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<h1>xsl:evaluate</h1>
<p>The <code>xsl:evaluate</code> instruction is new in XSLT 3.0. It allows dynamic evaluation
of XPath expressions constructed as a string, in the same way as the <code>saxon:evaluate()</code>
extension function that has been available in Saxon for many years.</p>
<p>The following example sorts product elements according to a sort key supplied (in the form of an XPath expression)
as a parameter to the stylesheet.</p>
<div class="codeblock"
style="border: solid thin; background-color: #B1CCC7; padding: 2px">
<pre>
<code>
<xsl:apply-templates select="product">
<xsl:sort>
<xsl:evaluate select="$product-sort-key"/>
</xsl:sort>
</xsl:apply-templates>
</code>
</pre>
</div>
<p>The functionality is available as an XSLT instruction, rather than a function, to allow more
flexibility in the syntax, in particular the ability to define parameters using <code>xsl:with-param</code>
child elements.</p>
<p>The instruction is fully implemented in Saxon 9.3 with the following exceptions:</p>
<ul>
<li content="para">
<p>Functions available only in XSLT, such as key(), cannot be used in the target XPath expression.</p>
</li>
</ul>
<p>The instruction may take an <code>xsl:fallback</code> to define fallback behaviour when using an
XSLT 2.0 processor.</p>
<p>Attributes:</p>
<ul>
<li content="para">
<p>xpath: an expression, which is evaluated to return the target expression as a string.</p>
</li>
<li content="para">
<p>base-uri: a string (as an AVT), gives the base URI for the target expression. Defaults to the base
URI of the stylesheet instruction.</p>
</li>
<li content="para">
<p>namespace-context: an expression returning a node; the in-scope namespaces of this node define
the namespace context for the XPath expression. Defaults to the namespace context of the <code>xsl:evaluate</code>
instruction in the stylesheet</p>
</li>
<li content="para">
<p>as: a SequenceType: defines the required type of the result of the XPath expression. Defaults to <code>item()*</code>
</p>
</li>
<li content="para">
<p>schema-aware: "yes" or "no", as an AVT: if "yes", the XPath expression has access to the schema components imported
into the stylesheet.</p>
</li>
</ul>
<p>Children:</p>
<ul>
<li content="para">
<p>
<code>xsl:with-param</code>: defines variables that the target expression can use.</p>
</li>
<li content="para">
<p>
<code>xsl:fallback</code>: defines fallback behaviour when using an
XSLT 2.0 (or 1.0) processor.</p>
</li>
</ul>
<div class="boxed"
style="border: solid thin; background-color: #B1CCC7; padding: 2px">Before using <code>xsl:evaluate</code>, consider whether higher-order functions (also new in XSLT 3.0) would
provide a better solution to the problem. Also think carefully about the possibility of injection attacks if the
expression to be evaluated is formed by string concatenation.</div>
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