99 lines
4.1 KiB
HTML
99 lines
4.1 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>A Simple Representation (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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<div class="subsection-level-extent" id="A-Simple-Representation">
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<p>
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Next: <a href="Faster-Integers.html" accesskey="n" rel="next">Faster Integers</a>, Up: <a href="Data-Representation.html" accesskey="u" rel="up">Data Representation</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="A-Simple-Representation-1"><span>9.2.1 A Simple Representation<a class="copiable-link" href="#A-Simple-Representation-1"> ¶</a></span></h4>
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<p>The simplest way to represent Scheme values in C would be to represent
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each value as a pointer to a structure containing a type indicator,
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followed by a union carrying the real value. Assuming that <code class="code">SCM</code> is
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the name of our universal type, we can write:
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</p>
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<div class="example">
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<pre class="example-preformatted">enum type { integer, pair, string, vector, ... };
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typedef struct value *SCM;
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struct value {
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enum type type;
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union {
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int integer;
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struct { SCM car, cdr; } pair;
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struct { int length; char *elts; } string;
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struct { int length; SCM *elts; } vector;
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...
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} value;
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};
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</pre></div>
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<p>with the ellipses replaced with code for the remaining Scheme types.
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</p>
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<p>This representation is sufficient to implement all of Scheme’s
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semantics. If <var class="var">x</var> is an <code class="code">SCM</code> value:
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</p><ul class="itemize mark-bullet">
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<li>To test if <var class="var">x</var> is an integer, we can write <code class="code"><var class="var">x</var>->type == integer</code>.
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</li><li>To find its value, we can write <code class="code"><var class="var">x</var>->value.integer</code>.
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</li><li>To test if <var class="var">x</var> is a vector, we can write <code class="code"><var class="var">x</var>->type == vector</code>.
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</li><li>If we know <var class="var">x</var> is a vector, we can write
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<code class="code"><var class="var">x</var>->value.vector.elts[0]</code> to refer to its first element.
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</li><li>If we know <var class="var">x</var> is a pair, we can write
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<code class="code"><var class="var">x</var>->value.pair.car</code> to extract its car.
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</li></ul>
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