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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <!-- Created by GNU Texinfo 6.8, https://www.gnu.org/software/texinfo/ --> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> <!-- This manual is for GNU Texinfo (version 6.8, 8 June 2021), a documentation system that can produce both online information and a printed manual from a single source using semantic markup. Copyright (C) 1988, 1990-1993, 1995-1999, 2001-2021 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License". --> <title>Optional Arguments (GNU Texinfo 6.8)</title> <meta name="description" content="Optional Arguments (GNU Texinfo 6.8)"> <meta name="keywords" content="Optional Arguments (GNU Texinfo 6.8)"> <meta name="resource-type" content="document"> <meta name="distribution" content="global"> <meta name="Generator" content="texi2any"> <meta name="viewport" content="width=device-width,initial-scale=1"> <link href="index.html" rel="start" title="Top"> <link href="Command-and-Variable-Index.html" rel="index" title="Command and Variable Index"> <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> <link href="Definition-Commands.html" rel="up" title="Definition Commands"> <link href="_0040deffnx.html" rel="next" title="@deffnx"> <link href="Def-Cmd-Continuation-Lines.html" rel="prev" title="Def Cmd Continuation Lines"> <style type="text/css"> <!-- a.copiable-anchor {visibility: hidden; text-decoration: none; line-height: 0em} a.summary-letter {text-decoration: none} blockquote.indentedblock {margin-right: 0em} div.display {margin-left: 3.2em} div.example {margin-left: 3.2em} kbd {font-style: oblique} pre.display {font-family: inherit} pre.format {font-family: inherit} pre.menu-comment {font-family: serif} pre.menu-preformatted {font-family: serif} span.nolinebreak {white-space: nowrap} span.roman {font-family: initial; font-weight: normal} span.sansserif {font-family: sans-serif; font-weight: normal} span:hover a.copiable-anchor {visibility: visible} ul.no-bullet {list-style: none} --> </style> </head> <body lang="en"> <div class="section" id="Optional-Arguments"> <div class="header"> <p> Next: <a href="_0040deffnx.html" accesskey="n" rel="next"><code>@deffnx</code>, et al.: Two or More ‘First’ Lines</a>, Previous: <a href="Def-Cmd-Continuation-Lines.html" accesskey="p" rel="prev">Definition Command Continuation Lines</a>, Up: <a href="Definition-Commands.html" accesskey="u" rel="up">Definition Commands</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Command-and-Variable-Index.html" title="Index" rel="index">Index</a>]</p> </div> <hr> <span id="Optional-and-Repeated-Arguments"></span><h3 class="section">13.3 Optional and Repeated Arguments</h3> <span id="index-Optional-and-repeated-arguments"></span> <span id="index-Repeated-and-optional-arguments"></span> <span id="index-Arguments_002c-repeated-and-optional"></span> <span id="index-Syntax_002c-optional-_0026-repeated-arguments"></span> <span id="index-Meta_002dsyntactic-chars-for-arguments"></span> <p>Some entities take optional or repeated arguments, conventionally specified by using square brackets and ellipses: an argument enclosed within square brackets is optional, and an argument followed by an ellipsis is optional and may be repeated more than once. </p> <p>Thus, [<var>optional-arg</var>] means that <var>optional-arg</var> is optional and <var>repeated-args</var><code>…</code> stands for zero or more arguments. Parentheses are used when several arguments are grouped into additional levels of list structure in Lisp. </p> <p>Here is the <code>@defspec</code> line of an example of an imaginary (complicated) special form: </p> <blockquote> <dl class="def"> <dt id="index-foobar"><span class="category">Special Form: </span><span><strong>foobar</strong> <em>(var [from to [inc]]) body…</em><a href='#index-foobar' class='copiable-anchor'> ¶</a></span></dt> </dl> </blockquote> <p>In this example, the arguments <var>from</var> and <var>to</var> are optional, but must both be present or both absent. If they are present, <var>inc</var> may optionally be specified as well. These arguments are grouped with the argument <var>var</var> into a list, to distinguish them from <var>body</var>, which includes all remaining elements of the form. </p> <p>In a Texinfo source file, this <code>@defspec</code> line is written like this: </p> <div class="example"> <pre class="example">@defspec foobar (var [from to [inc]]) body@dots{} </pre></div> <p>The function is listed in the Command and Variable Index under ‘<samp>foobar</samp>’. </p> </div> </body> </html>