152 lines
6.9 KiB
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152 lines
6.9 KiB
HTML
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Defining Macros (Guile Reference Manual)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Macros.html" rel="up" title="Macros">
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<link href="Syntax-Rules.html" rel="next" title="Syntax Rules">
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<div class="subsection-level-extent" id="Defining-Macros">
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<div class="nav-panel">
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<p>
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Next: <a href="Syntax-Rules.html" accesskey="n" rel="next">Syntax-rules 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>
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</div>
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<hr>
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<h4 class="subsection" id="Defining-Macros-1"><span>6.8.1 Defining Macros<a class="copiable-link" href="#Defining-Macros-1"> ¶</a></span></h4>
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<p>A macro is a binding between a keyword and a syntax transformer. Since it’s
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difficult to discuss <code class="code">define-syntax</code> without discussing the format of
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transformers, consider the following example macro definition:
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</p>
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<div class="example">
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<pre class="example-preformatted">(define-syntax when
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(syntax-rules ()
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((when condition exp ...)
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(if condition
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(begin exp ...)))))
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(when #t
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(display "hey ho\n")
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(display "let's go\n"))
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-| hey ho
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-| let's go
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</pre></div>
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<p>In this example, the <code class="code">when</code> binding is bound with <code class="code">define-syntax</code>.
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Syntax transformers are discussed in more depth in <a class="ref" href="Syntax-Rules.html">Syntax-rules Macros</a> and
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<a class="ref" href="Syntax-Case.html">Support for the <code class="code">syntax-case</code> System</a>.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-define_002dsyntax"><span class="category-def">Syntax: </span><span><strong class="def-name">define-syntax</strong> <var class="def-var-arguments">keyword transformer</var><a class="copiable-link" href="#index-define_002dsyntax"> ¶</a></span></dt>
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<dd><p>Bind <var class="var">keyword</var> to the syntax transformer obtained by evaluating
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<var class="var">transformer</var>.
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</p>
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<p>After a macro has been defined, further instances of <var class="var">keyword</var> in Scheme
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source code will invoke the syntax transformer defined by <var class="var">transformer</var>.
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</p></dd></dl>
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<p>One can also establish local syntactic bindings with <code class="code">let-syntax</code>.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-let_002dsyntax"><span class="category-def">Syntax: </span><span><strong class="def-name">let-syntax</strong> <var class="def-var-arguments">((keyword transformer) …) exp1 exp2 …</var><a class="copiable-link" href="#index-let_002dsyntax"> ¶</a></span></dt>
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<dd><p>Bind each <var class="var">keyword</var> to its corresponding <var class="var">transformer</var> while
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expanding <var class="var">exp1</var> <var class="var">exp2</var> <small class="enddots">...</small>.
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</p>
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<p>A <code class="code">let-syntax</code> binding only exists at expansion-time.
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</p>
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<div class="example">
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<pre class="example-preformatted">(let-syntax ((unless
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(syntax-rules ()
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((unless condition exp ...)
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(if (not condition)
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(begin exp ...))))))
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(unless #t
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(primitive-exit 1))
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"rock rock rock")
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⇒ "rock rock rock"
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</pre></div>
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</dd></dl>
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<p>A <code class="code">define-syntax</code> form is valid anywhere a definition may appear: at the
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top-level, or locally. Just as a local <code class="code">define</code> expands out to an instance
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of <code class="code">letrec</code>, a local <code class="code">define-syntax</code> expands out to
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<code class="code">letrec-syntax</code>.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-letrec_002dsyntax"><span class="category-def">Syntax: </span><span><strong class="def-name">letrec-syntax</strong> <var class="def-var-arguments">((keyword transformer) …) exp1 exp2 …</var><a class="copiable-link" href="#index-letrec_002dsyntax"> ¶</a></span></dt>
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<dd><p>Bind each <var class="var">keyword</var> to its corresponding <var class="var">transformer</var> while
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expanding <var class="var">exp1</var> <var class="var">exp2</var> <small class="enddots">...</small>.
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</p>
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<p>In the spirit of <code class="code">letrec</code> versus <code class="code">let</code>, an expansion produced by
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<var class="var">transformer</var> may reference a <var class="var">keyword</var> bound by the
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same <var class="var">letrec-syntax</var>.
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</p>
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<div class="example">
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<pre class="example-preformatted">(letrec-syntax ((my-or
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(syntax-rules ()
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((my-or)
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#t)
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((my-or exp)
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exp)
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((my-or exp rest ...)
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(let ((t exp))
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(if t
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t
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(my-or rest ...)))))))
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(my-or #f "rockaway beach"))
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⇒ "rockaway beach"
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</pre></div>
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</dd></dl>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="Syntax-Rules.html">Syntax-rules 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>
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</div>
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</body>
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