XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (315 page)

• By the expression
  $S cast as xs:boolean
  
• By the
  xs:boolean()
  constructor function (note the difference from the
  boolean()
  function in the core library, sometimes written as
  fn:boolean()
  to emphasize the difference)
  
• When an
  xs:untypedAtomic
  value is implicitly converted to an
  xs:boolean
  value in the course of a function call, where one of the function arguments has a required type of
  xs:boolean
  

Conditional expressions are one of the few places in the XPath language where you get a guarantee that an expression will or will not be evaluated. Specifically, if the condition is true then the
else
branch will not be evaluated, while if it is false, the
then
branch will not be evaluated. This means you can safely use expressions that could otherwise cause errors, for example:

Changes in XPath 2.0

Conditional expressions are new in XPath 2.0. In the context of an XSLT stylesheet, they often make it possible to replace a cumbersome

instruction. For example, the following:

can now be replaced with:

The rules for calculating the effective boolean value of an expression have been carefully chosen to be compatible with the rules for converting strings, numbers, or node-sets to booleans in XPath 1.0, while at the same time generalizing them to handle an arbitrary sequence. If they seem arbitrary, blame history.

Examples

Expression	Description
if (@x) then @x else 0	Returns the attribute node @x if it exists, or the xs:integer value zero otherwise. (This can also be expressed as (@x,0)[1] .)
if ($area/sales) then avg($area/sales/@value) else number(‘NaN’)	Returns the average sales value for the selected area if there were any sales, or the not-a-number value NaN otherwise.
if (normalize-space(.)) then string(.) else ()	Returns the context item converted to a string if it contains any non-whitespace characters; otherwise, returns the empty sequence. This relies on the fact that normalize-space() returns a zero-length string (which is treated as false ) if all the characters in the string are whitespace.