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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 MACRO-FUNCTION-ENVIRONMENT Writeup</TITLE>
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<H2>Issue MACRO-FUNCTION-ENVIRONMENT Writeup</H2>
<PRE><B>Issue:</B> <A HREF="iss230_m.htm">MACRO-FUNCTION-ENVIRONMENT</A><P>
<P>
<B>References:</B> <A REL=DEFINITION HREF="../Body/f_macro_.htm#macro-function"><B>MACRO-FUNCTION</B></A>, p. 144<P>
<A REL=DEFINITION HREF="../Body/s_flet_.htm#macrolet"><B>MACROLET</B></A>, pp. 113-4<P>
<A REL=DEFINITION HREF="../Body/03_dd.htm#AMenvironment"><B>&amp;ENVIRONMENT</B></A>, pp. 145-6<P>
<A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand"><B>MACROEXPAND</B></A> and <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand-1"><B>MACROEXPAND-1</B></A>, pp. 151-2<P>
<P>
<B>Category:</B> ADDITION<P>
<P>
<B>Edit history:</B> Version 1, Pavel, March 21, 1988<P>
Version 2, Masinter, 8-Jun-88, (as per cleanup discussion)<P>
<P>
<B>Problem description:<P>
</B><P>
The <A REL=DEFINITION HREF="../Body/03_dd.htm#AMenvironment"><B>&amp;ENVIRONMENT</B></A> argument to a macro-expansion function may only be used as<P>
the<P>
second argument to the functions <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand"><B>MACROEXPAND</B></A> and <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand-1"><B>MACROEXPAND-1</B></A>. It is<P>
sometimes<P>
more convenient, however, to be able to work directly with the more<P>
primitive<P>
function <A REL=DEFINITION HREF="../Body/f_macro_.htm#macro-function"><B>MACRO-FUNCTION</B></A>, on which <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand"><B>MACROEXPAND</B></A> and <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand-1"><B>MACROEXPAND-1</B></A> are<P>
presumably<P>
based. However, since <A REL=DEFINITION HREF="../Body/f_macro_.htm#macro-function"><B>MACRO-FUNCTION</B></A> does not take an environment<P>
argument, it<P>
cannot be used in situations in which that environment must be taken into<P>
account.<P>
<P>
<B>Proposal (MACRO-FUNCTION-ENVIRONMENT:YES):<P>
</B><P>
Add an optional second argument to <A REL=DEFINITION HREF="../Body/f_macro_.htm#macro-function"><B>MACRO-FUNCTION</B></A>, that argument being an<P>
environment that was passed as the <A REL=DEFINITION HREF="../Body/03_dd.htm#AMenvironment"><B>&amp;ENVIRONMENT</B></A> argument to some macro<P>
expansion<P>
function. If the argument is not given, or the argument is <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>, the null<P>
environment is used. <A REL=DEFINITION HREF="../Body/f_macro_.htm#macro-function"><B>MACRO-FUNCTION</B></A> will now consider macro definitions<P>
from<P>
that environment in preference to ones in the global environment. It is an<P>
error<P>
to supply the environment argument in a use of <A REL=DEFINITION HREF="../Body/f_macro_.htm#macro-function"><B>MACRO-FUNCTION</B></A> as a <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A><P>
location<P>
specifier.<P>
<P>
<B>Examples:<P>
</B><P>
(<A REL=DEFINITION HREF="../Body/s_flet_.htm#macrolet"><B>macrolet</B></A> ((foo (<A REL=DEFINITION HREF="../Body/03_dd.htm#AMenvironment"><B>&amp;environment</B></A> env)<P>
(<A REL=DEFINITION HREF="../Body/s_if.htm#if"><B>if</B></A> (<A REL=DEFINITION HREF="../Body/f_macro_.htm#macro-function"><B>macro-function</B></A> 'bar env)<P>
''yes<P>
''no)))<P>
(<A REL=DEFINITION HREF="../Body/a_list.htm#list"><B>list</B></A> (foo)<P>
(<A REL=DEFINITION HREF="../Body/s_flet_.htm#macrolet"><B>macrolet</B></A> ((bar () :beep))<P>
(foo))))<P>
<P>
=&gt; (no yes)<P>
<P>
(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/f_macro_.htm#macro-function"><B>macro-function</B></A> 'bar env) ...) is an error.<P>
<P>
<B>Rationale:<P>
</B><P>
Intuitively, the more primitive operation in macro expansion is<P>
<A REL=DEFINITION HREF="../Body/f_macro_.htm#macro-function"><B>MACRO-FUNCTION</B></A>,<P>
not <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand"><B>MACROEXPAND</B></A> or <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand-1"><B>MACROEXPAND-1</B></A>, yet the environment argument can only be<P>
supplied to the latter functions and not to the former one. By changing<P>
this<P>
state of affairs, the model of macro expansion becomes somewhat simpler.<P>
Also,<P>
more flexible use of the facility is enabled.<P>
<P>
<B>Current practice:<P>
</B><P>
Xerox Common Lisp already implements this proposal. Symbolics Common Lisp,<P>
and Kyoto Common Lisp do not. Lucid Common Lisp did not, but version 3.0<P>
does.<P>
<P>
<B>Cost to Implementors:<P>
</B><P>
This is presumably a simple change to make, a small matter of moving the<P>
environment-searching code from <A REL=DEFINITION HREF="../Body/f_mexp_.htm#macroexpand-1"><B>MACROEXPAND-1</B></A> to <A REL=DEFINITION HREF="../Body/f_macro_.htm#macro-function"><B>MACRO-FUNCTION</B></A>.<P>
<P>
<B>Cost to Users:<P>
</B><P>
The change is upward-compatible and so poses no cost to users.<P>
<P>
<B>Cost of non-adoption:<P>
</B><P>
One more (small) semantic wart on the language.<P>
<P>
<B>Benefits:<P>
</B><P>
The function that users think of as being more primitive really is.<P>
<P>
<B>Aesthetics:<P>
</B><P>
This slightly cleans up the language.<P>
<P>
<B>Discussion:<P>
</B><P>
This issue was discussed starting in January 1987, but got tangled in<P>
a web of other related proposals. In its current form, the only objections<P>
have been that some other proposal or committee might otherwise change<P>
macro handling, or that this proposal doesn't fix enough of the problems.<P>
<P>
<P>
<P>
</PRE>
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