127 lines
6.2 KiB
HTML
127 lines
6.2 KiB
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<!DOCTYPE html>
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<html>
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<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<!-- This manual documents Guile version 3.0.10.
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Inc.
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with no
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Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
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copy of the license is included in the section entitled "GNU Free
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Documentation License." -->
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<title>Local Evaluation (Guile Reference Manual)</title>
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<meta name="description" content="Local Evaluation (Guile Reference Manual)">
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<meta name="keywords" content="Local Evaluation (Guile Reference Manual)">
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<link href="index.html" rel="start" title="Top">
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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="Read_002fLoad_002fEval_002fCompile.html" rel="up" title="Read/Load/Eval/Compile">
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<link href="Local-Inclusion.html" rel="next" title="Local Inclusion">
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<link href="Delayed-Evaluation.html" rel="prev" title="Delayed Evaluation">
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<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
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</head>
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<body lang="en">
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<div class="subsection-level-extent" id="Local-Evaluation">
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<div class="nav-panel">
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<p>
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Next: <a href="Local-Inclusion.html" accesskey="n" rel="next">Local Inclusion</a>, Previous: <a href="Delayed-Evaluation.html" accesskey="p" rel="prev">Delayed Evaluation</a>, Up: <a href="Read_002fLoad_002fEval_002fCompile.html" accesskey="u" rel="up">Reading and Evaluating Scheme Code</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="Local-Evaluation-1"><span>6.16.11 Local Evaluation<a class="copiable-link" href="#Local-Evaluation-1"> ¶</a></span></h4>
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<p>Guile includes a facility to capture a lexical environment, and later
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evaluate a new expression within that environment. This code is
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implemented in a module.
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</p>
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<div class="example">
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<pre class="example-preformatted">(use-modules (ice-9 local-eval))
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</pre></div>
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<dl class="first-deffn">
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<dt class="deffn" id="index-the_002denvironment"><span class="category-def">syntax: </span><span><strong class="def-name">the-environment</strong><a class="copiable-link" href="#index-the_002denvironment"> ¶</a></span></dt>
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<dd><p>Captures and returns a lexical environment for use with
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<code class="code">local-eval</code> or <code class="code">local-compile</code>.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-local_002deval"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">local-eval</strong> <var class="def-var-arguments">exp env</var><a class="copiable-link" href="#index-local_002deval"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005flocal_005feval"><span class="category-def">C Function: </span><span><strong class="def-name">scm_local_eval</strong> <var class="def-var-arguments">(exp, env)</var><a class="copiable-link" href="#index-scm_005flocal_005feval"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-local_002dcompile"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">local-compile</strong> <var class="def-var-arguments">exp env [opts=()]</var><a class="copiable-link" href="#index-local_002dcompile"> ¶</a></span></dt>
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<dd><p>Evaluate or compile the expression <var class="var">exp</var> in the lexical environment
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<var class="var">env</var>.
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</p></dd></dl>
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<p>Here is a simple example, illustrating that it is the variable
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that gets captured, not just its value at one point in time.
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</p>
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<div class="example">
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<pre class="example-preformatted">(define e (let ((x 100)) (the-environment)))
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(define fetch-x (local-eval '(lambda () x) e))
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(fetch-x)
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⇒ 100
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(local-eval '(set! x 42) e)
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(fetch-x)
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⇒ 42
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</pre></div>
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<p>While <var class="var">exp</var> is evaluated within the lexical environment of
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<code class="code">(the-environment)</code>, it has the dynamic environment of the call to
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<code class="code">local-eval</code>.
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</p>
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<p><code class="code">local-eval</code> and <code class="code">local-compile</code> can only evaluate
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expressions, not definitions.
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</p>
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<div class="example">
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<pre class="example-preformatted">(local-eval '(define foo 42)
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(let ((x 100)) (the-environment)))
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⇒ syntax error: definition in expression context
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</pre></div>
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<p>Note that the current implementation of <code class="code">(the-environment)</code> only
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captures “normal” lexical bindings, and pattern variables bound by
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<code class="code">syntax-case</code>. It does not currently capture local syntax
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transformers bound by <code class="code">let-syntax</code>, <code class="code">letrec-syntax</code> or
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non-top-level <code class="code">define-syntax</code> forms. Any attempt to reference such
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captured syntactic keywords via <code class="code">local-eval</code> or
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<code class="code">local-compile</code> produces an error.
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</p>
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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="Local-Inclusion.html">Local Inclusion</a>, Previous: <a href="Delayed-Evaluation.html">Delayed Evaluation</a>, Up: <a href="Read_002fLoad_002fEval_002fCompile.html">Reading and Evaluating Scheme Code</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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</html>
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