100 lines
4.6 KiB
HTML
100 lines
4.6 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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Permission is granted to copy, distribute and/or modify this document
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<title>Latent Typing (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-Data.html" rel="up" title="About Data">
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<link href="Values-and-Variables.html" rel="next" title="Values and Variables">
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<body lang="en">
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<div class="subsection-level-extent" id="Latent-Typing">
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<div class="nav-panel">
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<p>
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Next: <a href="Values-and-Variables.html" accesskey="n" rel="next">Values and Variables</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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</div>
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<hr>
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<h4 class="subsection" id="Latent-Typing-1"><span>3.1.1 Latent Typing<a class="copiable-link" href="#Latent-Typing-1"> ¶</a></span></h4>
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<p>The term <em class="dfn">latent typing</em> is used to describe a computer language,
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such as Scheme, for which you cannot, <em class="emph">in general</em>, simply look at
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a program’s source code and determine what type of data will be
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associated with a particular variable, or with the result of a
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particular expression.
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</p>
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<p>Sometimes, of course, you <em class="emph">can</em> tell from the code what the type of
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an expression will be. If you have a line in your program that sets the
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variable <code class="code">x</code> to the numeric value 1, you can be certain that,
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immediately after that line has executed (and in the absence of multiple
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threads), <code class="code">x</code> has the numeric value 1. Or if you write a procedure
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that is designed to concatenate two strings, it is likely that the rest
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of your application will always invoke this procedure with two string
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parameters, and quite probable that the procedure would go wrong in some
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way if it was ever invoked with parameters that were not both strings.
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</p>
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<p>Nevertheless, the point is that there is nothing in Scheme which
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requires the procedure parameters always to be strings, or <code class="code">x</code>
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always to hold a numeric value, and there is no way of declaring in your
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program that such constraints should always be obeyed. In the same
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vein, there is no way to declare the expected type of a procedure’s
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return value.
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</p>
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<p>Instead, the types of variables and expressions are only known – in
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general – at run time. If you <em class="emph">need</em> to check at some point that
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a value has the expected type, Scheme provides run time procedures that
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you can invoke to do so. But equally, it can be perfectly valid for two
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separate invocations of the same procedure to specify arguments with
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different types, and to return values with different types.
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</p>
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<p>The next subsection explains what this means in practice, for the ways
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that Scheme programs use data types, values and variables.
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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="Values-and-Variables.html">Values and Variables</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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</body>
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