140 lines
5.2 KiB
HTML
140 lines
5.2 KiB
HTML
|
<!DOCTYPE html>
|
||
|
|
<html>
|
||
|
|
<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
|
||
|
|
<head>
|
||
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
||
|
|
<!-- This manual documents Guile version 3.0.10.
|
||
|
|
|
||
|
|
Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
|
||
|
|
Inc.
|
||
|
|
|
||
|
|
Copyright (C) 2021 Maxime Devos
|
||
|
|
|
||
|
|
Copyright (C) 2024 Tomas Volf
|
||
|
|
|
||
|
|
|
||
|
|
Permission is granted to copy, distribute and/or modify this document
|
||
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
||
|
|
any later version published by the Free Software Foundation; with no
|
||
|
|
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
|
||
|
|
copy of the license is included in the section entitled "GNU Free
|
||
|
|
Documentation License." -->
|
||
|
|
<title>Symbol Data (Guile Reference Manual)</title>
|
||
|
|
|
||
|
|
<meta name="description" content="Symbol Data (Guile Reference Manual)">
|
||
|
|
<meta name="keywords" content="Symbol Data (Guile Reference Manual)">
|
||
|
|
<meta name="resource-type" content="document">
|
||
|
|
<meta name="distribution" content="global">
|
||
|
|
<meta name="Generator" content=".texi2any-real">
|
||
|
|
<meta name="viewport" content="width=device-width,initial-scale=1">
|
||
|
|
|
||
|
|
<link href="index.html" rel="start" title="Top">
|
||
|
|
<link href="Concept-Index.html" rel="index" title="Concept Index">
|
||
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
||
|
|
<link href="Symbols.html" rel="up" title="Symbols">
|
||
|
|
<link href="Symbol-Keys.html" rel="next" title="Symbol Keys">
|
||
|
|
<style type="text/css">
|
||
|
|
<!--
|
||
|
|
a.copiable-link {visibility: hidden; text-decoration: none; line-height: 0em}
|
||
|
|
div.example {margin-left: 3.2em}
|
||
|
|
span:hover a.copiable-link {visibility: visible}
|
||
|
|
ul.mark-bullet {list-style-type: disc}
|
||
|
|
-->
|
||
|
|
</style>
|
||
|
|
<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
|
||
|
|
|
||
|
|
|
||
|
|
</head>
|
||
|
|
|
||
|
|
<body lang="en">
|
||
|
|
<div class="subsubsection-level-extent" id="Symbol-Data">
|
||
|
|
<div class="nav-panel">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Symbol-Keys.html" accesskey="n" rel="next">Symbols as Lookup Keys</a>, Up: <a href="Symbols.html" accesskey="u" rel="up">Symbols</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>
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<h4 class="subsubsection" id="Symbols-as-Discrete-Data"><span>6.6.6.1 Symbols as Discrete Data<a class="copiable-link" href="#Symbols-as-Discrete-Data"> ¶</a></span></h4>
|
||
|
|
|
||
|
|
<p>Numbers and symbols are similar to the extent that they both lend
|
||
|
|
themselves to <code class="code">eq?</code> comparison. But symbols are more descriptive
|
||
|
|
than numbers, because a symbol’s name can be used directly to describe
|
||
|
|
the concept for which that symbol stands.
|
||
|
|
</p>
|
||
|
|
<p>For example, imagine that you need to represent some colors in a
|
||
|
|
computer program. Using numbers, you would have to choose arbitrarily
|
||
|
|
some mapping between numbers and colors, and then take care to use that
|
||
|
|
mapping consistently:
|
||
|
|
</p>
|
||
|
|
<div class="example lisp">
|
||
|
|
<pre class="lisp-preformatted">;; 1=red, 2=green, 3=purple
|
||
|
|
|
||
|
|
(if (eq? (color-of vehicle) 1)
|
||
|
|
...)
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
<p>You can make the mapping more explicit and the code more readable by
|
||
|
|
defining constants:
|
||
|
|
</p>
|
||
|
|
<div class="example lisp">
|
||
|
|
<pre class="lisp-preformatted">(define red 1)
|
||
|
|
(define green 2)
|
||
|
|
(define purple 3)
|
||
|
|
|
||
|
|
(if (eq? (color-of vehicle) red)
|
||
|
|
...)
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
<p>But the simplest and clearest approach is not to use numbers at all, but
|
||
|
|
symbols whose names specify the colors that they refer to:
|
||
|
|
</p>
|
||
|
|
<div class="example lisp">
|
||
|
|
<pre class="lisp-preformatted">(if (eq? (color-of vehicle) 'red)
|
||
|
|
...)
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
<p>The descriptive advantages of symbols over numbers increase as the set
|
||
|
|
of concepts that you want to describe grows. Suppose that a car object
|
||
|
|
can have other properties as well, such as whether it has or uses:
|
||
|
|
</p>
|
||
|
|
<ul class="itemize mark-bullet">
|
||
|
|
<li>automatic or manual transmission
|
||
|
|
</li><li>leaded or unleaded fuel
|
||
|
|
</li><li>power steering (or not).
|
||
|
|
</li></ul>
|
||
|
|
|
||
|
|
<p>Then a car’s combined property set could be naturally represented and
|
||
|
|
manipulated as a list of symbols:
|
||
|
|
</p>
|
||
|
|
<div class="example lisp">
|
||
|
|
<pre class="lisp-preformatted">(properties-of vehicle1)
|
||
|
|
⇒
|
||
|
|
(red manual unleaded power-steering)
|
||
|
|
|
||
|
|
(if (memq 'power-steering (properties-of vehicle1))
|
||
|
|
(display "Unfit people can drive this vehicle.\n")
|
||
|
|
(display "You'll need strong arms to drive this vehicle!\n"))
|
||
|
|
-|
|
||
|
|
Unfit people can drive this vehicle.
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
<p>Remember, the fundamental property of symbols that we are relying on
|
||
|
|
here is that an occurrence of <code class="code">'red</code> in one part of a program is an
|
||
|
|
<em class="emph">indistinguishable</em> symbol from an occurrence of <code class="code">'red</code> in
|
||
|
|
another part of a program; this means that symbols can usefully be
|
||
|
|
compared using <code class="code">eq?</code>. At the same time, symbols have naturally
|
||
|
|
descriptive names. This combination of efficiency and descriptive power
|
||
|
|
makes them ideal for use as discrete data.
|
||
|
|
</p>
|
||
|
|
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<div class="nav-panel">
|
||
|
|
<p>
|
||
|
|
Next: <a href="Symbol-Keys.html">Symbols as Lookup Keys</a>, Up: <a href="Symbols.html">Symbols</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>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|