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<a href="index.html">XSLTC Design</a>
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<td valign="middle">&lt;xsl:choose&gt; / &lt;xsl:when&gt; / &lt;xsl:otherwise&gt;</td>
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<h2>&lt;xsl:choose&gt; / &lt;xsl:when&gt; / &lt;xsl:otherwise&gt;</h2>
<p align="right" size="2">
<a href="#content">(top)</a>
</p>
<h3>Contents</h3>
<ul>
<li>
<a href="#functionality">Functionality</a>
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<li>
<a href="#implementation">Implementation</a>
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<a name="functionality">‌</a>
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<h3>Functionality</h3>
<p>The <code>&lt;xsl:choose&gt;</code> element is used to determine one
course of action based on a series of tests. Each test is done inside an
<code>&lt;xsl:when&gt;</code> element. If a test succeeds, the body of the
<code>&lt;xsl:when&gt;</code> element is executed. If no tests fail then
a <code>&lt;xsl:otherwise&gt;</code> element can be used to specify a
default action:</p>
<blockquote class="source">
<pre>
&lt;xsl:choose&gt;
&lt;xsl:when test="element-available('some-extension')"&gt;
...
&lt;/xsl:when&gt;
&lt;xsl:when test="function-availabe('saxon:nodeset')"&gt;
...
&lt;/xsl:when&gt;
&lt;xsl:otherwise&gt;
...
&lt;/xsl:otherwise&gt;
&lt;/xsl:choose&gt;</pre>
</blockquote>
<a name="implementation">‌</a>
<p align="right" size="2">
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<h3>Implementation</h3>
<p>The <code>Choose</code> class places all <code>When</code> child-nodes
in a vector. The <code>Choose</code> class translates the "test"-attribute
of all <code>When</code> nodes (in strict order) and chains them together
in an if-else style. The expression that holds each test contains a true-
and a false-list. These lists are vectors of branch targets that should be
used if the test succeeds or fails, respectively. The first test's
false-list is pointed to the start of the next test (ie. if the first test
fails, then we run the next test). The last test's false-list points directly
to the code for the body of the <code>&lt;xsl:otherwise&gt;</code> element.
</p>
<p>Just as with the <code>&lt;xsl:if&gt;</code>-element, special care is
taken for the <code>element-available()</code> and
<code>function-available()</code> functions. These functions are evaluated at
compile-time (this can be done since all parameters for these functions are
literals) and the body of a <code>&lt;xsl:when&gt;</code> element is not
compiled if we know that it will never be needed at runtime.</p>
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<a href="#content">(top)</a>
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