XQuery, 2nd Edition

Chapter 15. Static Typing

Errors in a query can be raised in either the static analysis phase or the dynamic evaluation phase. These two phases are roughly analogous to compiling and running program code. Certain XQuery implementations take a more aggressive approach to finding type-related errors in the static analysis phase. These implementations are said to support static typing.

What Is Static Typing?

Static typing, as the term is used in XQuery, refers to raising all possible type errors at analysis (compile) time rather than evaluation (run) time. This is sometimes referred to as pessimistic static typing, where the philosophy is to raise any errors that could possibly happen, not just those that it knows will happen. The static typing feature of XQuery is optional; implementations are not required to support static typing, and many do not fully support it.

The fact that a processor doesn’t support this feature doesn’t mean that it is doing no compile-time analysis. It might use the analysis transparently for optimization purposes, or it might raise some errors at compile time; but in this case, it will raise errors optimistically. It will only raise errors when it can see that there is definitely something wrong, like in the expression "x" + 3, and not simply in cases of ambiguity, as in $x + 3, where the value of $x depends on some input data.

Static typing has the advantages of allowing type errors to be caught earlier and more reliably, and can help some implementations optimize ...