[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: Semantics was Re: XQuery types ...
Jonathan Robie wrote: > > The Formal Semantics is all about structural subsumption, not about > validation. In general there are two things about type or classification systems: 1) a way to determine whether individuals (or objects) have a particular type. In the so-called "static type" usages, an individual (or object) is explicitly declared to be a member of a particular class i.e. have a given type. In dynamic or run type usages, the system inspects the individual and classifies it. This is akin to validation, but as you say is not all that classifications are. 2) classes may be declared to be subclasses of other classes. Class hierarchies may be built using logical combinators which generally implement what XML Schema calls "derivation by restriction" and "derivation by extension". The logical combinators include "intersection" by which a subclass is defined to be the intersection of a set of super classes (derivation by restriction), "union" where a subclass is defined to be the union of a set of super classes (derivation by extention) etc. > In general, it tries to define structural subsumption so that > when it looks at a query expression and a schema, the conclusions it draws > would be compatible with the conclusions a schema processor would make when > validating instances from the universe of documents the schema defines. Having such a class hierarchy as described above does indeed allow one to draw logical inferences about the classes and class membership. Such inferences are indeed useful. Often one might not need to attempt to classify or validate a set of individuals (or objects) against a class definition, when such inferences can be drawn -- this is akin to query optimization. Jonathan
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