117 lines
5.6 KiB
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117 lines
5.6 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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<title>Variables and the VM (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="A-Virtual-Machine-for-Guile.html" rel="up" title="A Virtual Machine for Guile">
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<div class="subsection-level-extent" id="Variables-and-the-VM">
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<div class="nav-panel">
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<p>
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Next: <a href="VM-Programs.html" accesskey="n" rel="next">Compiled Procedures are VM Programs</a>, Previous: <a href="Stack-Layout.html" accesskey="p" rel="prev">Stack Layout</a>, Up: <a href="A-Virtual-Machine-for-Guile.html" accesskey="u" rel="up">A Virtual Machine for Guile</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="Variables-and-the-VM-1"><span>9.3.4 Variables and the VM<a class="copiable-link" href="#Variables-and-the-VM-1"> ¶</a></span></h4>
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<p>Consider the following Scheme code as an example:
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</p>
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<div class="example">
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<pre class="example-preformatted"> (define (foo a)
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(lambda (b) (vector foo a b)))
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</pre></div>
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<p>Within the lambda expression, <code class="code">foo</code> is a top-level variable,
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<code class="code">a</code> is a lexically captured variable, and <code class="code">b</code> is a local
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variable.
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</p>
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<p>Another way to refer to <code class="code">a</code> and <code class="code">b</code> is to say that <code class="code">a</code> is
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a “free” variable, since it is not defined within the lambda, and
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<code class="code">b</code> is a “bound” variable. These are the terms used in the
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<em class="dfn">lambda calculus</em>, a mathematical notation for describing functions.
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The lambda calculus is useful because it is a language in which to
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reason precisely about functions and variables. It is especially good
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at describing scope relations, and it is for that reason that we mention
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it here.
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</p>
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<p>Guile allocates all variables on the stack. When a lexically enclosed
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procedure with free variables—a <em class="dfn">closure</em>—is created, it copies
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those variables into its free variable vector. References to free
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variables are then redirected through the free variable vector.
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</p>
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<p>If a variable is ever <code class="code">set!</code>, however, it will need to be
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heap-allocated instead of stack-allocated, so that different closures
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that capture the same variable can see the same value. Also, this
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allows continuations to capture a reference to the variable, instead
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of to its value at one point in time. For these reasons, <code class="code">set!</code>
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variables are allocated in “boxes”—actually, in variable cells.
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See <a class="xref" href="Variables.html">Variables</a>, for more information. References to <code class="code">set!</code>
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variables are indirected through the boxes.
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</p>
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<p>Thus perhaps counterintuitively, what would seem “closer to the
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metal”, viz <code class="code">set!</code>, actually forces an extra memory allocation and
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indirection. Sometimes Guile’s optimizer can remove this allocation,
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but not always.
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</p>
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<p>Going back to our example, <code class="code">b</code> may be allocated on the stack, as
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it is never mutated.
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</p>
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<p><code class="code">a</code> may also be allocated on the stack, as it too is never
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mutated. Within the enclosed lambda, its value will be copied into
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(and referenced from) the free variables vector.
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</p>
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<p><code class="code">foo</code> is a top-level variable, because <code class="code">foo</code> is not
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lexically bound in this example.
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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="VM-Programs.html">Compiled Procedures are VM Programs</a>, Previous: <a href="Stack-Layout.html">Stack Layout</a>, Up: <a href="A-Virtual-Machine-for-Guile.html">A Virtual Machine for Guile</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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