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.

Category: declaration
Content: ( xsl:param* , sequence-constructor )
Permitted parent elements: xsl:package ; xsl:stylesheet ; xsl:transform ; xsl:override




Name of the function. Must be in a non-default namespace.



Defines the return type of the function.


"public" | "private" | "final" | "abstract"

New in XSLT 3.0. Determines the visibility of the function in other packages.


"unclassified" | "absorbing" | "inspection" | "filter" | "shallow-descent" | "deep-descent" | "ascent" | eqname

Not implemented in Saxon 9.7.



Determines what happens if this function has the same name and arity as another provided by the implementation or made available in the static context by an implementation-defined mechanism. If the value is yes (the default), then this function takes precedence; if the value is no, then the other function takes precedence.



A deprecated synonym of override-extension-function, retained for XSLT 2.0 compatibility.


"yes" | "true" | "1" | "no" | "false" | "0" | "maybe"

Assigns the function to one of three categories regarding the determinism of functions (relevant to optimization). The value no means the function is deterministic; the value yes means it is proactive; and the value maybe means it is elidable. Requires Saxon-PE or Saxon-EE.



Indicates whether the function is to cache its results. Requires Saxon-PE or Saxon-EE.



Specifying 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. Requires Saxon-PE or Saxon-EE. For details see saxon:memo-function.

Notes on the Saxon implementation

Saxon defines an extra attribute on xsl:function, namely saxon:memo-function. The attribute 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.

The extension attribute saxon:explain can also be used on an xsl:function element. If the attribute has value yes, then at compile time Saxon outputs (to the standard error output) a representation of the optimized expression tree for that function.

The attributes cache and new-each-time are interpreted in Saxon 9.7 (PE or higher) as follows: if the value of cache is full or partial, and the value of new-each-time is no then the function is implemented as a memo function, in the same way as when the extension attribute saxon:memo-function is set. Note that the cache used for a memo function in Saxon 9.7 is always a full cache, that is, it retains the results of all previous function calls within the scope of a query or transformation. In Saxon-HE, these attributes have no effect.

The visibility attribute is implemented since Saxon 9.6 as part of the implementation of XSLT 3.0 packages.

Saxon 9.8 introduces support for streamable stylesheet functions, based on the new XSLT 3.0 attribute streamability. Test coverage for this feature is rather slim at the time of release.


In limited circumstances, stylesheet functions (xsl:function) optimise tail-recursion. The circumstances are that the select expression of the xsl:sequence instruction which constitutes the enclosed sequence constructor 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. See the examples below.


Example 1

The following example is not tail-recursive, because the recursive call is within an arithmetic expression: the multiplication takes place on return from the recursive call.

<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)"/> </xsl:function>

Example 2

The previous example 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.

Links to W3C specifications

XSLT 2.0 Specification

XSLT 3.0 Specification

See also