155 lines
6.3 KiB
HTML
155 lines
6.3 KiB
HTML
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<!DOCTYPE html>
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<html>
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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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any later version published by the Free Software Foundation; with no
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copy of the license is included in the section entitled "GNU Free
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<title>Internal Definitions (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="Binding-Constructs.html" rel="up" title="Binding Constructs">
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<link href="Binding-Reflection.html" rel="next" title="Binding Reflection">
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<link href="Local-Bindings.html" rel="prev" title="Local Bindings">
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</head>
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<body lang="en">
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<div class="subsection-level-extent" id="Internal-Definitions">
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<div class="nav-panel">
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<p>
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Next: <a href="Binding-Reflection.html" accesskey="n" rel="next">Querying variable bindings</a>, Previous: <a href="Local-Bindings.html" accesskey="p" rel="prev">Local Variable Bindings</a>, Up: <a href="Binding-Constructs.html" accesskey="u" rel="up">Definitions and Variable Bindings</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="Internal-definitions"><span>6.10.3 Internal definitions<a class="copiable-link" href="#Internal-definitions"> ¶</a></span></h4>
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<p>A <code class="code">define</code> form which appears inside the body of a <code class="code">lambda</code>,
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<code class="code">let</code>, <code class="code">let*</code>, <code class="code">letrec</code>, <code class="code">letrec*</code> or equivalent
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expression is called an <em class="dfn">internal definition</em>. An internal
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definition differs from a top level definition (see <a class="pxref" href="Top-Level.html">Top Level Variable Definitions</a>),
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because the definition is only visible inside the complete body of the
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enclosing form. Let us examine the following example.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(let ((frumble "froz"))
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(define banana (lambda () (apple 'peach)))
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(define apple (lambda (x) x))
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(banana))
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⇒
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peach
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</pre></div>
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<p>Here the enclosing form is a <code class="code">let</code>, so the <code class="code">define</code>s in the
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<code class="code">let</code>-body are internal definitions. Because the scope of the
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internal definitions is the <strong class="strong">complete</strong> body of the
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<code class="code">let</code>-expression, the <code class="code">lambda</code>-expression which gets bound to
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the variable <code class="code">banana</code> may refer to the variable <code class="code">apple</code>, even
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though its definition appears lexically <em class="emph">after</em> the definition of
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<code class="code">banana</code>. This is because a sequence of internal definition acts
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as if it were a <code class="code">letrec*</code> expression.
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(let ()
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(define a 1)
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(define b 2)
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(+ a b))
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</pre></div>
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<p>is equivalent to
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(let ()
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(letrec* ((a 1) (b 2))
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(+ a b)))
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</pre></div>
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<p>Internal definitions may be mixed with non-definition expressions. If
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an expression precedes a definition, it is treated as if it were a
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definition of an unreferenced variable. So this:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(let ()
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(define a 1)
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(foo)
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(define b 2)
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(+ a b))
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</pre></div>
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<p>is equivalent to
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(let ()
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(letrec* ((a 1) (_ (begin (foo) #f)) (b 2))
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(+ a b)))
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</pre></div>
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<p>Another noteworthy difference to top level definitions is that within
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one group of internal definitions all variable names must be distinct.
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Whereas on the top level a second define for a given variable acts like
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a <code class="code">set!</code>, for internal definitions, duplicate bound identifiers
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signals an error.
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</p>
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<p>As a historical note, it used to be that internal bindings were expanded
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in terms of <code class="code">letrec</code>, not <code class="code">letrec*</code>. This was the situation
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for the R5RS report and before. However with the R6RS, it was recognized
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that sequential definition was a more intuitive expansion, as in the
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following case:
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</p>
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<div class="example lisp">
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<pre class="lisp-preformatted">(let ()
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(define a 1)
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(define b (+ a a))
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(+ a b))
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</pre></div>
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<p>Guile decided to follow the R6RS in this regard, and now expands
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internal definitions using <code class="code">letrec*</code>. Relatedly, it used to be
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that internal definitions had to precede all expressions in the body;
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this restriction was relaxed in Guile 3.0.
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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="Binding-Reflection.html">Querying variable bindings</a>, Previous: <a href="Local-Bindings.html">Local Variable Bindings</a>, Up: <a href="Binding-Constructs.html">Definitions and Variable Bindings</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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