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88 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>8. Macros</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Macros">
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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=tex2html2729 HREF="node98.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2727 HREF="clm.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2721 HREF="node96.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2731 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2732 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2730 HREF="node98.html"> Macro Definition</A>
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<B>Up:</B> <A NAME=tex2html2728 HREF="clm.html">Common Lisp the Language</A>
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<B> Previous:</B> <A NAME=tex2html2722 HREF="node96.html"> Dynamic Non-Local Exits</A>
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<HR> <P>
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<H1><A NAME=SECTION001200000000000000000>8. Macros</A></H1>
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<P>
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<A NAME=MACROS>The</A>
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Common Lisp macro facility allows the user to define arbitrary
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functions that convert certain Lisp forms into different forms before
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evaluating or compiling them. This is done at the expression level,
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not at the character-string level as in most other languages. Macros
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are important in the writing of good code: they make it possible to
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write code that is clear and elegant at the user level but that is
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converted to a more complex or more efficient internal form for
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execution.
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<P>
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When <tt>eval</tt> is given a list whose <i>car</i> is a symbol, it looks
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for local definitions of that symbol (by <tt>flet</tt>, <tt>labels</tt>,
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and <tt>macrolet</tt>); if that fails, it looks for a global definition.
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If the definition is a macro definition, then the original
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list is said to be a <i>macro call</i>. Associated with the definition
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will be a function of two arguments, called the <i>expansion function</i>.
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This function is called with the entire macro call as its first argument
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(the second argument is a lexical environment);
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it must return some new Lisp form, called the <i>expansion</i> of the
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macro call. (Actually, a more general mechanism is involved;
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see <tt>macroexpand</tt>.)
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This expansion is then evaluated in place of the original
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form.
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<P>
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When a function is being compiled, any macros it contains are expanded
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at compilation time. This means that a macro definition must be seen by the
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compiler before the first use of the macro.
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<P>
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More generally, an implementation of Common Lisp has great latitude in deciding
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exactly when to expand macro calls within a program. For example,
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it is acceptable for the <tt>defun</tt> special form to expand all macro
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calls within its body at the time the <tt>defun</tt> form is executed
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and record the fully expanded body as the body of the function
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being defined.
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(An implementation might even choose always to compile functions defined
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by <tt>defun</tt>, even when operating in an ``interpretive'' mode.)
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<P>
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Macros should be written so as to depend as little as possible
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on the execution environment to produce a correct expansion. To ensure
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consistent behavior, it is best to ensure that all macro definitions are
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available, whether to the interpreter or compiler, before any code
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containing calls to those macros is introduced.
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<P>
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In Common Lisp, macros are not functions.
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In particular, macros cannot be used as
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functional arguments to such functions as <tt>apply</tt>, <tt>funcall</tt>,
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or <tt>map</tt>; in such situations, the list representing the ``original macro
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call'' does not exist, and cannot exist, because in some sense the arguments
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have already been evaluated.
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<P>
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<HR>
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<UL>
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<LI> <A NAME=tex2html2733 HREF="node98.html#SECTION001210000000000000000"> Macro Definition</A>
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<LI> <A NAME=tex2html2734 HREF="node99.html#SECTION001220000000000000000"> Macro Expansion</A>
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<LI> <A NAME=tex2html2735 HREF="node100.html#SECTION001230000000000000000"> Destructuring</A>
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<LI> <A NAME=tex2html2736 HREF="node101.html#SECTION001240000000000000000"> Compiler Macros</A>
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<LI> <A NAME=tex2html2737 HREF="node102.html#SECTION001250000000000000000"> Environments</A>
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</UL>
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<BR> <HR><A NAME=tex2html2729 HREF="node98.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2727 HREF="clm.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2721 HREF="node96.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2731 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2732 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2730 HREF="node98.html"> Macro Definition</A>
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<B>Up:</B> <A NAME=tex2html2728 HREF="clm.html">Common Lisp the Language</A>
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<B> Previous:</B> <A NAME=tex2html2722 HREF="node96.html"> Dynamic Non-Local Exits</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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