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<TITLE>CLHS: Issue PUSHNEW-STORE-REQUIRED 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/iss279_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss281.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss282_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue PUSHNEW-STORE-REQUIRED Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss281.htm">PUSHNEW-STORE-REQUIRED</A><P>
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<B>References:</B> <A REL=DEFINITION HREF="../Body/m_pshnew.htm#pushnew"><B>PUSHNEW</B></A><P>
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Related issues: <A HREF="iss279_m.htm">PUSH-EVALUATION-ORDER</A><P>
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<A HREF="iss306.htm">SETF-GET-DEFAULT</A><P>
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<B>Category:</B> CLARIFICATION<P>
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<B>Edit history:</B> v1, 14 Feb 1991, Sandra Loosemore<P>
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v2, 26 Mar 1991, Sandra Loosemore (amendment from meeting)<P>
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<B>Status:</B> v2 (proposal UNSPECIFIED) accepted by X3J13, Mar 1991<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 description of the <A REL=DEFINITION HREF="../Body/m_pshnew.htm#pushnew"><B>PUSHNEW</B></A> macro says contradictory things about<P>
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whether the storing form for its place subform must or must not be <P>
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evaluated in the situation where the <item> is already a member of<P>
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the list.<P>
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<P>
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In CLtL, <A REL=DEFINITION HREF="../Body/m_pshnew.htm#pushnew"><B>PUSHNEW</B></A> is first described as:<P>
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<P>
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If the <item> is not already a member of the list (...), then the <P>
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<item> is consed onto the front of the list, and the augmented list is<P>
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stored back into <place> and returned; otherwise the unaugmented list<P>
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is returned.<P>
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<P>
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This passage implies that <A REL=DEFINITION HREF="../Body/m_pshnew.htm#pushnew"><B>PUSHNEW</B></A> is effectively (modulo order of<P>
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evaluation issues and keyword arguments): <P>
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<P>
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(<A REL=DEFINITION HREF="../Body/s_if.htm#if"><B>if</B></A> (<A REL=DEFINITION HREF="../Body/a_not.htm#not"><B>not</B></A> (<A REL=DEFINITION HREF="../Body/a_member.htm#member"><B>member</B></A> <item> <place>))<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> <place> (<A REL=DEFINITION HREF="../Body/a_cons.htm#cons"><B>cons</B></A> <item> <place>))<P>
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<place>)<P>
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<P>
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which executes the storing form for <place> only if the member test<P>
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fails.<P>
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<P>
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However, later on it's said that <A REL=DEFINITION HREF="../Body/m_pshnew.htm#pushnew"><B>PUSHNEW</B></A> is effectively (again modulo<P>
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order of evaluation issues and keyword arguments):<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>setf</B></A> <place> (<A REL=DEFINITION HREF="../Body/f_adjoin.htm#adjoin"><B>adjoin</B></A> <item> <place>))<P>
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<P>
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which executes the storing form for <place> unconditionally.<P>
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<P>
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So which of these two models for its expansion is correct?<P>
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<P>
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<P>
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<B>Proposal (PUSHNEW-STORE-REQUIRED:UNSPECIFIED):<P>
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</B><P>
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Clarify that it is not specified whether <A REL=DEFINITION HREF="../Body/m_pshnew.htm#pushnew"><B>PUSHNEW</B></A> executes the storing<P>
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form for its <place> subform in the situation where the <item> is<P>
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already a member of the list.<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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The store operation might be expensive and there's no point in doing<P>
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it if the value being stored hasn't changed. On the other hand, in<P>
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some implementations <A REL=DEFINITION HREF="../Body/m_pshnew.htm#pushnew"><B>PUSHNEW</B></A> always does the store.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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In both Lucid and Allegro, <A REL=DEFINITION HREF="../Body/m_pshnew.htm#pushnew"><B>PUSHNEW</B></A> always does the store.<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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None. No implementation is required to change.<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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Probably 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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The description of <A REL=DEFINITION HREF="../Body/m_pshnew.htm#pushnew"><B>PUSHNEW</B></A> is confusing.<P>
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<P>
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<B>Performance impact:<P>
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</B><P>
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Not doing the store when it's not necessary can speed things up.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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The cost of non-adoption is avoided.<P>
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<P>
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<B>Esthetics:<P>
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</B><P>
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<A REL=DEFINITION HREF="../Body/m_pshnew.htm#pushnew"><B>PUSHNEW</B></A> would be less confusing if we could just agree on one or <P>
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the other of the two models as being correct and remove all mention<P>
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of the other from its description 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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<B>Discussion:<P>
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</B><P>
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There is a potential for generalization to other read-modify-write <P>
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macros. See the discussion section of issue <A HREF="iss306.htm">SETF-GET-DEFAULT</A>.<P>
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<P>
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</PRE>
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