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153 lines
6.5 KiB
HTML
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<title>Eval When (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="Eval-When">
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<p>
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Next: <a href="Macro-Expansion.html" accesskey="n" rel="next">Macro Expansion</a>, Previous: <a href="Syntax-Parameters.html" accesskey="p" rel="prev">Syntax Parameters</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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<h4 class="subsection" id="Eval_002dwhen"><span>6.8.8 Eval-when<a class="copiable-link" href="#Eval_002dwhen"> ¶</a></span></h4>
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<p>As <code class="code">syntax-case</code> macros have the whole power of Scheme available to them,
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they present a problem regarding time: when a macro runs, what parts of the
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program are available for the macro to use?
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</p>
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<p>The default answer to this question is that when you import a module (via
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<code class="code">define-module</code> or <code class="code">use-modules</code>), that module will be loaded up at
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expansion-time, as well as at run-time. Additionally, top-level syntactic
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definitions within one compilation unit made by <code class="code">define-syntax</code> are also
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evaluated at expansion time, in the order that they appear in the compilation
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unit (file).
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</p>
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<p>But if a syntactic definition needs to call out to a normal procedure at
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expansion-time, it might well need special declarations to indicate that
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the procedure should be made available at expansion-time.
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</p>
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<p>For example, the following code tries to embed a compilation
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timestamp in the compiled bytecode using a macro that expands
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to the date as a string literal. It will work at a REPL, but
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not in a file, as it cannot be byte-compiled:
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</p>
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<div class="example">
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<pre class="example-preformatted">(use-modules (srfi srfi-19))
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(define start-date (date->string (current-date)))
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(define-syntax *compilation-date*
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(lambda (syntax)
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start-date))
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(display *compilation-date*)
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(newline)
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</pre></div>
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<p>It works at a REPL because the expressions are evaluated one-by-one, in order,
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but if placed in a file, the expressions are expanded one-by-one, but not
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evaluated until the compiled file is loaded.
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</p>
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<p>The fix is to use <code class="code">eval-when</code>.
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</p>
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<div class="example">
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<pre class="example-preformatted">(use-modules (srfi srfi-19))
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(eval-when (expand load eval)
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(define start-date (date->string (current-date))))
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(define-syntax *compilation-date*
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(lambda (syntax)
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start-date))
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(display *compilation-date*)
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(newline)
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</pre></div>
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<dl class="first-deffn">
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<dt class="deffn" id="index-eval_002dwhen"><span class="category-def">Syntax: </span><span><strong class="def-name">eval-when</strong> <var class="def-var-arguments">conditions exp...</var><a class="copiable-link" href="#index-eval_002dwhen"> ¶</a></span></dt>
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<dd><p>Evaluate <var class="var">exp...</var> under the given <var class="var">conditions</var>. Valid
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conditions include:
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</p>
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<dl class="table">
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<dt><code class="code">expand</code></dt>
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<dd><p>Evaluate during macro expansion, whether compiling or not.
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</p>
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</dd>
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<dt><code class="code">load</code></dt>
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<dd><p>Evaluate during the evaluation phase of compiled code, e.g. when loading
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a compiled module or running compiled code at the REPL.
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</p>
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</dd>
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<dt><code class="code">eval</code></dt>
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<dd><p>Evaluate during the evaluation phase of non-compiled code.
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</p>
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</dd>
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<dt><code class="code">compile</code></dt>
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<dd><p>Evaluate during macro expansion, but only when compiling.
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</p></dd>
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</dl>
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<p>In other words, when using the primitive evaluator, <code class="code">eval-when</code>
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expressions with <code class="code">expand</code> are run during macro expansion, and those
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with <code class="code">eval</code> are run during the evaluation phase.
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</p>
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<p>When using the compiler, <code class="code">eval-when</code> expressions with either
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<code class="code">expand</code> or <code class="code">compile</code> are run during macro expansion, and
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those with <code class="code">load</code> are run during the evaluation phase.
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</p>
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<p>When in doubt, use the three conditions <code class="code">(expand load eval)</code>, as in
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the example above. Other uses of <code class="code">eval-when</code> may void your
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warranty or poison your cat.
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</p></dd></dl>
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</div>
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<hr>
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<p>
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Next: <a href="Macro-Expansion.html">Macro Expansion</a>, Previous: <a href="Syntax-Parameters.html">Syntax Parameters</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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