XQuery

The Treat Expression

The treat expression, like the typeswitch expression, is used to assure the processor that only values of a certain type will participate in a particular function or operation. The syntax of a treat expression is shown in Figure 14-2.

Figure 14-2. Syntax of a treat expression

Building on the ProductType/HatType example from the previous section, suppose you would like to display the size of a product, if it is a hat. Although ProductType doesn't allow a size child, HatType does. You could use the query shown in Example 14-4.

Example 14-4. A query without a treat expression

if ($myProduct instance of element(*,prod:HatType))
then <p>The size is: {data($myProduct/size)}</p>
else ( )

It tests to see if the product is a hat, and if it is, constructs a p element that contains its size. Unfortunately, an implementation that supports static typing will raise a type error with this query. This is because, as far as the processor knows, $myProduct has type ProductType, which does not allow a size child. As discussed in the previous section, it does not matter that you check the type of $myProduct in the enclosing if expression.

Example 14-5 shows a revised query that uses a treat expression to assure the processor that $myProduct is indeed an element of type HatType.

Example 14-5. A query with a treat expression

if ($myProduct instance of element(*,prod:HatType)) then <p>The ...