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David G. Durand wrote:
> At the beginning of this thread Dare asserted that a weakness of
> XML-Query is that it's semantics is too weak to allow static analysis
> of the correctness of updates.
> What's interesting is that this assertion is in fact untrue, but
> people seem to be accepting it and instead arguing whether that's a
> fata flaw or not (and what the relevant definition of "type" is).
> One of the interesting things about regular tree grammars is that
> many of their properties are decideable. They have the same closure
> properties as regular langauges over strings. This has some nice
> consequences. For instance, Pierce and Hosoya's work on XDuce is
> admittedly an initial step into a research area, but some results are
> already crystal clear. You can use a regular tree grammar (and hence
> any of several popular schema languages) as a static type in the
> programming language sense, and use it to type check and validate
> programs operate on documents. In particular you can validate that a
> program is guaranteed to produce results conforming to another
> schema. I don't think anyone would argue that the best formalism,
> syntax, or algorithms have yet been determined for this kind of
> system yet; it's still bleeding edge. But one can already write
> programs and feed them to an automatic system that will predict and
> validate the relationship between input and output schemas.
> There are a variety of other schema checks that database systems
> perform (like referential integrity). Frequently these are dynamic,
> in fact, not static, though they are given declaratively. At least
> some of the complaints I hear relate to these issues. I'm not
> informed about these issues, but defects in aspects like this are not
> due to some inherent "lack of a type system" but rather to type
> systems that solve different problems.
> I'm not trying to argue the details of the XQuery semantics, as I'm
> not an expert there (I'm barely informed), but I think that it is
> very significant that we are seeing that one can get real mileage out
> of current scheme languages when they are considered as type systems.
> I'm interested in this because this has potential to make document
> processing a lot cleaner, more reliable, and pleasant -- but I'm one
> of the document-oriented old guard in the markup game.
> I should note that I mentioned Pierce and Hosoya's XDuce, but there
> are a number of other researchers exploring different aspects of this
> terrain. For the interested, starting from on of their XDuce papers
> in Citeseer will lead you to a lot of this other work.