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<TITLE>CLHS: Issue SETF-OF-VALUES 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/iss310_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss311.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss312_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue SETF-OF-VALUES Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss311.htm">SETF-OF-VALUES</A><P>
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<B>References:</B> <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A>, GET-SETF-METHOD-MULTIPLE-VALUE<P>
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Related issues: <A HREF="iss309.htm">SETF-MULTIPLE-STORE-VARIABLES</A><P>
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<A HREF="iss214.htm">LISP-SYMBOL-REDEFINITION</A><P>
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<A HREF="iss252.htm">PACKAGE-CLUTTER</A><P>
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<A HREF="iss245.htm">MULTIPLE-VALUE-SETQ-ORDER</A><P>
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<B>Category:</B> ADDITION, CHANGE<P>
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<B>Edit history:</B> v1, 12 Feb 1991, Sandra Loosemore<P>
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<P>
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<B>Problem description:<P>
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</B><P>
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The writeup for issue <A HREF="iss309.htm">SETF-MULTIPLE-STORE-VARIABLES</A> indicates that<P>
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adding support for multiple store variables makes it easy to write<P>
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a <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> for the <A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>VALUES</B></A> function, yielding a powerful<P>
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MULTIPLE-VALUE-SETF-like form. The proposal didn't actually<P>
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define <A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>VALUES</B></A> as a <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> place on the rationale that this could be<P>
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easily done by users in a portable fashion; however, issue<P>
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issue <A HREF="iss214.htm">LISP-SYMBOL-REDEFINITION</A> prohibits users from defining a <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> <P>
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<A REL=DEFINITION HREF="../Body/t_method.htm#method"><B>method</B></A> for <A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>VALUES</B></A> or any other symbol that is exported from the<P>
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COMMON-LISP package. (Issue <A HREF="iss252.htm">PACKAGE-CLUTTER</A> does not prohibit<P>
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implementations from defining <A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>VALUES</B></A> as a <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> place as an extension,<P>
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though.)<P>
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<P>
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There are two proposals, ADD and REMOVE-PROHIBITION. The two proposals<P>
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are not mutually exclusive.<P>
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<P>
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<P>
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<B>Proposal (SETF-OF-VALUES:ADD):<P>
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</B> <P>
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Define <A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>VALUES</B></A> as a <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> place in the <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A>.<P>
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<P>
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For a form such as<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> (<A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>values</B></A> <place1> .... <placen>) <value-producing-form>)<P>
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<P>
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the <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> methods for each of the nested <placei> are obtained as if by<P>
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GET-SETF-METHOD-MULTIPLE-VALUE. The order of evaluation is as follows:<P>
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<P>
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(1) subforms of the nested <placei> are evaluated in left-to-right order.<P>
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(2) The <value-producing-form> is evaluated, and the first store variable<P>
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from each <placei> bound to the values as by <A REL=DEFINITION HREF="../Body/m_multip.htm#multiple-value-bind"><B>MULTIPLE-VALUE-BIND</B></A>.<P>
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If the <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> for a nested <placei> involves more than one store<P>
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variable, then the additional store variables are bound to <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>.<P>
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(3) Finally the storing forms for the nested <placei> are evaluated in<P>
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left-to-right order.<P>
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<P>
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Note that (as required by CLtL), the storing form for <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> of <A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>VALUES</B></A><P>
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returns the values of the store variables as its values. (This might<P>
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be more or fewer values than what the <value-producing-form> returns.)<P>
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<P>
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Rationale for proposal ADD:<P>
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<P>
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<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> of <A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>VALUES</B></A> is a powerful feature. Even if it were made possible to<P>
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define it portably, it is useful enough to warrant being included as<P>
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a <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A>, built-in part of the language. Standardizing this feature<P>
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will prevent problems with a proliferation of similar features that all<P>
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have slightly different names or semantics.<P>
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<P>
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<P>
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<B>Proposal (SETF-OF-VALUES:REMOVE-PROHIBITION):<P>
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</B><P>
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Permit users to define <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> methods and/or <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> functions for places <P>
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whose names are external symbols in the COMMON-LISP package, provided <P>
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that the <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> does not already define the symbol as a name of a <P>
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<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> place.<P>
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<P>
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Rationale for proposal REMOVE-PROHIBITION:<P>
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<P>
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There are other potential <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> places besides <A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>VALUES</B></A> that a user might<P>
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conceivably wish to define <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> methods for, that are named by external<P>
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symbols in the COMMON-LISP package. The writeup for issue <P>
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SETF-MULTIPLE-STORE-VALUES suggested <A REL=DEFINITION HREF="../Body/a_cons.htm#cons"><B>CONS</B></A> and <A REL=DEFINITION HREF="../Body/a_vector.htm#vector"><B>VECTOR</B></A>, for example.<P>
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<P>
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<P>
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<B>Examples:<P>
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</B><P>
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<B>Current Practice:<P>
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</B><P>
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Chestnut's Lisp-to-C translator implements proposal ADD.<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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For proposal ADD, the <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> for <A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>VALUES</B></A> will need to be written.<P>
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<P>
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For proposal REMOVE-PROHIBITION, implementors will have to be careful<P>
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about defining additional <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> places on symbols exported from the<P>
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COMMON-LISP package, since those definitions might be overridden by<P>
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user-supplied definitions.<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. This is an upward-compatible extension.<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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Users are prohibited from doing something useful for no good reason.<P>
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<P>
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<B>Performance impact:<P>
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</B><P>
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Probably none.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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The costs of non-adoption are avoided.<P>
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<P>
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<B>Esthetics:<P>
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</B><P>
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It looks OK to me.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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CLtL-II notes that a proposal to add <A REL=DEFINITION HREF="../Body/a_values.htm#values"><B>VALUES</B></A> as a <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> place was <P>
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submitted in September 89, but I can find no record of it in the<P>
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minutes from either the November 89 or June 90 meetings.<P>
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<P>
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I have no idea what the semantics of a <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> on <A REL=DEFINITION HREF="../Body/a_cons.htm#cons"><B>CONS</B></A> or <A REL=DEFINITION HREF="../Body/a_vector.htm#vector"><B>VECTOR</B></A><P>
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might be.<P>
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<P>
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-------<P>
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