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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 SETF-METHOD-VS-SETF-METHOD Writeup</TITLE>
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<H2>Issue SETF-METHOD-VS-SETF-METHOD Writeup</H2>
<PRE><B>Issue:</B> <A HREF="iss308.htm">SETF-METHOD-VS-SETF-METHOD</A><P>
<B>Forum:</B> Editorial<P>
<B>References:</B> DEFINE-SETF-METHOD (CLtL, p106),<P>
GET-SETF-METHOD (CLtL, p106),<P>
GET-SETF-METHOD-MULTIPLE-VALUE (CLtL, p107),<P>
&quot;setf method&quot; (27.1 Glossary, page 27-27, Draft 8.81)<P>
<B>Category:</B> CHANGE<P>
<B>Edit history:</B> 08-Mar-91, Version 1 by Pitman<P>
15-Mar-91, Version 2 by Pitman (merge much discussion)<P>
08-Apr-91, Version 3 by Pitman<P>
(GET-SETF-EXPANDER =&gt; <A REL=DEFINITION HREF="../Body/f_get_se.htm#get-setf-expansion"><B>GET-SETF-EXPANSION</B></A>, per X3J13)<P>
<B>Status:</B> X3J13 passed option RENAME-OLD-TERMS on 10-3 vote, March 1991.<P>
(Note that option +ADD-ACCESSOR was not voted upon,<P>
and so it is not in effect.)<P>
<P>
<B>Problem Description:<P>
</B><P>
The terms &quot;@bold(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A>) @italic(<A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A>)&quot; and &quot;@italic(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> <A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A>)&quot;<P>
mean two different things. The former means a CLOS <A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A> defined<P>
using <A REL=DEFINITION HREF="../Body/m_defmet.htm#defmethod"><B>DEFMETHOD</B></A>, which among other things might be used by <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A>.<P>
The latter means a macro-like entity defined by <A REL=DEFINITION HREF="../Body/m_defset.htm#defsetf"><B>DEFSETF</B></A> or <P>
DEFINE-SETF-METHOD, data about which can be retrieved by<P>
GET-SETF-METHOD[-MULTIPLE-VALUE].<P>
<P>
Verbal confusion will almost surely result from the similarity of<P>
these two terms.<P>
<P>
Visual confusion will almost surely result if people don't happen to<P>
notice the font, or to realize the important distinction which it <P>
seeks to make.<P>
<P>
<B>Proposal (SETF-METHOD-VS-SETF-METHOD:RENAME-OLD-TERMS):<P>
</B><P>
Rename DEFINE-SETF-METHOD to <A REL=DEFINITION HREF="../Body/m_defi_3.htm#define-setf-expander"><B>DEFINE-SETF-EXPANDER</B></A>.<P>
Remove GET-SETF-METHOD.<P>
Rename GET-SETF-METHOD-MULTIPLE-VALUE to <A REL=DEFINITION HREF="../Body/f_get_se.htm#get-setf-expansion"><B>GET-SETF-EXPANSION</B></A>.<P>
<P>
Rename the glossary term &quot;setf method&quot; to &quot;setf expander&quot;.<P>
That is, a &quot;setf expander&quot; is a function which tells how to<P>
store into a generalized reference.<P>
<P>
Create a glossary term &quot;setf expansion&quot; to refer to the five<P>
magic values returned by a &quot;setf expander&quot;.<P>
<P>
Clarify that the glossary term &quot;setf method&quot; is to be synonymous<P>
with &quot;SETF method&quot; That is, a &quot;setf method&quot; is a CLOS <A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A> <P>
named (<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> &lt;something&gt;).<P>
<P>
Rationale:<P>
<P>
Avoids massive terminological confusion.<P>
<P>
Simplifies the language by promoting GET-SETF-METHOD-MULITPLE-VALUE,<P>
the general purpose routine, into active duty and removing its<P>
simpleton pal GET-SETF-METHOD.<P>
<P>
<B>Proposal (SETF-METHOD-VS-SETF-METHOD:+ADD-ACCESSOR):<P>
</B><P>
Add a new function:<P>
<P>
SETF-EXPANDER-FUNCTION name <A REL=DEFINITION HREF="../Body/03_da.htm#AMoptional"><B>&amp;OPTIONAL</B></A> environment [Function]<P>
<P>
If &lt;name&gt; has a <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> expander function (defined with <A REL=DEFINITION HREF="../Body/m_defset.htm#defsetf"><B>DEFSETF</B></A> or <P>
DEFINE-SETF-METHOD), and the global functional binding of &lt;name&gt; is<P>
not lexically shadowed within the &lt;environment&gt;, <P>
SETF-EXPANDER-FUNCTION returns that function. Otherwise <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> is <P>
returned.<P>
<P>
A <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> expander function is a function which accepts two arguments, <P>
the &quot;place&quot; form and an environment, and returns the five values <P>
representing the <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> expansion for that &quot;place&quot;.<P>
<P>
<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> may be used with SETF-EXPANDER-FUNCTION. The value must either<P>
be a <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> expander function or <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> (which removes any existing <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A><P>
expander function associated with the &lt;name&gt;). [The consequences of<P>
supplying an environment other than <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> when using <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> of <P>
SETF-EXPANDER-FUNCTION are not defined.]<P>
<P>
Rationale:<P>
<P>
This is symmetric with having <A REL=DEFINITION HREF="../Body/f_macro_.htm#macro-function"><B>MACRO-FUNCTION</B></A> and <P>
<A REL=DEFINITION HREF="../Body/f_cmp_ma.htm#compiler-macro-function"><B>COMPILER-MACRO-FUNCTION</B></A>.<P>
<P>
<B>Current Practice:<P>
</B><P>
Presumably no one already does this.<P>
<P>
<B>Cost to Implementors:<P>
</B><P>
RENAME-OLD-TERMS: Small. Trivial renaming of functions. <P>
Old names can be retained as an extension for compatibility<P>
if necessary to ease transition.<P>
<P>
+ADD-ACCESSOR: Probably small. Implementations have to represent<P>
this information somehow already, so this just provides access to<P>
what is already there.<P>
<P>
<B>Cost to Users:<P>
</B><P>
RENAME-OLD-TERMS:<P>
Small. It's clearly incompatible, but hopefully the motivation is strong<P>
enough that users will realize that the change is needed. A simple<P>
find-and-replace operation is likely to fix these, or compatibility names<P>
can be trivially established locally by users for code that can't easily <P>
be changed in implementations that don't already <A REL=DEFINITION HREF="../Body/f_provid.htm#provide"><B>provide</B></A> the compatibility<P>
support for old names.<P>
<P>
+ADD-ACCESSOR: None.<P>
<P>
<B>Cost of Non-Adoption:<P>
</B><P>
Risk of a `fonto' in the spec causing major havoc.<P>
Risk of implementors getting confused.<P>
Risk of confusing new users.<P>
Either some places in the spec could be harder to word unambiguously,<P>
or else glossary terms might have to be chosen which do not match<P>
function names in the language itself.<P>
<P>
<B>Benefits:<P>
</B><P>
Much easier to have casual verbal discussions.<P>
<P>
<B>Aesthetics:<P>
</B><P>
RENAME-OLD-TERMS: Major improvement in aesthetics.<P>
<P>
+ADD-ACCESSOR: Language is slightly more uniform.<P>
<P>
<B>Discussion:<P>
</B><P>
Pitman supports this proposal but is not fussy about the specific<P>
term that's used as long as it's not the status quo (`<A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A>').<P>
<P>
The terms &quot;strategy&quot;, &quot;technique&quot;, &quot;macroexpander&quot;, &quot;macro expander&quot;,<P>
&quot;expander&quot;, and &quot;transformer&quot; were discussed after v1. Eric Benson, <P>
JonL, Sandra, and Pitman all seemed happy with &quot;expander&quot;, so it <P>
v2 was changed to use that.<P>
<P>
There was also support (from Moon and Sandra) for merging <P>
GET-SETF-METHOD and GET-SETF-METHOD-MULTIPLE-VALUE back into one,<P>
so v2 added that as well.<P>
<P>
Sandra asked for SETF-EXPANDER-FUNCTION.<P>
</PRE>
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