140 lines
6.3 KiB
HTML
140 lines
6.3 KiB
HTML
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Symbols (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="Symbols">
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<p>
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Next: <a href="Keywords.html" accesskey="n" rel="next">Keywords</a>, Previous: <a href="Strings.html" accesskey="p" rel="prev">Strings</a>, Up: <a href="Data-Types.html" accesskey="u" rel="up">Data Types</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="Symbols-1"><span>6.6.6 Symbols<a class="copiable-link" href="#Symbols-1"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-Symbols"></a>
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<p>Symbols in Scheme are widely used in three ways: as items of discrete
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data, as lookup keys for alists and hash tables, and to denote variable
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references.
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</p>
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<p>A <em class="dfn">symbol</em> is similar to a string in that it is defined by a
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sequence of characters. The sequence of characters is known as the
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symbol’s <em class="dfn">name</em>. In the usual case — that is, where the symbol’s
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name doesn’t include any characters that could be confused with other
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elements of Scheme syntax — a symbol is written in a Scheme program by
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writing the sequence of characters that make up the name, <em class="emph">without</em>
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any quotation marks or other special syntax. For example, the symbol
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whose name is “multiply-by-2” is written, simply:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">multiply-by-2
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</pre></div>
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<p>Notice how this differs from a <em class="emph">string</em> with contents
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“multiply-by-2”, which is written with double quotation marks, like
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this:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">"multiply-by-2"
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</pre></div>
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<p>Looking beyond how they are written, symbols are different from strings
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in two important respects.
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</p>
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<p>The first important difference is uniqueness. If the same-looking
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string is read twice from two different places in a program, the result
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is two <em class="emph">different</em> string objects whose contents just happen to be
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the same. If, on the other hand, the same-looking symbol is read twice
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from two different places in a program, the result is the <em class="emph">same</em>
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symbol object both times.
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</p>
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<p>Given two read symbols, you can use <code class="code">eq?</code> to test whether they are
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the same (that is, have the same name). <code class="code">eq?</code> is the most
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efficient comparison operator in Scheme, and comparing two symbols like
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this is as fast as comparing, for example, two numbers. Given two
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strings, on the other hand, you must use <code class="code">equal?</code> or
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<code class="code">string=?</code>, which are much slower comparison operators, to
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determine whether the strings have the same contents.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(define sym1 (quote hello))
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(define sym2 (quote hello))
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(eq? sym1 sym2) ⇒ #t
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(define str1 "hello")
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(define str2 "hello")
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(eq? str1 str2) ⇒ #f
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(equal? str1 str2) ⇒ #t
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</pre></div>
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<p>The second important difference is that symbols, unlike strings, are not
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self-evaluating. This is why we need the <code class="code">(quote …)</code>s in the
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example above: <code class="code">(quote hello)</code> evaluates to the symbol named
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"hello" itself, whereas an unquoted <code class="code">hello</code> is <em class="emph">read</em> as the
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symbol named "hello" and evaluated as a variable reference … about
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which more below (see <a class="pxref" href="Symbol-Variables.html">Symbols as Denoting Variables</a>).
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</p>
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<ul class="mini-toc">
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<li><a href="Symbol-Data.html" accesskey="1">Symbols as Discrete Data</a></li>
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<li><a href="Symbol-Keys.html" accesskey="2">Symbols as Lookup Keys</a></li>
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<li><a href="Symbol-Variables.html" accesskey="3">Symbols as Denoting Variables</a></li>
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<li><a href="Symbol-Primitives.html" accesskey="4">Operations Related to Symbols</a></li>
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<li><a href="Symbol-Read-Syntax.html" accesskey="5">Extended Read Syntax for Symbols</a></li>
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<li><a href="Symbol-Uninterned.html" accesskey="6">Uninterned Symbols</a></li>
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</ul>
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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="Keywords.html">Keywords</a>, Previous: <a href="Strings.html">Strings</a>, Up: <a href="Data-Types.html">Data Types</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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