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>> So I'll try to explain my point in lay terms. The main >> benefit of a type system is so that things can be done >> *statically* at compile time instead dynamically at >> run time. Validation is a dynamic process. > > No. The main benefit of a "type system" is that individuals (e.g. > individual documents or fragments of documents) can be collected into > sets based on constraints. This is so simple and basic a concept that it > should hit you right between the eyes. No formal mumbo jumbo needed. In the CS context, "type system" doesn't just mean a way of collecting individuals into sets. For example, in his recent book "Types and Programming Languages" Benjamin Pierce (one of the XDuce authors) offers the following definition: "As with many terms shared by large communities, it is difficult to define "type system" in a way that covers its informal usage by programming language designers and implementors but is still specific enough to have any bite. One plausible definition it this: A type system is a tractable syntactic method for proving the absence of certain kinds of program behaviors by classifying phrases according to the kinds of values they compute." This is the sense in which the term "type system" is being used when the XQuery folks talk about the "XQuery type system". James
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