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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>B.3. Gatherers</TITLE>
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<meta name="description" value=" Gatherers">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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<P>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html6262 HREF="node366.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html6260 HREF="node362.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html6254 HREF="node364.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html6264 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html6265 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html6263 HREF="node366.html"> Discussion</A>
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<B>Up:</B> <A NAME=tex2html6261 HREF="node362.html"> Generators and Gatherers</A>
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<B> Previous:</B> <A NAME=tex2html6255 HREF="node364.html"> Generators</A>
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<HR> <P>
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<H1><A NAME=SECTION003530000000000000000>B.3. Gatherers</A></H1>
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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These functions create and process gatherers.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>gatherer <i>collector</i></tt><P>The <i>collector</i> must be a function of type
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<tt>(function ((series <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap44337.gif">)) <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap44339.gif">)</tt>. Given this function, <tt>gatherer</tt>
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returns a gatherer that accepts elements of type <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap44337.gif"> and returns a final
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result of type <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap44339.gif">. The method for combining elements used by the
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gatherer is the same as the one used by the <i>collector</i>.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>next-out <i>gatherer</i> <i>item</i></tt><P>Given a gatherer and a value, <tt>next-out</tt> enters the value into the
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gatherer.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>result-of <i>gatherer</i></tt><P><tt>result-of</tt> retrieves the net result from a gatherer. <tt>result-of</tt>
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can be applied at any time. However, it is an error to apply
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<tt>result-of</tt> twice to the same gatherer or to apply <tt>next-out</tt> to a
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gatherer once <tt>result-of</tt> has been applied.
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<P><pre>
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(let ((g (gatherer #'collect-sum)))
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(dolist (i '(1 2 3 4))
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(next-out g i)
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(if (evenp i) (next-out g (* 10 i))))
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(result-of g))
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=> 70
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</pre><P>
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>gathering ({(<i>var</i> <i>fn</i>)}*) {<i>form</i>}*</tt><P>The first subform must be a list of pairs. The first
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element of each pair, <i>var</i>, must be a variable name.
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The second element of each pair, <i>fn</i>,
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must be a form that when wrapped in <tt>(function ...)</tt> is
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acceptable as an argument to <tt>gatherer</tt>. Each symbol is bound to a
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gatherer constructed from the corresponding collector. The body
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(consisting of the <i>forms</i>) is evaluated in the scope of these bindings.
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When this evaluation is complete, <tt>gathering</tt> returns the <tt>result-of</tt> each
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gatherer. If there are <b>n</b> pairs in the binding list,
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<tt>gathering</tt> returns <b>n</b> values. For example:
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<P><pre>
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(defun examp (data)
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(gathering ((x collect) (y collect-sum))
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(iterate ((i (scan data)))
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(case (first i)
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(:slot (next-out x (second i)))
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(:part (dolist (j (second i)) (next-out x j))))
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(next-out y (third i)))))
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(examp '((:slot a 10) (:part (c d) 40))) => (a c d) and 50
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</pre><P>
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<P>
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As a further illustration of gatherers, consider the following definition for a
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simplified version of <tt>gathering</tt> that handles only one binding pair.
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<P><pre>
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(defmacro simple-gathering (((var collector)) &body body)
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`(let ((,var (gatherer (function ,collector))))
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,@body
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(result-of ,var)))
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</pre><P>
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The full capabilities of
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<tt>gathering</tt> can be supported in much the same way.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR> <HR><A NAME=tex2html6262 HREF="node366.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html6260 HREF="node362.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html6254 HREF="node364.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html6264 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html6265 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html6263 HREF="node366.html"> Discussion</A>
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<B>Up:</B> <A NAME=tex2html6261 HREF="node362.html"> Generators and Gatherers</A>
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<B> Previous:</B> <A NAME=tex2html6255 HREF="node364.html"> Generators</A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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