65 lines
3.4 KiB
HTML
65 lines
3.4 KiB
HTML
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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> 1.2.3. Evaluation, Expansion, and Equivalence</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Evaluation, Expansion, and Equivalence">
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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=tex2html1608 HREF="node11.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html1606 HREF="node7.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html1600 HREF="node9.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html1610 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html1611 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html1609 HREF="node11.html"> Errors</A>
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<B>Up:</B> <A NAME=tex2html1607 HREF="node7.html"> Notational Conventions</A>
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<B> Previous:</B> <A NAME=tex2html1601 HREF="node9.html"> NilFalse, and </A>
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<HR> <P>
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<H2><A NAME=SECTION00523000000000000000> 1.2.3. Evaluation, Expansion, and Equivalence</A></H2>
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<P>
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Execution of code in Lisp is called <i>evaluation</i> because executing a
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piece of code normally results in a data object called the <i>value</i>
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produced by the code. The symbol <tt>=></tt> is used in examples to
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indicate evaluation.
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For example,
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<P><pre>
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(+ 4 5) => 9
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</pre><P>
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means ``the result of evaluating the code <tt>(+ 4 5)</tt> is (or would be,
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or would have been) <tt>9</tt>.''
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<P>
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The symbol -> is used in examples to indicate macro expansion.
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For example,
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<P><pre>
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(push x v) -> (setf v (cons x v))
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</pre><P>
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means ``the result of expanding the macro-call form <tt>(push x v)</tt>
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is <tt>(setf v (cons x v))</tt>.'' This implies that the two pieces
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of code do the same thing; the second piece of code is
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the definition of what the first does.
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<P>
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The symbol == is used in examples to indicate code equivalence.
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For example,
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<P><pre>
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(gcd x (gcd y z)) == (gcd (gcd x y) z)
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</pre><P>
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means ``the value and effects of evaluating the form
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<tt>(gcd x (gcd y z))</tt> are always the same as the value
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and effects of
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<tt>(gcd (gcd x y) z)</tt> for any values of the
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variables <tt>x</tt>, <tt>y</tt>, and <tt>z</tt>.''
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This implies that the two pieces
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of code do the same thing; however, neither directly defines
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the other in the way macro expansion does.
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<P>
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<BR> <HR><A NAME=tex2html1608 HREF="node11.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html1606 HREF="node7.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html1600 HREF="node9.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html1610 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html1611 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html1609 HREF="node11.html"> Errors</A>
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<B>Up:</B> <A NAME=tex2html1607 HREF="node7.html"> Notational Conventions</A>
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<B> Previous:</B> <A NAME=tex2html1601 HREF="node9.html"> NilFalse, and </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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