133 lines
7.9 KiB
HTML
133 lines
7.9 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>8.4. Compiler Macros</TITLE>
|
|
</HEAD>
|
|
<BODY>
|
|
<meta name="description" value=" Compiler 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=tex2html2782 HREF="node102.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2780 HREF="node97.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2774 HREF="node100.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2784 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2785 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
|
<B> Next:</B> <A NAME=tex2html2783 HREF="node102.html"> Environments</A>
|
|
<B>Up:</B> <A NAME=tex2html2781 HREF="node97.html"> Macros</A>
|
|
<B> Previous:</B> <A NAME=tex2html2775 HREF="node100.html"> Destructuring</A>
|
|
<HR> <P>
|
|
<H1><A NAME=SECTION001240000000000000000>8.4. Compiler Macros</A></H1>
|
|
<P>
|
|
<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
|
|
X3J13 voted in June 1989 (DEFINE-COMPILER-MACRO) <A NAME=8630> </A>
|
|
to add a facility for defining <i>compiler macros</i> that
|
|
take effect only when compiling code, not when interpreting it.
|
|
<P>
|
|
The purpose of this facility is to permit selective source-code
|
|
transformations only when the compiler is processing the code.
|
|
When the compiler is about to compile a non-atomic form, it first calls
|
|
<tt>compiler-macroexpand-1</tt> repeatedly until there is no more expansion
|
|
(there might not be any to begin with). Then it continues its
|
|
remaining processing, which may include calling <tt>macroexpand-1</tt> and so on.
|
|
<P>
|
|
The compiler is required to expand compiler macros. It is unspecified
|
|
whether the interpreter does so. The intention is that only the
|
|
compiler will do so, but the range of possible ``compiled-only''
|
|
implementation strategies precludes any firm specification.
|
|
<P>
|
|
<BR><b>[Macro]</b><BR>
|
|
<pre>
|
|
define-compiler-macro <i>name</i> <i>lambda-list</i> {<i>declaration</i> | <i>doc-string</i>}* {<i>form</i>}*
|
|
</pre>
|
|
<P> This is just like <tt>defmacro</tt> except the definition is not stored in the
|
|
symbol function cell of <i>name</i> and is not seen by <tt>macroexpand-1</tt>.
|
|
It is, however, seen by <tt>compiler-macroexpand-1</tt>. As with <tt>defmacro</tt>, the
|
|
<i>lambda-list</i> may include <tt>&environment</tt> and <tt>&whole</tt>
|
|
and may include destructuring. The definition is
|
|
global. (There is no provision for defining local compiler
|
|
macros in the way that <tt>macrolet</tt> defines local macros.)
|
|
<P>
|
|
A top-level call to <tt>define-compiler-macro</tt> in a file being compiled by
|
|
<tt>compile-file</tt> has an effect on the compilation environment similar to
|
|
that of a call to <tt>defmacro</tt>, except it is noticed as a
|
|
compiler macro (see section <A HREF="node224.html#COMPILERSECTION">25.1</A>).
|
|
<P>
|
|
Note that compiler macro definitions do not appear in information returned by
|
|
<tt>function-information</tt>; they are global, and their interaction
|
|
with other lexical and global definitions can be reconstructed by
|
|
<tt>compiler-macro-function</tt>. It is up to code-walking programs to decide
|
|
whether to invoke compiler macro expansion.
|
|
<P>
|
|
X3J13 voted in March 1988 (FLET-IMPLICIT-BLOCK) <A NAME=8653> </A>
|
|
to specify that the body of the expander function defined
|
|
by <tt>defmacro</tt> is implicitly enclosed in a <tt>block</tt> construct
|
|
whose name is the same as the <i>name</i> of the defined macro;
|
|
presumably this applies also to <tt>define-compiler-macro</tt>.
|
|
Therefore <tt>return-from</tt> may be used to exit from the function.
|
|
<P>
|
|
<BR><b>[Function]</b><BR>
|
|
<tt>compiler-macro-function</tt> <tt><i>name</i></tt> <tt>&optional</tt> <tt><i>env</i></tt><P> The <i>name</i> must be a symbol.
|
|
If it has been defined as a compiler macro, then
|
|
<tt>compiler-macro-function</tt> returns the macro expansion
|
|
function; otherwise it returns <tt>nil</tt>. The
|
|
lexical environment <i>env</i> may override any global definition for <i>name</i>
|
|
by defining a local function or local macro (such as by <tt>flet</tt>, <tt>labels</tt>, or
|
|
<tt>macrolet</tt>) in which case <tt>nil</tt> is returned.
|
|
<P>
|
|
<tt>setf</tt> may be used with <tt>compiler-macro-function</tt> to install a function as
|
|
the expansion function for the compiler macro <i>name</i>, in the same manner as for
|
|
<tt>macro-function</tt>. Storing the value <tt>nil</tt> removes any existing
|
|
compiler macro definition. As with <tt>macro-function</tt>, a non-<tt>nil</tt> stored value
|
|
must be a function of two arguments, the entire macro call and
|
|
the environment. The second argument to <tt>compiler-macro-function</tt> must
|
|
be omitted when it is used with <tt>setf</tt>.
|
|
<P>
|
|
<BR><b>[Function]</b><BR>
|
|
<tt>compiler-macroexpand</tt> <tt><i>form</i></tt> <tt>&optional</tt> <tt><i>env</i></tt> <tt><BR></tt><tt>compiler-macroexpand-1</tt> <tt><i>form</i></tt> <tt>&optional</tt> <tt><i>env</i></tt><P> These are just like <tt>macroexpand</tt> and <tt>macroexpand-1</tt>
|
|
except that the expander function is obtained as if by a call to
|
|
<tt>compiler-macro-function</tt> on the <i>car</i> of the <i>form</i> rather than by a call to
|
|
<tt>macro-function</tt>.
|
|
Note that <tt>compiler-macroexpand</tt> performs repeated expansion
|
|
but <tt>compiler-macroexpand-1</tt> performs at most one expansion.
|
|
Two values are returned, the expansion (or the original <i>form</i>)
|
|
and a value that is true if any expansion occurred and <tt>nil</tt> otherwise.
|
|
<P>
|
|
There are three cases where no expansion happens:
|
|
<UL> <LI> There is no compiler macro definition for the <i>car</i> of <i>form</i>.
|
|
<LI> There is such a definition but there is also a <tt>notinline</tt>
|
|
declaration, either globally or in the lexical environment <i>env</i>.
|
|
<LI> A global compiler macro definition is shadowed by a local
|
|
function or macro definition (such as by <tt>flet</tt>, <tt>labels</tt>, or
|
|
<tt>macrolet</tt>).
|
|
</UL>
|
|
Note that if there is no expansion, the original <i>form</i> is returned as
|
|
the first value, and <tt>nil</tt> as the second value.
|
|
<P>
|
|
Any macro expansion performed by the function <tt>compiler-macroexpand</tt>
|
|
or by the function <tt>compiler-macroexpand-1</tt> is carried out
|
|
by calling the function that is the value of <tt>*macroexpand-hook*</tt>.
|
|
<P>
|
|
A compiler macro may decline to provide any expansion merely
|
|
by returning the original form. This is useful when using the facility
|
|
to put ``compiler optimizers'' on various function names. For example,
|
|
here is a compiler macro that ``optimizes'' (one would hope)
|
|
the zero-argument and one-argument cases of
|
|
a function called <tt>plus</tt>:
|
|
<P><pre>
|
|
(define-compiler-macro plus (&whole form &rest args)
|
|
(case (length args)
|
|
(0 0)
|
|
(1 (car args))
|
|
(t form)))
|
|
</pre><P>
|
|
<br><img align=bottom alt="change_end" src="gif/change_end.gif">
|
|
<P>
|
|
<BR> <HR><A NAME=tex2html2782 HREF="node102.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2780 HREF="node97.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2774 HREF="node100.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2784 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2785 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
|
|
<B> Next:</B> <A NAME=tex2html2783 HREF="node102.html"> Environments</A>
|
|
<B>Up:</B> <A NAME=tex2html2781 HREF="node97.html"> Macros</A>
|
|
<B> Previous:</B> <A NAME=tex2html2775 HREF="node100.html"> Destructuring</A>
|
|
<HR> <P>
|
|
<HR>
|
|
<P><ADDRESS>
|
|
AI.Repository@cs.cmu.edu
|
|
</ADDRESS>
|
|
</BODY>
|