101 lines
4.8 KiB
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101 lines
4.8 KiB
HTML
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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<title>Values and Variables (Guile Reference Manual)</title>
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<div class="subsection-level-extent" id="Values-and-Variables">
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<p>
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Next: <a href="Definition.html" accesskey="n" rel="next">Defining and Setting Variables</a>, Previous: <a href="Latent-Typing.html" accesskey="p" rel="prev">Latent Typing</a>, Up: <a href="About-Data.html" accesskey="u" rel="up">Data Types, Values and Variables</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="Values-and-Variables-1"><span>3.1.2 Values and Variables<a class="copiable-link" href="#Values-and-Variables-1"> ¶</a></span></h4>
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<p>Scheme provides many data types that you can use to represent your data.
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Primitive types include characters, strings, numbers and procedures.
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Compound types, which allow a group of primitive and compound values to
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be stored together, include lists, pairs, vectors and multi-dimensional
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arrays. In addition, Guile allows applications to define their own data
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types, with the same status as the built-in standard Scheme types.
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</p>
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<p>As a Scheme program runs, values of all types pop in and out of
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existence. Sometimes values are stored in variables, but more commonly
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they pass seamlessly from being the result of one computation to being
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one of the parameters for the next.
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</p>
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<p>Consider an example. A string value is created because the interpreter
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reads in a literal string from your program’s source code. Then a
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numeric value is created as the result of calculating the length of the
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string. A second numeric value is created by doubling the calculated
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length. Finally the program creates a list with two elements – the
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doubled length and the original string itself – and stores this list in
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a program variable.
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</p>
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<p>All of the values involved here – in fact, all values in Scheme –
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carry their type with them. In other words, every value “knows,” at
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runtime, what kind of value it is. A number, a string, a list,
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whatever.
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</p>
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<p>A variable, on the other hand, has no fixed type. A variable –
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<code class="code">x</code>, say – is simply the name of a location – a box – in which
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you can store any kind of Scheme value. So the same variable in a
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program may hold a number at one moment, a list of procedures the next,
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and later a pair of strings. The “type” of a variable – insofar as
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the idea is meaningful at all – is simply the type of whatever value
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the variable happens to be storing at a particular moment.
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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="Definition.html">Defining and Setting Variables</a>, Previous: <a href="Latent-Typing.html">Latent Typing</a>, Up: <a href="About-Data.html">Data Types, Values and Variables</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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