117 lines
5.9 KiB
HTML
117 lines
5.9 KiB
HTML
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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>Chaining (Guile Reference Manual)</title>
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<meta name="description" content="Chaining (Guile Reference Manual)">
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<meta name="keywords" content="Chaining (Guile Reference Manual)">
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<meta name="viewport" content="width=device-width,initial-scale=1">
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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="About-Closure.html" rel="up" title="About Closure">
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<link href="Lexical-Scope.html" rel="next" title="Lexical Scope">
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<link href="Local-Variables.html" rel="prev" title="Local Variables">
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<style type="text/css">
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a.copiable-link {visibility: hidden; text-decoration: none; line-height: 0em}
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</style>
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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="Chaining">
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<div class="nav-panel">
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<p>
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Next: <a href="Lexical-Scope.html" accesskey="n" rel="next">Lexical Scope</a>, Previous: <a href="Local-Variables.html" accesskey="p" rel="prev">Local Variables and Environments</a>, Up: <a href="About-Closure.html" accesskey="u" rel="up">The Concept of Closure</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="Environment-Chaining"><span>3.4.3 Environment Chaining<a class="copiable-link" href="#Environment-Chaining"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-shadowing-an-imported-variable-binding"></a>
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<a class="index-entry-id" id="index-chaining-environments"></a>
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<p>In the example of the previous subsection, we glossed over an important
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point. The body of the <code class="code">let</code> expression in that example refers not
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only to the local variable <code class="code">s</code>, but also to the top level variables
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<code class="code">a</code>, <code class="code">b</code>, <code class="code">c</code> and <code class="code">sqrt</code>. (<code class="code">sqrt</code> is the
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standard Scheme procedure for calculating a square root.) If the body
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of the <code class="code">let</code> expression is evaluated in the context of the
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<em class="emph">local</em> <code class="code">let</code> environment, how does the evaluation get at the
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values of these top level variables?
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</p>
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<p>The answer is that the local environment created by a <code class="code">let</code>
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expression automatically has a reference to its containing environment
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— in this case the top level environment — and that the Scheme
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interpreter automatically looks for a variable binding in the containing
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environment if it doesn’t find one in the local environment. More
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generally, every environment except for the top level one has a
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reference to its containing environment, and the interpreter keeps
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searching back up the chain of environments — from most local to top
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level — until it either finds a variable binding for the required
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identifier or exhausts the chain.
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</p>
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<p>This description also determines what happens when there is more than
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one variable binding with the same name. Suppose, continuing the
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example of the previous subsection, that there was also a pre-existing
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top level variable <code class="code">s</code> created by the expression:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define s "Some beans, my lord!")
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</pre></div>
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<p>Then both the top level environment and the local <code class="code">let</code> environment
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would contain bindings for the name <code class="code">s</code>. When evaluating code
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within the <code class="code">let</code> body, the interpreter looks first in the local
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<code class="code">let</code> environment, and so finds the binding for <code class="code">s</code> created by
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the <code class="code">let</code> syntax. Even though this environment has a reference to
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the top level environment, which also has a binding for <code class="code">s</code>, the
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interpreter doesn’t get as far as looking there. When evaluating code
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outside the <code class="code">let</code> body, the interpreter looks up variable names in
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the top level environment, so the name <code class="code">s</code> refers to the top level
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variable.
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</p>
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<p>Within the <code class="code">let</code> body, the binding for <code class="code">s</code> in the local
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environment is said to <em class="dfn">shadow</em> the binding for <code class="code">s</code> in the top
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level environment.
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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="Lexical-Scope.html">Lexical Scope</a>, Previous: <a href="Local-Variables.html">Local Variables and Environments</a>, Up: <a href="About-Closure.html">The Concept of Closure</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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