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<h1>saxon:function()</h1>
<p><b>saxon:function($arg1 as xs:string, $arg2 as xs:integer) ==> function()</b></p>
<p><i>This function is available only in Saxon-PE and Saxon-EE. It is obsolescent, since the
call <code>saxon:function("abc", 3)</code> can be replaced by the standard XPath 3.0 syntax
<code>abc#3</code>.</i></p>
<div class="boxed"
style="border: solid thin; background-color: #B1CCC7; padding: 2px">From Saxon 9.2, provided XQuery 3.0 support is enabled and the query prolog specifies version "3.0",
the syntax <code>my:function#3</code> can be used in place of the call <code>saxon:function('my:function', 3)</code>.
The implementation (with either syntax) has also been extended so that it works with all functions, not only
with user-written functions.</div>
<p>This function takes as its arguments the name and arity of a function,
and returns a value that represents the function and
can be used to invoke the function using
<a class="bodylink" href="../../extensions/functions/call.xml">saxon:call</a>. This allows higher-order
functions to be implemented in XSLT and XQuery, that is, functions that take other functions as arguments. An
example of such a higher-order function is
<a class="bodylink" href="../../extensions/functions/for-each-group.xml">saxon:for-each-group</a>, which
provides grouping capability in XQuery similar to that of the <code>xsl:for-each-group</code> instruction in XSLT.</p>
<p>The arguments must be specified as literals (this function is always evaluated at compile time). The first argument
gives the name of the function as a lexical QName (using the default function namespace if unprefixed), the second
gives the function arity (number of arguments).</p>
<p>Here is an example, the textbook <code>fold</code> function in XSLT:</p>
<div class="codeblock"
style="border: solid thin; background-color: #B1CCC7; padding: 2px">
<pre>
<code><xsl:function name="f:fold">
<xsl:param name="sequence"/>
<xsl:param name="operation"/>
<xsl:param name="start-value"/>
<xsl:sequence select="if (empty($sequence))
then $start-value
else f:fold(remove($sequence, 1),
$operation,
saxon:call($operation,
$start-value,
$sequence[1])"/>
</xsl:function>
<xsl:function name="f:plus">
<xsl:param name="a"/>
<xsl:param name="b"/>
<xsl:sequence select="$a + $b"/>
</xsl:function>
<xsl:function name="f:times">
<xsl:param name="a"/>
<xsl:param name="b"/>
<xsl:sequence select="$a * $b"/>
</xsl:function>
<xsl:function name="f:sum">
<xsl:param name="sequence"/>
<xsl:sequence select="f:fold($sequence, saxon:function('f:plus', 2), 0)"/>
</xsl:function>
<xsl:function name="f:product">
<xsl:param name="sequence"/>
<xsl:sequence select="f:fold($sequence, saxon:function('f:times', 2), 0)"/>
</xsl:function>
</code>
</pre>
</div>
<p>Here is the same example in XQuery (using XQuery 3.0 syntax):</p>
<div class="codeblock"
style="border: solid thin; background-color: #B1CCC7; padding: 2px">
<pre>
<code>xquery version "3.0";
declare function f:fold (
$sequence as xs:double*, $operation, $start-value as xs:double) {
if (empty($sequence))
then $start-value
else f:fold(remove($sequence, 1),
$operation,
saxon:call($operation, $start-value, $sequence[1]))
};
declare function f:plus ($a as xs:double, $b as xs:double) {$a + $b};
declare function f:times ($a as xs:double, $b as xs:double) {$a * $b};
declare function f:sum ($sequence as xs:double*) as xs:double {
f:fold($sequence, f:plus#2, 0)
};
declare function f:product ($sequence as xs:double*) as xs:double {
f:fold($sequence, f:times#2, 1)
};
</code>
</pre>
</div>
<p>The result of <code>f:sum(1 to 4)</code> is 10, while the result of <code>f:product(1 to 4)</code>
is 24.</p>
<p>Higher-order functions allow many generic functions such as <code>fold</code> to
be written, and their availability in Saxon-EE turns XSLT and XQuery into fully-fledged
functional programming languages.</p>
<p>The type of the result of saxon:function is <code>function()</code>, a new type introduced in
XQuery 1.1 - it is a third subtype of <code>item()</code> alongside nodes and atomic values.</p>
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