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<TITLE>CLHS: Issue ASSOC-RASSOC-IF-KEY 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/iss016_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss017_m.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss019_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue ASSOC-RASSOC-IF-KEY Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss017_m.htm">ASSOC-RASSOC-IF-KEY</A><P>
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<B>References:</B> <A REL=DEFINITION HREF="../Body/f_assocc.htm#assoc-if"><B>ASSOC-IF</B></A> (p280), <A REL=DEFINITION HREF="../Body/f_assocc.htm#assoc-if-not"><B>ASSOC-IF-NOT</B></A> (p280), <A REL=DEFINITION HREF="../Body/f_rassoc.htm#rassoc-if"><B>RASSOC-IF</B></A> (p281),<P>
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<A REL=DEFINITION HREF="../Body/f_rassoc.htm#rassoc-if-not"><B>RASSOC-IF-NOT</B></A> (p281)<P>
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<B>Category:</B> ENHANCEMENT<P>
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<B>Edit history:</B> 22-Apr-87, Version 1 by Pitman<P>
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20-Nov-87, Versions 2,3 by Masinter<P>
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23-Nov-87, Version 4 by Masinter<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 descriptions of <A REL=DEFINITION HREF="../Body/f_assocc.htm#assoc-if"><B>ASSOC-IF</B></A>, <A REL=DEFINITION HREF="../Body/f_assocc.htm#assoc-if-not"><B>ASSOC-IF-NOT</B></A>, <A REL=DEFINITION HREF="../Body/f_rassoc.htm#rassoc-if"><B>RASSOC-IF</B></A>, and <A REL=DEFINITION HREF="../Body/f_rassoc.htm#rassoc-if-not"><B>RASSOC-IF-NOT</B></A> do not<P>
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mention a :KEY option, although <A REL=DEFINITION HREF="../Body/f_assocc.htm#assoc"><B>ASSOC</B></A> and <A REL=DEFINITION HREF="../Body/f_rassoc.htm#rassoc"><B>RASSOC</B></A> have one. <P>
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<P>
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This is often reported as an inconsistency in Common Lisp.<P>
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<P>
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<B>Proposal (ASSOC-RASSOC-IF-KEY:YES):<P>
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</B><P>
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Allow a :KEY keyword for <A REL=DEFINITION HREF="../Body/f_assocc.htm#assoc-if"><B>ASSOC-IF</B></A>, <A REL=DEFINITION HREF="../Body/f_assocc.htm#assoc-if-not"><B>ASSOC-IF-NOT</B></A>, <A REL=DEFINITION HREF="../Body/f_rassoc.htm#rassoc-if"><B>RASSOC-IF</B></A>, and <A REL=DEFINITION HREF="../Body/f_rassoc.htm#rassoc-if-not"><B>RASSOC-IF-NOT</B></A>.<P>
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If not supplied, it should default to #'<A REL=DEFINITION HREF="../Body/f_identi.htm#identity"><B>IDENTITY</B></A> as do the :KEY keywords for<P>
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other -IF and -IF-NOT functions. The function, as with the :KEY argument for<P>
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<A REL=DEFINITION HREF="../Body/f_assocc.htm#assoc"><B>ASSOC</B></A> and <A REL=DEFINITION HREF="../Body/f_rassoc.htm#rassoc"><B>RASSOC</B></A>, is applied to the <A REL=DEFINITION HREF="../Body/f_car_c.htm#car"><B>CAR</B></A> of the pair in the association list for<P>
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<A REL=DEFINITION HREF="../Body/f_assocc.htm#assoc-if"><B>ASSOC-IF</B></A> and <A REL=DEFINITION HREF="../Body/f_assocc.htm#assoc-if-not"><B>ASSOC-IF-NOT</B></A> and the <A REL=DEFINITION HREF="../Body/f_car_c.htm#cdr"><B>CDR</B></A> of the pair for <A REL=DEFINITION HREF="../Body/f_rassoc.htm#rassoc-if"><B>RASSOC-IF</B></A> and<P>
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<A REL=DEFINITION HREF="../Body/f_rassoc.htm#rassoc-if-not"><B>RASSOC-IF-NOT</B></A>.<P>
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<P>
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<B>Documentation impact:<P>
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</B><P>
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A better description of the intent might be to say that the <A REL=DEFINITION HREF="../Body/f_car_c.htm#car"><B>car</B></A> /contains/ the<P>
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key of the association, and by default the <A REL=DEFINITION HREF="../Body/f_car_c.htm#car"><B>car</B></A> /is/ the key of the association.<P>
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<P>
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<B>Example:<P>
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</B><P>
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(<A REL=DEFINITION HREF="../Body/f_assocc.htm#assoc-if"><B>assoc-if</B></A> #'<A REL=DEFINITION HREF="../Body/f_zerop.htm#zerop"><B>zerop</B></A> pathnames :key #'<A REL=DEFINITION HREF="../Body/f_pn_hos.htm#pathname-version"><B>pathname-version</B></A>)<P>
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<P>
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could be used to search a list indexed by pathnames finding one with zero<P>
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version. <P>
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<P>
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<B>Rationale:<P>
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</B><P>
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This is an inconsistency in the language that is simple to fix.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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Symbolics implements :KEY for the -IF and -IF-NOT <A REL=DEFINITION HREF="../Body/f_assocc.htm#assoc"><B>assoc</B></A> functions. Others follow<P>
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the book and allow :KEY only for <A REL=DEFINITION HREF="../Body/f_assocc.htm#assoc"><B>ASSOC</B></A>.<P>
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<P>
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<B>Cost to Common Lisp implementors:<P>
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</B><P>
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A small amount of additional code is necessary to support this in<P>
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implementations not already offering it as an extension.<P>
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<P>
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<B>Cost to Common Lisp users:<P>
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</B><P>
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The change is essentially upward compatible with user code.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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This would make the set of -IF and -IF-NOT functions be more regular in their<P>
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calling conventions.<P>
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<P>
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<B>Aesthetics:<P>
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</B><P>
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All the other -IF and -IF-NOT variations of list operations omit the :TEST and<P>
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:TEST-NOT keywords, but allow :KEY. For example, consider the family of <A REL=DEFINITION HREF="../Body/a_member.htm#member"><B>MEMBER</B></A>,<P>
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<A REL=DEFINITION HREF="../Body/f_mem_m.htm#member-if"><B>MEMBER-IF</B></A>, and <A REL=DEFINITION HREF="../Body/f_mem_m.htm#member-if-not"><B>MEMBER-IF-NOT</B></A>. Although this introduces additional mechanism, it<P>
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does so in a way that probably makes it easier to think about which functions do<P>
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what, so it would likely be seen as a simplification.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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The omission of :KEY in this situation in CLtL was probably an oversight.<P>
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<P>
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The cleanup committee supports this proposal.<P>
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<P>
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</PRE>
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