<xsl:choose> / <xsl:when> / <xsl:otherwise>
Contents
Functionality
The <xsl:choose> element is used to determine one
course of action based on a series of tests. Each test is done inside an
<xsl:when> element. If a test succeeds, the body of the
<xsl:when> element is executed. If no tests fail then
a <xsl:otherwise> element can be used to specify a
default action:
<xsl:choose>
<xsl:when test="element-available('some-extension')">
...
</xsl:when>
<xsl:when test="function-availabe('saxon:nodeset')">
...
</xsl:when>
<xsl:otherwise>
...
</xsl:otherwise>
</xsl:choose>
Implementation
The Choose class places all When child-nodes
in a vector. The Choose class translates the "test"-attribute
of all When nodes (in strict order) and chains them together
in an if-else style. The expression that holds each test contains a true-
and a false-list. These lists are vectors of branch targets that should be
used if the test succeeds or fails, respectively. The first test's
false-list is pointed to the start of the next test (ie. if the first test
fails, then we run the next test). The last test's false-list points directly
to the code for the body of the <xsl:otherwise> element.
Just as with the <xsl:if>-element, special care is
taken for the element-available() and
function-available() functions. These functions are evaluated at
compile-time (this can be done since all parameters for these functions are
literals) and the body of a <xsl:when> element is not
compiled if we know that it will never be needed at runtime.