XQuery, 2nd Edition

Chapter 11. A Closer Look at Types

Chapter 2 briefly introduced the use of types in XQuery. This chapter delves deeper into the XQuery type system and its set of built-in types. It explains the automatic type conversions performed by the processors and describes the expressions that are relevant to types, namely type constructors, cast and castable expressions, and instance of expressions.

The XQuery Type System

XQuery is a strongly typed language, meaning that each function and operator is expecting its arguments or operands to be of a particular type. This means, for example, that you cannot perform arithmetic operations on strings, without explicitly telling the processor to treat your strings like numbers. This is similar to some common object-oriented programming languages, like Java and C#. It is in contrast to most scripting languages, like JavaScript, which will automatically coerce values to the appropriate type.

Advantages of a Strong Type System

There are several advantages to a strong type system. One of them is the early and reliable identification of errors in a query. Potential errors in the query can be determined statically before the query is even executed. For example, if you are trying to double a value that is a string (e.g., a product name), there is probably an error in the query. In addition, a strict type system allows for the identification of errors in the values of input data. This identification of errors can make queries easier to debug, and results in ...