>non-terminating</I></A> <A REL=DEFINITION HREF="26_glo_m.htm#macro_character"><I>macro character</I></A> occurs in the middle of a <A REL=DEFINITION HREF="26_glo_t.htm#token"><I>token</I></A>, the <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> associated with the <A REL=DEFINITION HREF="26_glo_n.htm#non-terminating"><I>non-terminating</I></A> <A REL=DEFINITION HREF="26_glo_m.htm#macro_character"><I>macro character</I></A> is not called, and the <A REL=DEFINITION HREF="26_glo_n.htm#non-terminating"><I>non-terminating</I></A> <A REL=DEFINITION HREF="26_glo_m.htm#macro_character"><I>macro character</I></A> does not terminate the <A REL=DEFINITION HREF="26_glo_t.htm#token"><I>token</I></A>'s name; it becomes part of the name as if the <A REL=DEFINITION HREF="26_glo_m.htm#macro_character"><I>macro character</I></A> were really a constituent character. A <A REL=DEFINITION HREF="26_glo_t.htm#terminating"><I>terminating</I></A> <A REL=DEFINITION HREF="26_glo_m.htm#macro_character"><I>macro character</I></A> terminates any <A REL=DEFINITION HREF="26_glo_t.htm#token"><I>token</I></A>, and its associated <A REL=DEFINITION HREF="26_glo_r.htm#reader_macro_function"><I>reader macro function</I></A> is called no matter where the <A REL=DEFINITION HREF="26_glo_c.htm#character"><I>character</I></A> appears. The only <A REL=DEFINITION HREF="26_glo_n.htm#non-terminating"><I>non-terminating</I></A> <A REL=DEFINITION HREF="26_glo_m.htm#macro_character"><I>macro character</I></A> in <A REL=DEFINITION HREF="26_glo_s.htm#standard_syntax"><I>standard syntax</I></A> is <A REL=DEFINITION HREF="26_glo_s.htm#sharpsign"><I>sharpsign</I></A>. <P>
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If a <A REL=DEFINITION HREF="26_glo_c.htm#character"><I>character</I></A> is a <A REL=DEFINITION HREF="26_glo_d.htm#dispatching_macro_character"><I>dispatching macro character</I></A> C1, its <A REL=DEFINITION HREF="26_glo_r.htm#reader_macro_function"><I>reader macro function</I></A> is a <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> supplied by the <A REL=DEFINITION HREF="26_glo_i.htm#implementation"><I>implementation</I></A>. This <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> reads decimal <A REL=DEFINITION HREF="26_glo_d.htm#digit"><I>digit</I></A> <A REL=DEFINITION HREF="26_glo_c.htm#character"><I>characters</I></A> until a non-<A REL=DEFINITION HREF="26_glo_d.htm#digit"><I>digit</I></A> C2 is read. If any <A REL=DEFINITION HREF="26_glo_d.htm#digit"><I>digits</I></A> were read, they are converted into a corresponding <A REL=DEFINITION HREF="26_glo_i.htm#integer"><I>integer</I></A> infix parameter P; otherwise, the infix parameter P is <A REL=DEFINITION HREF="a_nil.htm#nil"><B>nil</B></A>. The terminating non-<A REL=DEFINITION HREF="26_glo_d.htm#digit"><I>digit</I></A> C2 is a <A REL=DEFINITION HREF="26_glo_c.htm#character"><I>character</I></A> (sometimes called a ``sub-character'' to emphasize its subordinate role in the dispatching) that is looked up in the dispatch table associated with the <A REL=DEFINITION HREF="26_glo_d.htm#dispatching_macro_character"><I>dispatching macro character</I></A> C1. The <A REL=DEFINITION HREF="26_glo_r.htm#reader_macro_function"><I>reader macro function</I></A> associated with the sub-character C2 is invoked with three arguments: the <A REL=DEFINITION HREF="26_glo_s.htm#stream"><I>stream</I></A>, the sub-character C2, and the infix parameter P. For more information about dispatch characters, see the <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> <A REL=DEFINITION HREF="f_set__1.htm#set-dispatch-macro-character"><B>set-dispatch-macro-character</B></A>. <P>
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For information about the <A REL=DEFINITION HREF="26_glo_m.htm#macro_character"><I>macro characters</I></A> that are available in <A REL=DEFINITION HREF="26_glo_s.htm#standard_syntax"><I>standard syntax</I></A>, see <A REL=CHILD HREF="02_d.htm">Section 2.4 (Standard Macro Characters)</A>. <P>
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