128 lines
5.9 KiB
HTML
128 lines
5.9 KiB
HTML
|
<!DOCTYPE html>
|
||
|
|
<html>
|
||
|
|
<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
|
||
|
|
<head>
|
||
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
||
|
|
<!-- This manual documents Guile version 3.0.10.
|
||
|
|
|
||
|
|
Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
|
||
|
|
Inc.
|
||
|
|
|
||
|
|
Copyright (C) 2021 Maxime Devos
|
||
|
|
|
||
|
|
Copyright (C) 2024 Tomas Volf
|
||
|
|
|
||
|
|
|
||
|
|
Permission is granted to copy, distribute and/or modify this document
|
||
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
||
|
|
any later version published by the Free Software Foundation; with no
|
||
|
|
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
|
||
|
|
copy of the license is included in the section entitled "GNU Free
|
||
|
|
Documentation License." -->
|
||
|
|
<title>Defmacros (Guile Reference Manual)</title>
|
||
|
|
|
||
|
|
<meta name="description" content="Defmacros (Guile Reference Manual)">
|
||
|
|
<meta name="keywords" content="Defmacros (Guile Reference Manual)">
|
||
|
|
<meta name="resource-type" content="document">
|
||
|
|
<meta name="distribution" content="global">
|
||
|
|
<meta name="Generator" content=".texi2any-real">
|
||
|
|
<meta name="viewport" content="width=device-width,initial-scale=1">
|
||
|
|
|
||
|
|
<link href="index.html" rel="start" title="Top">
|
||
|
|
<link href="Concept-Index.html" rel="index" title="Concept Index">
|
||
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
||
|
|
<link href="Macros.html" rel="up" title="Macros">
|
||
|
|
<link href="Identifier-Macros.html" rel="next" title="Identifier Macros">
|
||
|
|
<link href="Syntax-Transformer-Helpers.html" rel="prev" title="Syntax Transformer Helpers">
|
||
|
|
<style type="text/css">
|
||
|
|
<!--
|
||
|
|
a.copiable-link {visibility: hidden; text-decoration: none; line-height: 0em}
|
||
|
|
div.example {margin-left: 3.2em}
|
||
|
|
span:hover a.copiable-link {visibility: visible}
|
||
|
|
-->
|
||
|
|
</style>
|
||
|
|
<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
|
||
|
|
|
||
|
|
|
||
|
|
</head>
|
||
|
|
|
||
|
|
<body lang="en">
|
||
|
|
<div class="subsection-level-extent" id="Defmacros">
|
||
|
|
<div class="nav-panel">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Identifier-Macros.html" accesskey="n" rel="next">Identifier Macros</a>, Previous: <a href="Syntax-Transformer-Helpers.html" accesskey="p" rel="prev">Syntax Transformer Helpers</a>, Up: <a href="Macros.html" accesskey="u" rel="up">Macros</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<h4 class="subsection" id="Lisp_002dstyle-Macro-Definitions"><span>6.8.5 Lisp-style Macro Definitions<a class="copiable-link" href="#Lisp_002dstyle-Macro-Definitions"> ¶</a></span></h4>
|
||
|
|
|
||
|
|
<p>The traditional way to define macros in Lisp is very similar to procedure
|
||
|
|
definitions. The key differences are that the macro definition body should
|
||
|
|
return a list that describes the transformed expression, and that the definition
|
||
|
|
is marked as a macro definition (rather than a procedure definition) by the use
|
||
|
|
of a different definition keyword: in Lisp, <code class="code">defmacro</code> rather than
|
||
|
|
<code class="code">defun</code>, and in Scheme, <code class="code">define-macro</code> rather than <code class="code">define</code>.
|
||
|
|
</p>
|
||
|
|
<a class="index-entry-id" id="index-defmacro"></a>
|
||
|
|
<a class="index-entry-id" id="index-define_002dmacro"></a>
|
||
|
|
<p>Guile supports this style of macro definition using both <code class="code">defmacro</code>
|
||
|
|
and <code class="code">define-macro</code>. The only difference between them is how the
|
||
|
|
macro name and arguments are grouped together in the definition:
|
||
|
|
</p>
|
||
|
|
<div class="example lisp">
|
||
|
|
<pre class="lisp-preformatted">(defmacro <var class="var">name</var> (<var class="var">args</var> ...) <var class="var">body</var> ...)
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
<p>is the same as
|
||
|
|
</p>
|
||
|
|
<div class="example lisp">
|
||
|
|
<pre class="lisp-preformatted">(define-macro (<var class="var">name</var> <var class="var">args</var> ...) <var class="var">body</var> ...)
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
<p>The difference is analogous to the corresponding difference between
|
||
|
|
Lisp’s <code class="code">defun</code> and Scheme’s <code class="code">define</code>.
|
||
|
|
</p>
|
||
|
|
<p>Having read the previous section on <code class="code">syntax-case</code>, it’s probably clear that
|
||
|
|
Guile actually implements defmacros in terms of <code class="code">syntax-case</code>, applying the
|
||
|
|
transformer on the expression between invocations of <code class="code">syntax->datum</code> and
|
||
|
|
<code class="code">datum->syntax</code>. This realization leads us to the problem with defmacros,
|
||
|
|
that they do not preserve referential transparency. One can be careful to not
|
||
|
|
introduce bindings into expanded code, via liberal use of <code class="code">gensym</code>, but
|
||
|
|
there is no getting around the lack of referential transparency for free
|
||
|
|
bindings in the macro itself.
|
||
|
|
</p>
|
||
|
|
<p>Even a macro as simple as our <code class="code">when</code> from before is difficult to get right:
|
||
|
|
</p>
|
||
|
|
<div class="example">
|
||
|
|
<pre class="example-preformatted">(define-macro (when cond exp . rest)
|
||
|
|
`(if ,cond
|
||
|
|
(begin ,exp . ,rest)))
|
||
|
|
|
||
|
|
(when #f (display "Launching missiles!\n"))
|
||
|
|
⇒ #f
|
||
|
|
|
||
|
|
(let ((if list))
|
||
|
|
(when #f (display "Launching missiles!\n")))
|
||
|
|
-| Launching missiles!
|
||
|
|
⇒ (#f #<unspecified>)
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
<p>Guile’s perspective is that defmacros have had a good run, but that modern
|
||
|
|
macros should be written with <code class="code">syntax-rules</code> or <code class="code">syntax-case</code>. There
|
||
|
|
are still many uses of defmacros within Guile itself, but we will be phasing
|
||
|
|
them out over time. Of course we won’t take away <code class="code">defmacro</code> or
|
||
|
|
<code class="code">define-macro</code> themselves, as there is lots of code out there that uses
|
||
|
|
them.
|
||
|
|
</p>
|
||
|
|
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<div class="nav-panel">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Identifier-Macros.html">Identifier Macros</a>, Previous: <a href="Syntax-Transformer-Helpers.html">Syntax Transformer Helpers</a>, Up: <a href="Macros.html">Macros</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|