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content/glyph/book/text_editing/conditionals.html

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<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
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		<title>Glyph - Conditional Macros</title>
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						<h2>Conditional Macros</h2>
						<hr />
						<div id="content-body">
							<div class="navigation"><a href="/glyph/book/text_editing/evaluation.html">Evaluating Ruby code and Configuration Settings &larr;</a><a href="/glyph/book/index.html">Contents</a><a href="/glyph/book/compiling/compiling.html">&rarr; Compiling a project</a></div>
								<p>Sometimes you may want text to be included in a document only if certain conditions are satisfied. For example, you may want to display a disclaimer section only if the document is a draft (see the <a href="/glyph/book/config/document.html#s_document_draft"><code>document.draft</code></a> setting), or use a particular stylesheet only if when you generate a <span class="caps">PDF</span> document.</p>
<p>To do so, you can use the <a href="/glyph/book/macros/macros_core.html#m_condition"><code>condition</code></a> macro (aliased by <code>?</code>), and a set of additional macros that can be used as conditional operators i.e.:</p>
<ul>
	<li><a href="/glyph/book/macros/macros_core.html#m_eq"><code>eq</code></a> macro</li>
	<li><a href="/glyph/book/macros/macros_core.html#m_not"><code>not</code></a> macro</li>
	<li><a href="/glyph/book/macros/macros_core.html#m_and"><code>and</code></a> macro</li>
	<li><a href="/glyph/book/macros/macros_core.html#m_or"><code>or</code></a> macro</li>
	<li><a href="/glyph/book/macros/macros_core.html#m_match"><code>match</code></a> macro</li>
</ul>
<p>Consider the following code:</p>
	<pre class="lazy"><span class="line-numbers">   1 </span> ?[$[document.draft]|
<span class="line-numbers">   2 </span> This is a first draft of the Glyph Book]
<span class="line-numbers">   3 </span> ?[not[$[document.draft]]|
<span class="line-numbers">   4 </span> This is the official version of the Glyph Book]
</pre>
	<p>In this case, if <code>document.draft</code> is set to <code>true</code>, &#8220;This is a first draft of the Glyph Book&#8221; will be displayed; if not, &#8220;This is the official version of the Glyph Book&#8221; will be displayed instead.</p>
<p>The <a href="/glyph/book/macros/macros_core.html#m_condition"><code>condition</code></a> macro takes two parameters:</p>
<ul>
	<li>the first one is the condition to evaluate</li>
	<li>the second one is the text to include in the document only if the condition is satisfied.</li>
</ul>
<p>Note that <em>both</em> parameters can contain macros, of course, so you can write things like:</p>
	<pre class="lazy"><span class="line-numbers">   1 </span> ?[and[
<span class="line-numbers">   2 </span>     eq[$[document.output]|pdf]
<span class="line-numbers">   3 </span>     |
<span class="line-numbers">   4 </span>     eq[$[tools.pdf_generator]|prince]
<span class="line-numbers">   5 </span>     ]
<span class="line-numbers">   6 </span>   |
<span class="line-numbers">   7 </span>   style[pagination.css]]
</pre>
	<p>In this case, the <code>pagination.css</code> stylesheet is included only when you're generating a PDF document using Prince XML.</p>
  <div class="section">
<h4 id="h_38">Results of conditional expressions</h4>
<p>The <a href="/glyph/book/macros/macros_core.html#m_condition"><code>condition</code></a> macro in Glyph works in a similar way as conditionals in programming languages: if the conditional expression (supplied as first parameter) is satisfied then the second parameter is executed or displayed. But when is a conditional expression satisfied? Glyph is a simple mini-language to perform text manipulation, and has no types, it can only understand text, therefore:</p>
<ul>
	<li>A conditional expression is satisfied if it evaluates to a non-empty string except &#8220;false&#8221;.</li>
	<li>A conditional expression is not satisfied if it evaluates to an empty string or the string &#8220;false&#8221;.</li>
</ul>

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