The xsl:function element defines a function within a stylesheet. The function is written in XSLT but it may be called from any XPath expression in the stylesheet. It must have a non-default namespace prefix.


<xsl:function name="my:factorial" as="xs:integer"> <xsl:param name="number" as="xs:integer"/> <xsl:sequence select="if ($number=0) then 1 else $number * my:factorial($number-1)"/>

In limited circumstances, stylesheet functions (xsl:function) optimise tail-recursion. The circumstances are that the select expression of the xsl:result instruction must contain a call on the same function as the then or else part of a conditional expression (which may be nested in further conditional expressions). It may require a little care to write functions to exploit this. The example above is not tail-recursive, because the recursive call is within an arithmetic expression: the multiplication takes place on return from the recursive call. It can be recast in tail-recursive form by adding an extra parameter (which should be set to 1 on the initial call):

<xsl:function name="x:factorial"> <xsl:param name="acc" as="xs:integer?"/> <xsl:param name="n" as="xs:integer"/> <xsl:sequence as="xs:integer" select="if ($n = 1) then $acc else x:factorial($acc*$n, $n - 1)" /> </xsl:function>

The call x:factorial(1, 5) returns 120.

Saxon defines an extra attribute on xsl:function: saxon:memo-function="yes" indicates that Saxon should remember the results of calling the function in a cache, and if the function is called again with the same arguments, the result is retrieved from the cache rather than being recalculated. Further details: see saxon:memo-function.

Saxon, as at version 9.5, does not yet implement the new XSLT 3.0 attributes visibility, cache, override-extension-functions, or identity-sensitive.