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73 lines
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<TITLE>CLHS: Macro DEFINE-SYMBOL-MACRO</TITLE>
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<A NAME="define-symbol-macro"><I>Macro</I> <B>DEFINE-SYMBOL-MACRO</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>define-symbol-macro</B> <I>symbol expansion</I><P> => <I>symbol</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>symbol</I>---a <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A>. <P>
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<I>expansion</I>---a <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A>. <P>
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<P><B>Description:</B><P>
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<P>
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Provides a mechanism for globally affecting the <A REL=DEFINITION HREF="26_glo_m.htm#macro_expansion"><I>macro expansion</I></A> of the indicated <I>symbol</I>. <P>
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Globally establishes an expansion function for the <A REL=DEFINITION HREF="26_glo_s.htm#symbol_macro"><I>symbol macro</I></A> named by <I>symbol</I>. The only guaranteed property of an expansion <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> for a <A REL=DEFINITION HREF="26_glo_s.htm#symbol_macro"><I>symbol macro</I></A> is that when it is applied to the <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A> and the <A REL=DEFINITION HREF="26_glo_e.htm#environment"><I>environment</I></A> it returns the correct expansion. (In particular, it is <A REL=DEFINITION HREF="26_glo_i.htm#implementation-dependent"><I>implementation-dependent</I></A> whether the expansion is conceptually stored in the expansion function, the <A REL=DEFINITION HREF="26_glo_e.htm#environment"><I>environment</I></A>, or both.) <P>
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Each global reference to <I>symbol</I> (i.e., not <I>shadowed</I>[2] by a <A REL=DEFINITION HREF="26_glo_b.htm#binding"><I>binding</I></A> for a <A REL=DEFINITION HREF="26_glo_v.htm#variable"><I>variable</I></A> or <A REL=DEFINITION HREF="26_glo_s.htm#symbol_macro"><I>symbol macro</I></A> named by the same <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A>) is expanded by the normal macro expansion process; see <A REL=CHILD HREF="03_abaa.htm">Section 3.1.2.1.1 (Symbols as Forms)</A>. The expansion of a <A REL=DEFINITION HREF="26_glo_s.htm#symbol_macro"><I>symbol macro</I></A> is subject to further <A REL=DEFINITION HREF="26_glo_m.htm#macro_expansion"><I>macro expansion</I></A> in the same <A REL=DEFINITION HREF="26_glo_l.htm#lexical_environment"><I>lexical environment</I></A> as the <A REL=DEFINITION HREF="26_glo_s.htm#symbol_macro"><I>symbol macro</I></A> reference, exactly analogous to normal <A REL=DEFINITION HREF="26_glo_m.htm#macro"><I>macros</I></A>. <P>
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The consequences are unspecified if a <A REL=DEFINITION HREF="d_specia.htm#special"><B>special</B></A> declaration is made for <I>symbol</I> while in the scope of this definition (i.e., when it is not <I>shadowed</I>[2] by a <A REL=DEFINITION HREF="26_glo_b.htm#binding"><I>binding</I></A> for a <A REL=DEFINITION HREF="26_glo_v.htm#variable"><I>variable</I></A> or <A REL=DEFINITION HREF="26_glo_s.htm#symbol_macro"><I>symbol macro</I></A> named by the same <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A>). <P>
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Any use of <A REL=DEFINITION HREF="s_setq.htm#setq"><B>setq</B></A> to set the value of the <I>symbol</I> while in the scope of this definition is treated as if it were a <A REL=DEFINITION HREF="m_setf_.htm#setf"><B>setf</B></A>. <A REL=DEFINITION HREF="m_psetq.htm#psetq"><B>psetq</B></A> of <I>symbol</I> is treated as if it were a <A REL=DEFINITION HREF="m_setf_.htm#psetf"><B>psetf</B></A>, and <A REL=DEFINITION HREF="m_mult_2.htm#multiple-value-setq"><B>multiple-value-setq</B></A> is treated as if it were a <A REL=DEFINITION HREF="m_setf_.htm#setf"><B>setf</B></A> of <A REL=DEFINITION HREF="f_values.htm#values"><B>values</B></A>. <P>
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A <A REL=DEFINITION HREF="26_glo_b.htm#binding"><I>binding</I></A> for a <A REL=DEFINITION HREF="26_glo_s.htm#symbol_macro"><I>symbol macro</I></A> can be <I>shadowed</I>[2] by <A REL=DEFINITION HREF="s_let_l.htm#let"><B>let</B></A> or <A REL=DEFINITION HREF="s_symbol.htm#symbol-macrolet"><B>symbol-macrolet</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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(defvar *things* (list 'alpha 'beta 'gamma)) => *THINGS*
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(define-symbol-macro thing1 (first *things*)) => THING1
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(define-symbol-macro thing2 (second *things*)) => THING2
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(define-symbol-macro thing3 (third *things*)) => THING3
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thing1 => ALPHA
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(setq thing1 'ONE) => ONE
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*things* => (ONE BETA GAMMA)
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(multiple-value-setq (thing2 thing3) (values 'two 'three)) => TWO
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thing3 => THREE
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*things* => (ONE TWO THREE)
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(list thing2 (let ((thing2 2)) thing2)) => (TWO 2)
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</PRE>
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</TT> <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><P>
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<P>
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If <I>symbol</I> is already defined as a <A REL=DEFINITION HREF="26_glo_g.htm#global_variable"><I>global variable</I></A>, an error of <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>type</I></A> <A REL=DEFINITION HREF="e_progra.htm#program-error"><B>program-error</B></A> is signaled. <P>
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<P><B>See Also:</B><P>
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<P>
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<A REL=DEFINITION HREF="s_symbol.htm#symbol-macrolet"><B>symbol-macrolet</B></A>, <A REL=DEFINITION HREF="f_mexp_.htm#macroexpand"><B>macroexpand</B></A> <P>
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<P><B>Notes:</B> None.
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