51 lines
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51 lines
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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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<TITLE>20. The Evaluator</TITLE>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html3761 HREF="node180.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3759 HREF="clm.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3753 HREF="node178.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3763 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3764 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3762 HREF="node180.html"> Run-Time Evaluation of </A>
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<B>Up:</B> <A NAME=tex2html3760 HREF="clm.html">Common Lisp the Language</A>
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<B> Previous:</B> <A NAME=tex2html3754 HREF="node178.html"> Other Aspects of </A>
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<HR> <P>
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<H1><A NAME=SECTION002400000000000000000>20. The Evaluator</A></H1>
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<P>
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The mechanism that executes Lisp programs is called the evaluator.
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More precisely, the evaluator accepts a form and performs the
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computation specified by the form. This mechanism is made available
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to the user through the function <tt>eval</tt>.
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<P>
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The evaluator is typically implemented as an interpreter
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that traverses the given form recursively, performing each step
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of the computation as it goes. An interpretive implementation is not
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required, however. A permissible alternative approach is
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for the evaluator first to completely compile the form into
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machine-executable code and then invoke the resulting code.
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This technique virtually eliminates incompatibilities
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between interpreted and compiled code but also renders the <tt>evalhook</tt>
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mechanism relatively useless.
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Various mixed strategies are also possible. All of these approaches
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should produce the same results when executing a correct program
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but may produce different results for incorrect programs.
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For example, the approaches may differ as to when macro calls
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are expanded; macro definitions should not depend on the time
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at which they are expanded. Implementors should
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document the evaluation strategy for each implementation.
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<P>
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<HR>
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<UL>
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<LI> <A NAME=tex2html3765 HREF="node180.html#SECTION002410000000000000000"> Run-Time Evaluation of Forms</A>
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<LI> <A NAME=tex2html3766 HREF="node181.html#SECTION002420000000000000000"> The Top-Level Loop</A>
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</UL>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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