79 lines
4.5 KiB
HTML
79 lines
4.5 KiB
HTML
|
<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
|
||
|
|
<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
|
||
|
|
<HEAD>
|
||
|
|
<TITLE>5.1.4. Macros</TITLE>
|
||
|
|
</HEAD>
|
||
|
|
<BODY>
|
||
|
|
<meta name="description" value=" Macros">
|
||
|
|
<meta name="keywords" value="clm">
|
||
|
|
<meta name="resource-type" value="document">
|
||
|
|
<meta name="distribution" value="global">
|
||
|
|
<P>
|
||
|
|
<b>Common Lisp the Language, 2nd Edition</b>
|
||
|
|
<BR> <HR><A NAME=tex2html2263 HREF="node61.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2261 HREF="node56.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2255 HREF="node59.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2265 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2266 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
||
|
|
<B> Next:</B> <A NAME=tex2html2264 HREF="node61.html"> Function Calls</A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html2262 HREF="node56.html"> Forms</A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html2256 HREF="node59.html"> Special Forms</A>
|
||
|
|
<HR> <P>
|
||
|
|
<H2><A NAME=SECTION00914000000000000000>5.1.4. Macros</A></H2>
|
||
|
|
<P>
|
||
|
|
If a form is a list and the first element is not the name of a special
|
||
|
|
form, it may be the name of a <i>macro</i>; if so, the form is said
|
||
|
|
to be a <i>macro call</i>. A macro is essentially a function from
|
||
|
|
forms to forms that will, given a call to that macro, compute
|
||
|
|
a new form to be evaluated in place of the macro call.
|
||
|
|
(This computation is sometimes referred to as <i>macro expansion</i>.)
|
||
|
|
For example, the macro named <tt>return</tt> will take a form such as
|
||
|
|
<tt>(return x)</tt> and from that form compute a new form
|
||
|
|
<tt>(return-from <tt>nil</tt> x)</tt>. We say that the old
|
||
|
|
form <i>expands</i> into the new form. The new form is then evaluated in
|
||
|
|
place of the original form; the value of the new form is returned as the
|
||
|
|
value of the original form.
|
||
|
|
<P>
|
||
|
|
<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
|
||
|
|
X3J13 voted in January 1989
|
||
|
|
(DOTTED-MACRO-FORMS) <A NAME=3136> </A>
|
||
|
|
to clarify that macro calls, and subforms
|
||
|
|
of macro calls, need not be proper lists, but that use of dotted forms
|
||
|
|
requires the macro definition to use ``<tt>. <i>var</i></tt>'' or
|
||
|
|
``<tt>&rest <i>var</i></tt>'' in order to match them properly.
|
||
|
|
It is then the responsibility of the macro definition to recognize
|
||
|
|
and appropriately handle such dotted forms or subforms.
|
||
|
|
<br><img align=bottom alt="change_end" src="gif/change_end.gif">
|
||
|
|
<P>
|
||
|
|
There are a number of standard macros in Common Lisp, and the user can define more
|
||
|
|
by using <tt>defmacro</tt>.
|
||
|
|
<P>
|
||
|
|
Macros provided by a Common Lisp implementation as described herein may expand
|
||
|
|
into code that is not portable among differing implementations.
|
||
|
|
That is, a macro call may be implementation-independent because
|
||
|
|
the macro is defined in this book, but the expansion need not be.
|
||
|
|
<P>
|
||
|
|
<hr>
|
||
|
|
<b>Implementation note:</b> Implementors are encouraged to implement the macros
|
||
|
|
defined in this book, as far as is possible, in such a way that
|
||
|
|
the expansion will not contain any implementation-dependent
|
||
|
|
special forms, nor contain as forms data objects that
|
||
|
|
are not considered to be forms in Common Lisp.
|
||
|
|
The purpose of this restriction is to ensure that the expansion
|
||
|
|
can be processed by a program-analyzing program in an
|
||
|
|
implementation-independent manner.
|
||
|
|
There is no problem with a macro expansion containing
|
||
|
|
calls to implementation-dependent functions.
|
||
|
|
This restriction is not a requirement of Common Lisp; it is recognized
|
||
|
|
that certain complex macros may be able to expand into significantly
|
||
|
|
more efficient code in certain implementations
|
||
|
|
by using implementation-dependent special forms in the macro expansion.
|
||
|
|
<hr>
|
||
|
|
<P>
|
||
|
|
<BR> <HR><A NAME=tex2html2263 HREF="node61.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2261 HREF="node56.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2255 HREF="node59.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2265 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2266 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
||
|
|
<B> Next:</B> <A NAME=tex2html2264 HREF="node61.html"> Function Calls</A>
|
||
|
|
<B>Up:</B> <A NAME=tex2html2262 HREF="node56.html"> Forms</A>
|
||
|
|
<B> Previous:</B> <A NAME=tex2html2256 HREF="node59.html"> Special Forms</A>
|
||
|
|
<HR> <P>
|
||
|
|
<HR>
|
||
|
|
<P><ADDRESS>
|
||
|
|
AI.Repository@cs.cmu.edu
|
||
|
|
</ADDRESS>
|
||
|
|
</BODY>
|