109 lines
4.9 KiB
HTML
109 lines
4.9 KiB
HTML
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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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<title>About Environments (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="About-Closure.html" rel="up" title="About Closure">
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<div class="subsection-level-extent" id="About-Environments">
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<p>
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Next: <a href="Local-Variables.html" accesskey="n" rel="next">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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<hr>
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<h4 class="subsection" id="Names_002c-Locations_002c-Values-and-Environments"><span>3.4.1 Names, Locations, Values and Environments<a class="copiable-link" href="#Names_002c-Locations_002c-Values-and-Environments"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-location"></a>
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<a class="index-entry-id" id="index-environment"></a>
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<a class="index-entry-id" id="index-vcell"></a>
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<a class="index-entry-id" id="index-top-level-environment"></a>
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<a class="index-entry-id" id="index-environment_002c-top-level"></a>
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<p>We said earlier that a variable name in a Scheme program is associated
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with a location in which any kind of Scheme value may be stored.
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(Incidentally, the term “vcell” is often used in Lisp and Scheme
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circles as an alternative to “location”.) Thus part of what we mean
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when we talk about “creating a variable” is in fact establishing an
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association between a name, or identifier, that is used by the Scheme
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program code, and the variable location to which that name refers.
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Although the value that is stored in that location may change, the
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location to which a given name refers is always the same.
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</p>
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<p>We can illustrate this by breaking down the operation of the
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<code class="code">define</code> syntax into three parts: <code class="code">define</code>
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</p>
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<ul class="itemize mark-bullet">
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<li>creates a new location
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</li><li>establishes an association between that location and the name specified
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as the first argument of the <code class="code">define</code> expression
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</li><li>stores in that location the value obtained by evaluating the second
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argument of the <code class="code">define</code> expression.
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</li></ul>
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<p>A collection of associations between names and locations is called an
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<em class="dfn">environment</em>. When you create a top level variable in a program
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using <code class="code">define</code>, the name-location association for that variable is
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added to the “top level” environment. The “top level” environment
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also includes name-location associations for all the procedures that are
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supplied by standard Scheme.
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</p>
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<p>It is also possible to create environments other than the top level one,
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and to create variable bindings, or name-location associations, in those
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environments. This ability is a key ingredient in the concept of
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closure; the next subsection shows how it is done.
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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-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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