151 lines
7.3 KiB
HTML
151 lines
7.3 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>Syntax Parameters (Guile Reference Manual)</title>
|
||
|
|
|
||
|
|
<meta name="description" content="Syntax Parameters (Guile Reference Manual)">
|
||
|
|
<meta name="keywords" content="Syntax Parameters (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="Eval-When.html" rel="next" title="Eval When">
|
||
|
|
<link href="Identifier-Macros.html" rel="prev" title="Identifier Macros">
|
||
|
|
<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}
|
||
|
|
strong.def-name {font-family: monospace; font-weight: bold; font-size: larger}
|
||
|
|
-->
|
||
|
|
</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="Syntax-Parameters">
|
||
|
|
<div class="nav-panel">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Eval-When.html" accesskey="n" rel="next">Eval-when</a>, Previous: <a href="Identifier-Macros.html" accesskey="p" rel="prev">Identifier Macros</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="Syntax-Parameters-1"><span>6.8.7 Syntax Parameters<a class="copiable-link" href="#Syntax-Parameters-1"> ¶</a></span></h4>
|
||
|
|
|
||
|
|
<p>Syntax parameters<a class="footnote" id="DOCF16" href="#FOOT16"><sup>16</sup></a> are a
|
||
|
|
mechanism for rebinding a macro definition within the dynamic extent of
|
||
|
|
a macro expansion. This provides a convenient solution to one of the
|
||
|
|
most common types of unhygienic macro: those that introduce a unhygienic
|
||
|
|
binding each time the macro is used. Examples include a <code class="code">lambda</code>
|
||
|
|
form with a <code class="code">return</code> keyword, or class macros that introduce a
|
||
|
|
special <code class="code">self</code> binding.
|
||
|
|
</p>
|
||
|
|
<p>With syntax parameters, instead of introducing the binding
|
||
|
|
unhygienically each time, we instead create one binding for the keyword,
|
||
|
|
which we can then adjust later when we want the keyword to have a
|
||
|
|
different meaning. As no new bindings are introduced, hygiene is
|
||
|
|
preserved. This is similar to the dynamic binding mechanisms we have at
|
||
|
|
run-time (see <a class="pxref" href="SRFI_002d39.html">parameters</a>), except that the dynamic binding
|
||
|
|
only occurs during macro expansion. The code after macro expansion
|
||
|
|
remains lexically scoped.
|
||
|
|
</p>
|
||
|
|
<dl class="first-deffn">
|
||
|
|
<dt class="deffn" id="index-define_002dsyntax_002dparameter"><span class="category-def">Syntax: </span><span><strong class="def-name">define-syntax-parameter</strong> <var class="def-var-arguments">keyword transformer</var><a class="copiable-link" href="#index-define_002dsyntax_002dparameter"> ¶</a></span></dt>
|
||
|
|
<dd><p>Binds <var class="var">keyword</var> to the value obtained by evaluating
|
||
|
|
<var class="var">transformer</var>. The <var class="var">transformer</var> provides the default expansion
|
||
|
|
for the syntax parameter, and in the absence of
|
||
|
|
<code class="code">syntax-parameterize</code>, is functionally equivalent to
|
||
|
|
<code class="code">define-syntax</code>. Usually, you will just want to have the
|
||
|
|
<var class="var">transformer</var> throw a syntax error indicating that the <var class="var">keyword</var>
|
||
|
|
is supposed to be used in conjunction with another macro, for example:
|
||
|
|
</p><div class="example">
|
||
|
|
<pre class="example-preformatted">(define-syntax-parameter return
|
||
|
|
(lambda (stx)
|
||
|
|
(syntax-violation 'return "return used outside of a lambda^" stx)))
|
||
|
|
</pre></div>
|
||
|
|
</dd></dl>
|
||
|
|
|
||
|
|
<dl class="first-deffn">
|
||
|
|
<dt class="deffn" id="index-syntax_002dparameterize"><span class="category-def">Syntax: </span><span><strong class="def-name">syntax-parameterize</strong> <var class="def-var-arguments">((keyword transformer) …) exp …</var><a class="copiable-link" href="#index-syntax_002dparameterize"> ¶</a></span></dt>
|
||
|
|
<dd><p>Adjusts <var class="var">keyword</var> … to use the values obtained by evaluating
|
||
|
|
their <var class="var">transformer</var> …, in the expansion of the <var class="var">exp</var>
|
||
|
|
… forms. Each <var class="var">keyword</var> must be bound to a syntax-parameter.
|
||
|
|
<code class="code">syntax-parameterize</code> differs from <code class="code">let-syntax</code>, in that the
|
||
|
|
binding is not shadowed, but adjusted, and so uses of the keyword in the
|
||
|
|
expansion of <var class="var">exp</var> … use the new transformers. This is
|
||
|
|
somewhat similar to how <code class="code">parameterize</code> adjusts the values of
|
||
|
|
regular parameters, rather than creating new bindings.
|
||
|
|
</p>
|
||
|
|
<div class="example">
|
||
|
|
<pre class="example-preformatted">(define-syntax lambda^
|
||
|
|
(syntax-rules ()
|
||
|
|
[(lambda^ argument-list body body* ...)
|
||
|
|
(lambda argument-list
|
||
|
|
(call-with-current-continuation
|
||
|
|
(lambda (escape)
|
||
|
|
;; In the body we adjust the 'return' keyword so that calls
|
||
|
|
;; to 'return' are replaced with calls to the escape
|
||
|
|
;; continuation.
|
||
|
|
(syntax-parameterize ([return (syntax-rules ()
|
||
|
|
[(return vals (... ...))
|
||
|
|
(escape vals (... ...))])])
|
||
|
|
body body* ...))))]))
|
||
|
|
|
||
|
|
;; Now we can write functions that return early. Here, 'product' will
|
||
|
|
;; return immediately if it sees any 0 element.
|
||
|
|
(define product
|
||
|
|
(lambda^ (list)
|
||
|
|
(fold (lambda (n o)
|
||
|
|
(if (zero? n)
|
||
|
|
(return 0)
|
||
|
|
(* n o)))
|
||
|
|
1
|
||
|
|
list)))
|
||
|
|
</pre></div>
|
||
|
|
</dd></dl>
|
||
|
|
|
||
|
|
|
||
|
|
</div>
|
||
|
|
<div class="footnotes-segment">
|
||
|
|
<hr>
|
||
|
|
<h4 class="footnotes-heading">Footnotes</h4>
|
||
|
|
|
||
|
|
<h5 class="footnote-body-heading"><a id="FOOT16" href="#DOCF16">(16)</a></h5>
|
||
|
|
<p>Described in the paper <cite class="cite">Keeping it Clean
|
||
|
|
with Syntax Parameters</cite> by Barzilay, Culpepper and Flatt.</p>
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<div class="nav-panel">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Eval-When.html">Eval-when</a>, Previous: <a href="Identifier-Macros.html">Identifier Macros</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>
|