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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: XQuery types was Re: Yet another plea for XUpd
 David G. Durand wrote: well said. > 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. > Jonathan 
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