119 lines
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119 lines
5.1 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>Exceptions (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="Control-Mechanisms.html" rel="up" title="Control Mechanisms">
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<div class="subsection-level-extent" id="Exceptions">
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<div class="nav-panel">
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<p>
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Next: <a href="Error-Reporting.html" accesskey="n" rel="next">Procedures for Signaling Errors</a>, Previous: <a href="Multiple-Values.html" accesskey="p" rel="prev">Returning and Accepting Multiple Values</a>, Up: <a href="Control-Mechanisms.html" accesskey="u" rel="up">Controlling the Flow of Program Execution</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="Exceptions-1"><span>6.11.8 Exceptions<a class="copiable-link" href="#Exceptions-1"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-error-handling"></a>
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<a class="index-entry-id" id="index-exception-handling"></a>
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<p>What happens when things go wrong? Guile’s exception facility exists to
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help answer this question, allowing programs to describe the problem and
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to handle the situation in a flexible way.
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</p>
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<p>When a program runs into a problem, such as division by zero, it will
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raise an exception. Sometimes exceptions get raised by Guile on a
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program’s behalf. Sometimes a program will want to raise exceptions of
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its own. Raising an exception stops the current computation and instead
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invokes the current exception handler, passing it an exception object
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describing the unexpected situation.
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</p>
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<p>Usually an exception handler will unwind the computation back to some
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kind of safe point. For example, typical logic for a key press driven
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application might look something like this:
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</p>
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<div class="example">
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<pre class="example-preformatted">main-loop:
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read the next key press and call dispatch-key
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dispatch-key:
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lookup the key in a keymap and call an appropriate procedure,
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say find-file
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find-file:
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interactively read the required file name, then call
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find-specified-file
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find-specified-file:
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check whether file exists; if not, raise an exception
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...
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</pre></div>
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<p>In this case, <code class="code">main-loop</code> can install an exception handler that
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would cause any exception raised inside <code class="code">dispatch-key</code> to print a
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warning and jump back to the main loop.
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</p>
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<p>The following subsections go into more detail about exception objects,
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raising exceptions, and handling exceptions. It also presents a
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historical interface that was used in Guile’s first 25 years and which
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won’t be going away any time soon.
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</p>
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<ul class="mini-toc">
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<li><a href="Exception-Objects.html" accesskey="1">Exception Objects</a></li>
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<li><a href="Raising-and-Handling-Exceptions.html" accesskey="2">Raising and Handling Exceptions</a></li>
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<li><a href="Throw-and-Catch.html" accesskey="3">Throw and Catch</a></li>
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<li><a href="Exceptions-and-C.html" accesskey="4">Exceptions and C</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="Error-Reporting.html">Procedures for Signaling Errors</a>, Previous: <a href="Multiple-Values.html">Returning and Accepting Multiple Values</a>, Up: <a href="Control-Mechanisms.html">Controlling the Flow of Program Execution</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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