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78 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: Special Operator SYMBOL-MACROLET</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="m_defi_1.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="c_evalua.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="v_mexp_h.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<A NAME="symbol-macrolet"><I>Special Operator</I> <B>SYMBOL-MACROLET</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>symbol-macrolet</B> <I>((symbol expansion)<B>*</B>) <I>declaration</I><B>*</B> <I>form</I><B>*</B></I><P> => <I><I>result</I><B>*</B></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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<I>declaration</I>---a <A REL=DEFINITION HREF="s_declar.htm#declare"><B>declare</B></A> <A REL=DEFINITION HREF="26_glo_e.htm#expression"><I>expression</I></A>; not evaluated. <P>
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<I>forms</I>---an <A REL=DEFINITION HREF="26_glo_i.htm#implicit_progn"><I>implicit progn</I></A>. <P>
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<I>results</I>---the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>values</I></A> returned by the <I>forms</I>. <P>
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<P><B>Description:</B><P>
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<P>
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<A REL=DEFINITION HREF="#symbol-macrolet"><B>symbol-macrolet</B></A> provides a mechanism for affecting the <A REL=DEFINITION HREF="26_glo_m.htm#macro_expansion"><I>macro expansion</I></A> environment for <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbols</I></A>. <P>
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<A REL=DEFINITION HREF="#symbol-macrolet"><B>symbol-macrolet</B></A> lexically establishes expansion functions for each of the <A REL=DEFINITION HREF="26_glo_s.htm#symbol_macro"><I>symbol macros</I></A> named by <I>symbols</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 reference to <I>symbol</I> as a variable within the lexical <A REL=DEFINITION HREF="26_glo_s.htm#scope"><I>scope</I></A> of <A REL=DEFINITION HREF="#symbol-macrolet"><B>symbol-macrolet</B></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 symbol macro is subject to further macro expansion in the same lexical environment as the symbol macro invocation, exactly analogous to normal <A REL=DEFINITION HREF="26_glo_m.htm#macro"><I>macros</I></A>. <P>
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Exactly the same <I>declarations</I> are allowed as for <A REL=DEFINITION HREF="s_let_l.htm#let"><B>let</B></A> with one exception: <A REL=DEFINITION HREF="#symbol-macrolet"><B>symbol-macrolet</B></A> signals an error if a <A REL=DEFINITION HREF="d_specia.htm#special"><B>special</B></A> declaration names one of the <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbols</I></A> being defined by <A REL=DEFINITION HREF="#symbol-macrolet"><B>symbol-macrolet</B></A>. <P>
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When the <I>forms</I> of the <A REL=DEFINITION HREF="#symbol-macrolet"><B>symbol-macrolet</B></A> form are expanded, any use of <A REL=DEFINITION HREF="s_setq.htm#setq"><B>setq</B></A> to set the value of one of the specified variables 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 a <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A> defined as a symbol macro 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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The use of <A REL=DEFINITION HREF="#symbol-macrolet"><B>symbol-macrolet</B></A> can be shadowed by <A REL=DEFINITION HREF="s_let_l.htm#let"><B>let</B></A>. In other words, <A REL=DEFINITION HREF="#symbol-macrolet"><B>symbol-macrolet</B></A> only substitutes for occurrences of <I>symbol</I> that would be in the <A REL=DEFINITION HREF="26_glo_s.htm#scope"><I>scope</I></A> of a lexical binding of <I>symbol</I> surrounding the <I>forms</I>. <P>
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<P><B>Examples:</B><P>
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<P>
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<PRE>
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;;; The following is equivalent to
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;;; (list 'foo (let ((x 'bar)) x)),
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;;; not
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;;; (list 'foo (let (('foo 'bar)) 'foo))
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(symbol-macrolet ((x 'foo))
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(list x (let ((x 'bar)) x)))
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=> (foo bar)
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NOT=> (foo foo)
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(symbol-macrolet ((x '(foo x)))
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(list x))
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=> ((FOO X))
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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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If an attempt is made to bind a <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A> that is 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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If <I>declaration</I> contains a <A REL=DEFINITION HREF="d_specia.htm#special"><B>special</B></A> declaration that names one of the <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbols</I></A> being bound by <A REL=DEFINITION HREF="#symbol-macrolet"><B>symbol-macrolet</B></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="m_w_slts.htm#with-slots"><B>with-slots</B></A>, <A REL=DEFINITION HREF="f_mexp_.htm#macroexpand"><B>macroexpand</B></A> <P>
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<P><B>Notes:</B><P>
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<P>
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The special form <A REL=DEFINITION HREF="#symbol-macrolet"><B>symbol-macrolet</B></A> is the basic mechanism that is used to implement <A REL=DEFINITION HREF="m_w_slts.htm#with-slots"><B>with-slots</B></A>. <P>
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If a <A REL=DEFINITION HREF="#symbol-macrolet"><B>symbol-macrolet</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A> is a <A REL=DEFINITION HREF="26_glo_t.htm#top_level_form"><I>top level form</I></A>, the <I>forms</I> are also processed as <A REL=DEFINITION HREF="26_glo_t.htm#top_level_form"><I>top level forms</I></A>. See <A REL=CHILD HREF="03_bc.htm">Section 3.2.3 (File Compilation)</A>. <P>
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<P>
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<P><HR>The following <A REL=META HREF="../Front/X3J13Iss.htm">X3J13 cleanup issues</A>, <I>not part of the specification</I>, apply to this section:<P><UL><LI> <A REL=CHILD HREF="../Issues/iss214.htm">LISP-SYMBOL-REDEFINITION:MAR89-X3J13</A><LI> <A REL=CHILD HREF="../Issues/iss340.htm">SYMBOL-MACROS-AND-PROCLAIMED-SPECIALS:SIGNALS-AN-ERROR</A><LI> <A REL=CHILD HREF="../Issues/iss337.htm">SYMBOL-MACROLET-DECLARE:ALLOW</A><LI> <A REL=CHILD HREF="../Issues/iss338.htm">SYMBOL-MACROLET-SEMANTICS:SPECIAL-FORM</A><LI> <A REL=CHILD HREF="../Issues/iss097.htm">DECLS-AND-DOC</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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