69 lines
2.4 KiB
HTML
69 lines
2.4 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Originally converted to HTML using LaTeX2HTML 95 (Thu Jan 19 1995) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE> Functions with Extended Bodies</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Functions with Extended Bodies">
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<meta name="keywords" value="lp">
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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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<BR> <HR>
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<A HREF="node21.html"><IMG ALIGN=BOTTOM ALT="next" SRC="next_motif.gif"></A>
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<A HREF="node15.html"><IMG ALIGN=BOTTOM ALT="up" SRC="up_motif.gif"></A>
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<A HREF="node19.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="previous_motif.gif"></A> <BR>
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<A HREF="lp.html"><B>Contents</B></A>
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<B> Next:</B>
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<A HREF="node21.html"> Conditional Control</A>
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<B>Up:</B>
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<A HREF="node15.html"> Defining Lisp functions</A>
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<B> Previous:</B>
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<A HREF="node19.html"> Using Your Own </A>
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<BR> <HR> <P>
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<H1> Functions with Extended Bodies</H1>
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<P>
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As mentioned before, a function definition may contain an indefinite number of expressions in its body, one after the other. Take the following definition, which has two:
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<BLOCKQUOTE>
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<PRE>> (defun powers-of (x)
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(square x)
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(fourth-power x))
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POWERS-OF
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> (powers-of 2)
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16
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</PRE>
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</BLOCKQUOTE>
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Notice that this function only returns the value of the last expression in its body. In this case the last expression in the body is (fourth-power x) so only the value 16 gets printed in the example above.
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<P>
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What is the point of having more than one expression in the body of a function if it only ever returns the last? The answer to this question is that we may be interested in side effects of the intermediate evaluations.
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<P>
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Powers-of does not have any side effects as it is, but change the definition as follows:
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<BLOCKQUOTE>
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<PRE>> (defun powers-of (x)
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(setq y (square x))
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(fourth-power x))
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POWERS-OF
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</PRE>
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</BLOCKQUOTE>
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Watch what happens here:
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<BLOCKQUOTE>
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<PRE>> y
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27
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> (powers-of 7)
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2401
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> y
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49
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</PRE>
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</BLOCKQUOTE>
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The side effect of powers-of was to set the value of the variable y to 49. Since y did not appear in the parameter list of powers-of, it is treated as a global variable, and the effect of the set lasts beyond the life of your call to powers-of.
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<P>
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<BR> <HR>
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<P>
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<ADDRESS>
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<I>© Colin Allen & Maneesh Dhagat <BR>
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March 2007 </I>
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</ADDRESS>
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</BODY>
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