Typechecking is a static analysis on programs that enables the detection of potential dynamic type errors. Since it is already a standard technique in programming languages and since XML's schemas are analogous to data types in programming languages, we might expect existing typechecking techniques to be usable also for the verification of XML processing programs. However, a concrete analysis algorithm must be renewed entirely since schemas have a mathematical structure completely different from conventional data types. In this chapter, we first overview various approaches to XML typechecking and then consider, as a case study, a core of the XDuce type system in detail.

Compact taxonomy

Since the aim of typechecking is the rejection of possibly incorrect programs, it is a language feature clearly visible to the user. Therefore, in constructing a typechecker we must take into account not only the internal analysis algorithm but also the external specification, which further influences the design of the whole language.

The first design consideration is the kind of type error that should be detected. That is, what sort of type error should be raised from a program in the target language? The most typical kind of error occurs when the final result does not conform to the specified “output type.” However, some languages also check intermediate results against types. For example, the XDuce language has a pattern match facility, according to which a match error occurs when a form of conformance test on an intermediate result fails (Section 5.1).