xsl:assert

Used to make assertions in the form of XPath expressions, causing a dynamic error if the assertion turns out to be false.

Category: instruction
Content: sequence-constructor
Permitted parent elements: any XSLT element whose content model is sequence-constructor; any literal result element

Attributes

`test`	`expression`	XPath expression to be asserted.
`select?`	`expression`	Can be used to specify (part of) the content of the error message produced when the assertion is false.
`error-code?`	`{ eqname }`	Specifies the error code associated with the error message produced when the assertion is false.

Notes on the Saxon implementation

Following a change in the W3C specification, from Saxon 9.7 assertions are now disabled by default. To enable assertions, use the -ea command line option, or the FeatureKeys.XSLT_ENABLE_ASSERTIONS configuration property.

Details

It may be useful to add an attribute such as use-when="$DEBUG" to xsl:assert instructions; the global static variable DEBUG can then be used to switch assertion processing on and off. The variable might be declared as <xsl:variable name="DEBUG" as="xs:boolean" select="true()"/>.

Examples

The following code tests whether the value of $x is non-negative, and if not, fails with an error message:

<xsl:assert test="$x > 0">Argument $x must be greater than zero</xsl:assert>

Links to W3C specifications

XSLT 3.0 Specification