168 lines
6.7 KiB
HTML
168 lines
6.7 KiB
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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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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>Tail Calls (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="About-Expressions.html" rel="up" title="About Expressions">
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<link href="The-REPL.html" rel="next" title="The REPL">
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<link href="Evaluating.html" rel="prev" title="Evaluating">
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</head>
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<body lang="en">
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<div class="subsection-level-extent" id="Tail-Calls">
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<div class="nav-panel">
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<p>
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Next: <a href="The-REPL.html" accesskey="n" rel="next">Using the Guile REPL</a>, Previous: <a href="Evaluating.html" accesskey="p" rel="prev">Evaluating Expressions and Executing Programs</a>, Up: <a href="About-Expressions.html" accesskey="u" rel="up">Expressions and Evaluation</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="Tail-calls"><span>3.3.2 Tail calls<a class="copiable-link" href="#Tail-calls"> ¶</a></span></h4>
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<a class="index-entry-id" id="index-tail-calls"></a>
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<a class="index-entry-id" id="index-recursion"></a>
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<p>Scheme is “properly tail recursive”, meaning that tail calls or
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recursions from certain contexts do not consume stack space or other
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resources and can therefore be used on arbitrarily large data or for
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an arbitrarily long calculation. Consider for example,
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</p>
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<div class="example">
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<pre class="example-preformatted">(define (foo n)
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(display n)
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(newline)
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(foo (1+ n)))
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(foo 1)
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-|
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1
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2
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3
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...
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</pre></div>
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<p><code class="code">foo</code> prints numbers infinitely, starting from the given <var class="var">n</var>.
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It’s implemented by printing <var class="var">n</var> then recursing to itself to print
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<em class="math"><var class="var">n</var>+1</em> and so on. This recursion is a tail call, it’s the
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last thing done, and in Scheme such tail calls can be made without
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limit.
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</p>
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<p>Or consider a case where a value is returned, a version of the SRFI-1
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<code class="code">last</code> function (see <a class="pxref" href="SRFI_002d1-Selectors.html">Selectors</a>) returning the last
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element of a list,
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</p>
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<div class="example">
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<pre class="example-preformatted">(define (my-last lst)
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(if (null? (cdr lst))
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(car lst)
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(my-last (cdr lst))))
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(my-last '(1 2 3)) ⇒ 3
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</pre></div>
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<p>If the list has more than one element, <code class="code">my-last</code> applies itself
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to the <code class="code">cdr</code>. This recursion is a tail call, there’s no code
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after it, and the return value is the return value from that call. In
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Scheme this can be used on an arbitrarily long list argument.
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</p>
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<br>
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<p>A proper tail call is only available from certain contexts, namely the
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following special form positions,
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</p>
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<ul class="itemize mark-bullet">
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<li><code class="code">and</code> — last expression
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</li><li><code class="code">begin</code> — last expression
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</li><li><code class="code">case</code> — last expression in each clause
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</li><li><code class="code">cond</code> — last expression in each clause, and the call to a
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<code class="code">=></code> procedure is a tail call
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</li><li><code class="code">do</code> — last result expression
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</li><li><code class="code">if</code> — “true” and “false” leg expressions
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</li><li><code class="code">lambda</code> — last expression in body
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</li><li><code class="code">let</code>, <code class="code">let*</code>, <code class="code">letrec</code>, <code class="code">let-syntax</code>,
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<code class="code">letrec-syntax</code> — last expression in body
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</li><li><code class="code">or</code> — last expression
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</li></ul>
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<p>The following core functions make tail calls,
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</p>
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<ul class="itemize mark-bullet">
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<li><code class="code">apply</code> — tail call to given procedure
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</li><li><code class="code">call-with-current-continuation</code> — tail call to the procedure
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receiving the new continuation
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</li><li><code class="code">call-with-values</code> — tail call to the values-receiving
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procedure
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</li><li><code class="code">eval</code> — tail call to evaluate the form
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</li><li><code class="code">string-any</code>, <code class="code">string-every</code> — tail call to predicate on
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the last character (if that point is reached)
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</li></ul>
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<br>
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<p>The above are just core functions and special forms. Tail calls in
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other modules are described with the relevant documentation, for
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example SRFI-1 <code class="code">any</code> and <code class="code">every</code> (see <a class="pxref" href="SRFI_002d1-Searching.html">Searching</a>).
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</p>
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<p>It will be noted there are a lot of places which could potentially be
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tail calls, for instance the last call in a <code class="code">for-each</code>, but only
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those explicitly described are guaranteed.
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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="The-REPL.html">Using the Guile REPL</a>, Previous: <a href="Evaluating.html">Evaluating Expressions and Executing Programs</a>, Up: <a href="About-Expressions.html">Expressions and Evaluation</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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