Functions

Instead of lingering on theory too much, let's do something useful while learning XSL functions.

In XSLT 2.0 you can write the following:

<!-- example of a short `if-then-else` statement -->
<xsl:variable
  as="xs:boolean"
  name="shortIf"
  select="if ('a' = 'b') then 'wrong?' else 'right!'" />

Unfortunately, using BI Publisher, the exact same expression in the <xsl:value-of> element fails to compile:

<!-- example of a short `if-then-else` statement (does not compile)  -->
<xsl:value-of select="if ('a' = 'b') then 'wrong!' else 'right!'" />

Unless I read it wrong, this is non-compliant with the XSLT 2.0 specification. In any case, it is very annoying as writing it the long way, i.e. using <xsl:choose> would make your style sheets twice as long. Using <xsl:variable> every single time to achieve this would be slightly better but still very cumbersome and verbose.

What I would like to be able to write instead is the following:

<!-- helper function usage (declared below) -->
<xsl:value-of select="utils:ifElse('a' = 'b', 'wrong!', 'right!')" />

This is, actually, even more concise - and readable! - than the original short if-then-else expression! Let's create a function that enables us to achieve this:

<!-- helper function declaration -->
<xsl:function name="utils:ifElse">
  <xsl:param name="condition" />
  <xsl:param name="true" />
  <xsl:param name="false" />
  <xsl:variable name="result" select="if ($condition) then $true else $false" />

  <xsl:value-of select="$result" />
</xsl:function>

In order for this code to compile, you need to add the following namespace declaration to the opening tag of your XSL style sheet:

xmlns:utils="http://jayway.com/utils"

... and you're good to go!

Explanation

Even though I feel most of the code is quite self-explanatory, let's make sure everybody's on the same page:

1. <xsl:function name="utils:ifElse"> declares a function called ifElse in the utils namespace.
2. Individual parameters of a function are specified using the <xsl:param> which can be accessed in the scope of a function using their name.
3. A function returns the evaluation of the last <xsl:value-of> (or the last node or set of nodes present in the function declaration).