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<TITLE>CLHS: Issue REDUCE-ARGUMENT-EXTRACTION 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/iss291_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss292.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss293_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue REDUCE-ARGUMENT-EXTRACTION Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss292.htm">REDUCE-ARGUMENT-EXTRACTION</A><P>
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<B>References:</B> <A REL=DEFINITION HREF="../Body/f_reduce.htm#reduce"><B>REDUCE</B></A> (pp. 251-252), :KEY arguments (p. 246), <P>
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the astronaut <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> (pp. 312-313)<P>
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<B>Category:</B> ADDITION<P>
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<B>Edit history:</B> Version 1 by Pierson 5-Dec-87<P>
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Version 2 by Pierson 30-Dec-87<P>
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Version 3 by Masinter 13-Feb-88<P>
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<P>
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<B>Problem description:<P>
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</B><P>
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<A REL=DEFINITION HREF="../Body/f_reduce.htm#reduce"><B>REDUCE</B></A> is the only one of the Common Lisp functions that modify or<P>
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search lists and sequences which does not accept a :KEY argument.<P>
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This complicates many uses of <A REL=DEFINITION HREF="../Body/f_reduce.htm#reduce"><B>REDUCE</B></A>.<P>
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<P>
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<B>Proposal (REDUCE-ARGUMENT-EXTRACTION:KEY):<P>
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</B><P>
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Change the definition of <A REL=DEFINITION HREF="../Body/f_reduce.htm#reduce"><B>REDUCE</B></A> to take a :KEY keyword described as<P>
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follows: <P>
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<P>
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If a :KEY argument is supplied, its value must be a function of one<P>
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argument which will be used to extract the values to reduce. The :KEY<P>
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function will be applied exactly once to each element of the sequence<P>
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in the order implied by the reduction order but not to the value of<P>
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the :INITIAL-VALUE argument, if any.<P>
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<P>
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<B>Example:<P>
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</B><P>
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Using <A REL=DEFINITION HREF="../Body/f_reduce.htm#reduce"><B>REDUCE</B></A> to obtain the total of the ages of the possibly empty<P>
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sequence of astronauts ASTROS, would currently require:<P>
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<P>
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(<A REL=DEFINITION HREF="../Body/f_reduce.htm#reduce"><B>REDUCE</B></A> #'+ (<A REL=DEFINITION HREF="../Body/f_map.htm#map"><B>MAP</B></A> '<A REL=DEFINITION HREF="../Body/a_list.htm#list"><B>LIST</B></A> #'PERSON-AGE ASTROS))<P>
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<P>
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If this proposal is adopted, the same result could be obtained without<P>
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creating a new list by: <P>
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<P>
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(<A REL=DEFINITION HREF="../Body/f_reduce.htm#reduce"><B>REDUCE</B></A> #'+ ASTROS :KEY #'PERSON-AGE)<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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This proposal makes many common situations where <A REL=DEFINITION HREF="../Body/f_reduce.htm#reduce"><B>REDUCE</B></A> would be useful<P>
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much less cumbersome.<P>
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<P>
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<B>Current practice:<P>
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</B><P>
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We know of no implementation which currently supports this feature.<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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This will <A REL=DEFINITION HREF="../Body/f_provid.htm#require"><B>require</B></A> most implementations to make a trivial modification<P>
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to <A REL=DEFINITION HREF="../Body/f_reduce.htm#reduce"><B>REDUCE</B></A>. Implementations which wish to use this as an opportunity to<P>
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further optimize compiled calls to <A REL=DEFINITION HREF="../Body/f_reduce.htm#reduce"><B>REDUCE</B></A> will have to undertake more<P>
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work (which would be much more difficult today).<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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<A REL=DEFINITION HREF="../Body/f_reduce.htm#reduce"><B>REDUCE</B></A> will continue to be more difficult to use than other sequence<P>
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functions on sequences of complex objects.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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<A REL=DEFINITION HREF="../Body/f_reduce.htm#reduce"><B>REDUCE</B></A> will become easier to use on sequences of complex objects. It<P>
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will be easier for compilers to convert some calls to <A REL=DEFINITION HREF="../Body/f_reduce.htm#reduce"><B>REDUCE</B></A> into<P>
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efficient loops.<P>
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<P>
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<B>Aesthetics:<P>
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</B><P>
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Slightly damaged in one way. All :KEY arguments are currently defined<P>
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to be used for predicates, this proposal will implicitly broaden :KEY<P>
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to support general extraction for any purpose.<P>
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<P>
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Slightly improved in another way. Many common situations where <A REL=DEFINITION HREF="../Body/f_reduce.htm#reduce"><B>REDUCE</B></A><P>
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could be used would be easier to write and easier to later read.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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Several members of the committee feel that the increased functionality<P>
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outweighs the damage to the definition of :KEY. No one has objected<P>
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to this change in the recent round of discussions.<P>
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<P>
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There is some controversy over whether the "definition" of :KEY<P>
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arguments on page 246 of CLtL really constitutes a definition or just<P>
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an "unwritten rule".<P>
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<P>
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</PRE>
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