172 lines
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172 lines
7.4 KiB
HTML
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Identifier Macros (Guile Reference Manual)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<div class="subsection-level-extent" id="Identifier-Macros">
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<p>
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Next: <a href="Syntax-Parameters.html" accesskey="n" rel="next">Syntax Parameters</a>, Previous: <a href="Defmacros.html" accesskey="p" rel="prev">Lisp-style Macro Definitions</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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<hr>
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<h4 class="subsection" id="Identifier-Macros-1"><span>6.8.6 Identifier Macros<a class="copiable-link" href="#Identifier-Macros-1"> ¶</a></span></h4>
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<p>When the syntax expander sees a form in which the first element is a macro, the
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whole form gets passed to the macro’s syntax transformer. One may visualize this
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as:
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</p>
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<div class="example">
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<pre class="example-preformatted">(define-syntax foo foo-transformer)
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(foo <var class="var">arg</var>...)
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;; expands via
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(foo-transformer #'(foo <var class="var">arg</var>...))
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</pre></div>
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<p>If, on the other hand, a macro is referenced in some other part of a form, the
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syntax transformer is invoked with only the macro reference, not the whole form.
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</p>
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<div class="example">
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<pre class="example-preformatted">(define-syntax foo foo-transformer)
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foo
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;; expands via
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(foo-transformer #'foo)
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</pre></div>
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<p>This allows bare identifier references to be replaced programmatically via a
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macro. <code class="code">syntax-rules</code> provides some syntax to effect this transformation
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more easily.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-identifier_002dsyntax"><span class="category-def">Syntax: </span><span><strong class="def-name">identifier-syntax</strong> <var class="def-var-arguments">exp</var><a class="copiable-link" href="#index-identifier_002dsyntax"> ¶</a></span></dt>
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<dd><p>Returns a macro transformer that will replace occurrences of the macro with
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<var class="var">exp</var>.
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</p></dd></dl>
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<p>For example, if you are importing external code written in terms of <code class="code">fx+</code>,
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the fixnum addition operator, but Guile doesn’t have <code class="code">fx+</code>, you may use the
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following to replace <code class="code">fx+</code> with <code class="code">+</code>:
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</p>
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<div class="example">
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<pre class="example-preformatted">(define-syntax fx+ (identifier-syntax +))
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</pre></div>
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<p>There is also special support for recognizing identifiers on the
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left-hand side of a <code class="code">set!</code> expression, as in the following:
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</p>
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<div class="example">
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<pre class="example-preformatted">(define-syntax foo foo-transformer)
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(set! foo <var class="var">val</var>)
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;; expands via
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(foo-transformer #'(set! foo <var class="var">val</var>))
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;; if foo-transformer is a "variable transformer"
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</pre></div>
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<p>As the example notes, the transformer procedure must be explicitly
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marked as being a “variable transformer”, as most macros aren’t
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written to discriminate on the form in the operator position.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-make_002dvariable_002dtransformer"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">make-variable-transformer</strong> <var class="def-var-arguments">transformer</var><a class="copiable-link" href="#index-make_002dvariable_002dtransformer"> ¶</a></span></dt>
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<dd><p>Mark the <var class="var">transformer</var> procedure as being a “variable
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transformer”. In practice this means that, when bound to a syntactic
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keyword, it may detect references to that keyword on the left-hand-side
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of a <code class="code">set!</code>.
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</p>
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<div class="example">
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<pre class="example-preformatted">(define bar 10)
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(define-syntax bar-alias
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(make-variable-transformer
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(lambda (x)
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(syntax-case x (set!)
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((set! var val) #'(set! bar val))
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((var arg ...) #'(bar arg ...))
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(var (identifier? #'var) #'bar)))))
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bar-alias ⇒ 10
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(set! bar-alias 20)
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bar ⇒ 20
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(set! bar 30)
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bar-alias ⇒ 30
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</pre></div>
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</dd></dl>
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<p>There is an extension to identifier-syntax which allows it to handle the
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<code class="code">set!</code> case as well:
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-identifier_002dsyntax-1"><span class="category-def">Syntax: </span><span><strong class="def-name">identifier-syntax</strong> <var class="def-var-arguments">(var exp1) ((set! var val) exp2)</var><a class="copiable-link" href="#index-identifier_002dsyntax-1"> ¶</a></span></dt>
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<dd><p>Create a variable transformer. The first clause is used for references
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to the variable in operator or operand position, and the second for
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appearances of the variable on the left-hand-side of an assignment.
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</p>
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<p>For example, the previous <code class="code">bar-alias</code> example could be expressed
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more succinctly like this:
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</p>
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<div class="example">
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<pre class="example-preformatted">(define-syntax bar-alias
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(identifier-syntax
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(var bar)
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((set! var val) (set! bar val))))
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</pre></div>
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<p>As before, the templates in <code class="code">identifier-syntax</code> forms do not need
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wrapping in <code class="code">#'</code> syntax forms.
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</p></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-Parameters.html">Syntax Parameters</a>, Previous: <a href="Defmacros.html">Lisp-style Macro Definitions</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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