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69 lines
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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.4. Discussion</TITLE>
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<BODY>
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<meta name="description" value=" Discussion">
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<meta name="keywords" value="clm">
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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=tex2html6272 HREF="node367.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html6270 HREF="node362.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html6266 HREF="node365.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html6274 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html6275 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html6273 HREF="node367.html"> Backquote</A>
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<B>Up:</B> <A NAME=tex2html6271 HREF="node362.html"> Generators and Gatherers</A>
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<B> Previous:</B> <A NAME=tex2html6267 HREF="node365.html"> Gatherers</A>
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<HR> <P>
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<H1><A NAME=SECTION003540000000000000000>B.4. Discussion</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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The idea of generators and gatherers was first proposed by Pavel
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Curtis. A key aspect of his proposal was the realization that
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generators and gatherers can be implemented simply and elegantly as
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closures and that these closures can be compiled very
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efficiently if certain conditions are met.
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<P>
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First, the compiler must support an optimization Curtis calls
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``<tt>let</tt> eversion'' in addition to the optimization methods presented
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in [<A HREF="node368.html#RABBIT">45</A>]. If a closure is created and used entirely within a
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limited lexical scope, the scopes of any bound variables nested in the
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closure can be enlarged (everted) to enclose all the uses of the
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closure. This allows the variables to be allocated on the stack
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rather than the heap.
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<P>
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Second, for a generator/gatherer closure to be compiled efficiently,
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it must be possible to determine at compile time exactly what closure
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is involved and exactly what the scope of use of the closure is.
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There are several aspects to this. The expression creating the
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generator/gatherer cannot refer to a free series variable. The
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generator/gatherer must be stored in a local variable. This
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variable must be used only in calls of <tt>next-in</tt>,
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<tt>next-out</tt>, and <tt>result-of</tt>, and not inside a closure. In
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particular the generator/gatherer cannot be stored in a data
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structure, stored in a special variable, or returned as a result
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value.
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<P>
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All of the examples above satisfy these restrictions. For instance,
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once the uses of <tt>gathering</tt> and <tt>iterate</tt> have been
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optimized, the body of <tt>examp</tt> is as efficient as any loop
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performing the same computation.
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<P>
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The implementation discussed in [<A HREF="node368.html#WATERSSERIESDESIGN">52</A>] includes a
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portable Common Lisp implementation of generators and gatherers. Although
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the implementation does not support optimizations of the kind discussed
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in [<A HREF="node368.html#RABBIT">45</A>], it fully optimizes uses of <tt>gathering</tt>.
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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=tex2html6272 HREF="node367.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html6270 HREF="node362.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html6266 HREF="node365.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html6274 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html6275 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html6273 HREF="node367.html"> Backquote</A>
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<B>Up:</B> <A NAME=tex2html6271 HREF="node362.html"> Generators and Gatherers</A>
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<B> Previous:</B> <A NAME=tex2html6267 HREF="node365.html"> Gatherers</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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</ADDRESS>
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</BODY>
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