107 lines
4.8 KiB
HTML
107 lines
4.8 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>Evaluation Model (Guile Reference Manual)</title>
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<meta name="description" content="Evaluation Model (Guile Reference Manual)">
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<meta name="keywords" content="Evaluation Model (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="Debugging.html" rel="up" title="Debugging">
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<link href="Source-Properties.html" rel="next" title="Source Properties">
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</head>
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<body lang="en">
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<div class="subsection-level-extent" id="Evaluation-Model">
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<div class="nav-panel">
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<p>
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Next: <a href="Source-Properties.html" accesskey="n" rel="next">Source Properties</a>, Up: <a href="Debugging.html" accesskey="u" rel="up">Debugging Infrastructure</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="Evaluation-and-the-Scheme-Stack"><span>6.26.1 Evaluation and the Scheme Stack<a class="copiable-link" href="#Evaluation-and-the-Scheme-Stack"> ¶</a></span></h4>
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<p>The idea of the Scheme stack is central to a lot of debugging. The
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Scheme stack is a reified representation of the pending function returns
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in an expression’s continuation. As Guile implements function calls
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using a stack, this reification takes the form of a number of nested
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stack frames, each of which corresponds to the application of a
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procedure to a set of arguments.
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</p>
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<p>A Scheme stack always exists implicitly, and can be summoned into
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concrete existence as a first-class Scheme value by the
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<code class="code">make-stack</code> call, so that an introspective Scheme program – such
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as a debugger – can present it in some way and allow the user to query
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its details. The first thing to understand, therefore, is how Guile’s
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function call convention creates the stack.
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</p>
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<p>Broadly speaking, Guile represents all control flow on a stack. Calling
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a function involves pushing an empty frame on the stack, then evaluating
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the procedure and its arguments, then fixing up the new frame so that it
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points to the old one. Frames on the stack are thus linked together. A
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tail call is the same, except it reuses the existing frame instead of
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pushing on a new one.
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</p>
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<p>In this way, the only frames that are on the stack are “active”
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frames, frames which need to do some work before the computation is
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complete. On the other hand, a function that has tail-called another
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function will not be on the stack, as it has no work left to do.
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</p>
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<p>Therefore, when an error occurs in a running program, or the program
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hits a breakpoint, or in fact at any point that the programmer chooses,
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its state at that point can be represented by a <em class="dfn">stack</em> of all the
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procedure applications that are logically in progress at that time, each
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of which is known as a <em class="dfn">frame</em>. The programmer can learn more about
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the program’s state at that point by inspecting the stack and its
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frames.
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</p>
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<ul class="mini-toc">
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<li><a href="Stack-Capture.html" accesskey="1">Stack Capture</a></li>
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<li><a href="Stacks.html" accesskey="2">Stacks</a></li>
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<li><a href="Frames.html" accesskey="3">Frames</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="Source-Properties.html">Source Properties</a>, Up: <a href="Debugging.html">Debugging Infrastructure</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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