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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: Issue LOCALLY-TOP-LEVEL Writeup</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="../Issues/iss218_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss219.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss220_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue LOCALLY-TOP-LEVEL Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss219.htm">LOCALLY-TOP-LEVEL</A><P>
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<P>
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<B>References:</B> None<P>
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<P>
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Related issues: <A HREF="iss147.htm">EVAL-WHEN-NON-TOP-LEVEL</A>, <A HREF="iss092.htm">DECLARATION-SCOPE</A><P>
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<P>
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<B>Category:</B> CLARIFICATION / ADDITION<P>
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<P>
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<B>Edit history:</B> Version 1, 9-Mar-89, by Moon<P>
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Version 2, 16-Mar-89, by Moon, fix referenced proposal name<P>
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<P>
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<B>Problem description:<P>
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</B><P>
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It is desirable to be able to wrap <A REL=DEFINITION HREF="../Body/s_locall.htm#locally"><B>LOCALLY</B></A> around one or more<P>
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top-level forms and have them continue to be treated as top-level<P>
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forms. Three examples of how this is useful:<P>
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<P>
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- to put an <A REL=DEFINITION HREF="../Body/d_optimi.htm#optimize"><B>OPTIMIZE</B></A> or <A REL=DEFINITION HREF="../Body/d_inline.htm#inline"><B>INLINE</B></A> declaration into force around<P>
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several related forms.<P>
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<P>
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- to put declarations into force around <A REL=DEFINITION HREF="../Body/m_defcla.htm#defclass"><B>DEFCLASS</B></A>, or any other<P>
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top-level form that lacks a syntax for embedded declarations.<P>
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<P>
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- <A HREF="iss092.htm">DECLARATION-SCOPE:NO-HOISTING</A>, which passed in January,<P>
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removed the ability to use a <A REL=DEFINITION HREF="../Body/s_declar.htm#declare"><B>DECLARE</B></A> at the head of the body of a<P>
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<A REL=DEFINITION HREF="../Body/m_defun.htm#defun"><B>DEFUN</B></A> or <A REL=DEFINITION HREF="../Body/m_defmac.htm#defmacro"><B>DEFMACRO</B></A> to make a declaration that applies to the entire<P>
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form, including the lambda-list. We are supposed to use <A REL=DEFINITION HREF="../Body/s_locall.htm#locally"><B>LOCALLY</B></A><P>
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instead, but forms in the body of <A REL=DEFINITION HREF="../Body/s_locall.htm#locally"><B>LOCALLY</B></A> are not top-level,<P>
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and that changes the semantics of <A REL=DEFINITION HREF="../Body/m_defmac.htm#defmacro"><B>DEFMACRO</B></A>.<P>
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<P>
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Issue <A HREF="iss147.htm">EVAL-WHEN-NON-TOP-LEVEL</A> could not define <A REL=DEFINITION HREF="../Body/s_locall.htm#locally"><B>LOCALLY</B></A> to treat<P>
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its body as top-level forms, because only a special form can do<P>
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that and <A REL=DEFINITION HREF="../Body/s_locall.htm#locally"><B>LOCALLY</B></A> is a macro.<P>
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<P>
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<B>Proposal (LOCALLY-TOP-LEVEL:SPECIAL-FORM):<P>
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</B> <P>
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Change <A REL=DEFINITION HREF="../Body/s_locall.htm#locally"><B>LOCALLY</B></A> from a macro to a special form, and change the<P>
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definition of compiler processing (in <A HREF="iss147.htm">EVAL-WHEN-NON-TOP-LEVEL</A>)<P>
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so that when a <A REL=DEFINITION HREF="../Body/s_locall.htm#locally"><B>LOCALLY</B></A> form appears at top level the forms in<P>
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its body are processed at top level.<P>
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<P>
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<B>Examples:<P>
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</B><P>
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(<A REL=DEFINITION HREF="../Body/s_locall.htm#locally"><B>locally</B></A> (<A REL=DEFINITION HREF="../Body/s_declar.htm#declare"><B>declare</B></A> (<A REL=DEFINITION HREF="../Body/d_optimi.htm#optimize"><B>optimize</B></A> (<A REL=DEFINITION HREF="../Body/d_optimi.htm#safety"><B>safety</B></A> 3) (<A REL=DEFINITION HREF="../Body/d_optimi.htm#space"><B>space</B></A> 3) (<A REL=DEFINITION HREF="../Body/d_optimi.htm#speed"><B>speed</B></A> 0)))<P>
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(<A REL=DEFINITION HREF="../Body/m_defmac.htm#defmacro"><B>defmacro</B></A> frob (<A REL=DEFINITION HREF="../Body/03_dd.htm#AMenvironment"><B>&environment</B></A> e x y <A REL=DEFINITION HREF="../Body/03_da.htm#AMoptional"><B>&optional</B></A> (z (foo x y)))<P>
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(mumble x y z e)))<P>
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<P>
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Without this proposal, this would have to be written<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/m_defmac.htm#defmacro"><B>defmacro</B></A> frob (<A REL=DEFINITION HREF="../Body/03_dd.htm#AMenvironment"><B>&environment</B></A> e x y <A REL=DEFINITION HREF="../Body/03_da.htm#AMoptional"><B>&optional</B></A> (z (<A REL=DEFINITION HREF="../Body/s_locall.htm#locally"><B>locally</B></A><P>
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(<A REL=DEFINITION HREF="../Body/s_declar.htm#declare"><B>declare</B></A><P>
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(<A REL=DEFINITION HREF="../Body/d_optimi.htm#optimize"><B>optimize</B></A><P>
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(<A REL=DEFINITION HREF="../Body/d_optimi.htm#safety"><B>safety</B></A> 3)<P>
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(<A REL=DEFINITION HREF="../Body/d_optimi.htm#space"><B>space</B></A> 3)<P>
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(<A REL=DEFINITION HREF="../Body/d_optimi.htm#speed"><B>speed</B></A> 0)))<P>
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(foo x y))))<P>
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(<A REL=DEFINITION HREF="../Body/s_locall.htm#locally"><B>locally</B></A> (<A REL=DEFINITION HREF="../Body/s_declar.htm#declare"><B>declare</B></A> (<A REL=DEFINITION HREF="../Body/d_optimi.htm#optimize"><B>optimize</B></A> (<A REL=DEFINITION HREF="../Body/d_optimi.htm#safety"><B>safety</B></A> 3) (<A REL=DEFINITION HREF="../Body/d_optimi.htm#space"><B>space</B></A> 3) (<A REL=DEFINITION HREF="../Body/d_optimi.htm#speed"><B>speed</B></A> 0)))<P>
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(mumble x y z e)))<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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Wrapping <A REL=DEFINITION HREF="../Body/s_locall.htm#locally"><B>LOCALLY</B></A> around a form should not change its semantics except<P>
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as specified by the declarations, hence the body of a top-level<P>
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<A REL=DEFINITION HREF="../Body/s_locall.htm#locally"><B>LOCALLY</B></A> should be top-level.<P>
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<P>
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A macro cannot have a top-level body unless it expands into a special<P>
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form that has a top-level body; otherwise the macro invocation and<P>
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the macro expansion would not have identical semantics as top-level<P>
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forms. There is no available special form for <A REL=DEFINITION HREF="../Body/s_locall.htm#locally"><B>LOCALLY</B></A> to <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand"><B>macroexpand</B></A><P>
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into (CLtL doesn't say, but presumably the intent was to expand into<P>
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a <A REL=DEFINITION HREF="../Body/s_let_l.htm#let"><B>LET</B></A> with an empty binding list).<P>
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<P>
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<B>Current practice:<P>
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</B><P>
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The Zetalisp equivalent of <A REL=DEFINITION HREF="../Body/s_locall.htm#locally"><B>LOCALLY</B></A> worked to surround top-level forms,<P>
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because it was a macro that expanded into COMPILER-LET (stashing the<P>
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declarations in a special variable the compiler would look at). This<P>
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is of course the wrong way to do declarations, but it shows that the<P>
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idea was that you could wrap declarations around a bunch of top-level<P>
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forms.<P>
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<P>
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Symbolics Genera 7.4.0 does not implement the proposal (but it does<P>
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not implement <A HREF="iss092.htm">DECLARATION-SCOPE:NO-HOISTING</A> either). I did<P>
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not survey any other implementations.<P>
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<P>
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<B>Cost to Implementors:<P>
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</B><P>
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A half dozen lines of code in the compiler and a smaller amount<P>
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in the interpreter and any program-analyzing programs.<P>
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<P>
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<B>Cost to Users:<P>
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</B><P>
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None.<P>
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<P>
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<B>Cost of non-adoption:<P>
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</B><P>
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See the horrible example above.<P>
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<P>
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<B>Performance impact:<P>
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</B><P>
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None.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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More consistent language.<P>
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<P>
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<B>Esthetics:<P>
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</B><P>
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Improved.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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None.<P>
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<P>
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</PRE>
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