141 lines
6.6 KiB
HTML
141 lines
6.6 KiB
HTML
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<!DOCTYPE html>
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<html>
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<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<!-- This manual documents Guile version 3.0.10.
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Inc.
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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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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with no
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Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
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copy of the license is included in the section entitled "GNU Free
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Documentation License." -->
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<title>Pre-Unwind Debugging (Guile Reference Manual)</title>
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<meta name="description" content="Pre-Unwind Debugging (Guile Reference Manual)">
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<meta name="keywords" content="Pre-Unwind Debugging (Guile Reference Manual)">
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<link href="index.html" rel="start" title="Top">
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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="Programmatic-Error-Handling.html" rel="up" title="Programmatic Error Handling">
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<link href="Standard-Error-Handling.html" rel="next" title="Standard Error Handling">
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<link href="Catching-Exceptions.html" rel="prev" title="Catching Exceptions">
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<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
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</head>
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<body lang="en">
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<div class="subsubsection-level-extent" id="Pre_002dUnwind-Debugging">
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<div class="nav-panel">
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<p>
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Next: <a href="Standard-Error-Handling.html" accesskey="n" rel="next">call-with-error-handling</a>, Previous: <a href="Catching-Exceptions.html" accesskey="p" rel="prev">Catching Exceptions</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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</div>
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<hr>
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<h4 class="subsubsection" id="Pre_002dUnwind-Debugging-1"><span>6.26.3.2 Pre-Unwind Debugging<a class="copiable-link" href="#Pre_002dUnwind-Debugging-1"> ¶</a></span></h4>
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<p>Sometimes when something goes wrong, what you want is not just a
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representation of the exceptional situation, but the context that
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brought about that situation. The example in the previous section
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passed <code class="code">#:unwind #t</code> to <code class="code">with-exception-handler</code>, indicating
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that <code class="code">raise-exception</code> should unwind the stack before invoking the
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exception handler. However if you don’t take this approach and instead
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let the exception handler be invoked in the context of the
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<code class="code">raise-exception</code>, you can print a backtrace, launch a recursive
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debugger, or take other “pre-unwind” actions.
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</p>
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<p>The most basic idea would be to simply print a backtrace:
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</p>
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<div class="example">
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<pre class="example-preformatted">(define (call-with-backtrace thunk)
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(with-exception-handler
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(lambda (exn)
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(backtrace)
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(raise-exception exn))
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thunk))
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</pre></div>
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<p>Here we use the built-in <code class="code">backtrace</code> procedure to print the
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backtrace.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-backtrace-1"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">backtrace</strong> <var class="def-var-arguments">[highlights]</var><a class="copiable-link" href="#index-backtrace-1"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fbacktrace_005fwith_005fhighlights"><span class="category-def">C Function: </span><span><strong class="def-name">scm_backtrace_with_highlights</strong> <var class="def-var-arguments">(highlights)</var><a class="copiable-link" href="#index-scm_005fbacktrace_005fwith_005fhighlights"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fbacktrace"><span class="category-def">C Function: </span><span><strong class="def-name">scm_backtrace</strong> <var class="def-var-arguments">()</var><a class="copiable-link" href="#index-scm_005fbacktrace"> ¶</a></span></dt>
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<dd><p>Display a backtrace of the current stack to the current output port. If
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<var class="var">highlights</var> is given it should be a list; the elements of this list
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will be highlighted wherever they appear in the backtrace.
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</p></dd></dl>
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<p>By re-raising the exception, <code class="code">call-with-backtrace</code> doesn’t actually
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handle the error. We could define a version that instead aborts the
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computation:
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</p>
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<div class="example">
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<pre class="example-preformatted">(use-modules (ice-9 control))
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(define (call-with-backtrace thunk)
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(let/ec cancel
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(with-exception-handler
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(lambda (exn)
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(backtrace)
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(cancel #f))
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thunk)))
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</pre></div>
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<p>In this second example, we use an escape continuation to abort the
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computation after printing the backtrace, returning <code class="code">#f</code> instead.
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</p>
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<p>It could be that you want to only print a limited backtrace. In that
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case, use <code class="code">start-stack</code>:
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</p>
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<div class="example">
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<pre class="example-preformatted">(use-modules (ice-9 control))
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(define (call-with-backtrace thunk)
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(let/ec cancel
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(start-stack 'stack-with-backtrace
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(with-exception-handler
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(lambda (exn)
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(backtrace)
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(cancel #f))
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thunk))))
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</pre></div>
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<p>There are also more powerful, programmatic ways to walk the stack using
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<code class="code">make-stack</code> and friends; see the API described in <a class="ref" href="Stacks.html">Stacks</a> and
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<a class="ref" href="Frames.html">Frames</a>.
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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="Standard-Error-Handling.html">call-with-error-handling</a>, Previous: <a href="Catching-Exceptions.html">Catching Exceptions</a>, Up: <a href="Programmatic-Error-Handling.html">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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</div>
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</body>
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</html>
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