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99 lines
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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>Catching Exceptions (Guile Reference Manual)</title>
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<link href="Programmatic-Error-Handling.html" rel="up" title="Programmatic Error Handling">
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<div class="subsubsection-level-extent" id="Catching-Exceptions">
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<p>
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Next: <a href="Pre_002dUnwind-Debugging.html" accesskey="n" rel="next">Pre-Unwind Debugging</a>, Up: <a href="Programmatic-Error-Handling.html" accesskey="u" rel="up">Programmatic Error Handling</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="subsubsection" id="Catching-Exceptions-1"><span>6.26.3.1 Catching Exceptions<a class="copiable-link" href="#Catching-Exceptions-1"> ¶</a></span></h4>
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<p>A common requirement is to be able to show as much useful context as
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possible when a Scheme program hits an error. The most immediate
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information about an error is the kind of error that it is – such as
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“division by zero” – and any parameters that the code which signaled
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the error chose explicitly to provide. This information originates with
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the <code class="code">error</code> or <code class="code">raise-exception</code> call (or their C code
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equivalents, if the error is detected by C code) that signals the error,
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and is passed automatically to the handler procedure of the innermost
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applicable exception handler.
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</p>
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<p>Therefore, to catch errors that occur within a chunk of Scheme code, and
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to intercept basic information about those errors, you need to execute
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that code inside the dynamic context of a <code class="code">with-exception-handler</code>,
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or the equivalent in C.
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</p>
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<p>For example, to print out a message and return #f when an error occurs,
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you might use:
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</p>
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<div class="example smalllisp lisp">
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<pre class="lisp-preformatted">(define (catch-all thunk)
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(with-exception-handler
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(lambda (exn)
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(format (current-error-port)
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"Uncaught exception: ~s\n" exn)
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#f)
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thunk
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#:unwind? #t))
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(catch-all
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(lambda () (error "Not a vegetable: tomato")))
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-| Uncaught exception: #<&exception-with-kind-and-args ...>
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⇒ #f
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</pre></div>
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<p>See <a class="xref" href="Exceptions.html">Exceptions</a>, for full details.
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</p>
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