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PhD-defence Artur Wilk

Title: Types for XML with Application to Xcerpt
XML data is often accompanied by type information, usually expressed by
some schema language. Sometimes XML data can be related to
ontologies defining classes of objects, such classes can also be
interpreted as types. Type systems proved to be extremely useful in
programming languages, for instance to automatically discover certain
kinds of errors. This thesis deals with an XML query language Xcerpt,
which originally has no underlying type system nor any provision for
taking advantage of existing type information. We provide a type system
for Xcerpt; it makes possible type inference and checking type
correctness.

The system is descriptive: the types associated with Xcerpt constructs are
sets of data terms and approximate the semantics of the constructs. A
formalism of Type Definitions is adapted to specify such sets. The
formalism may be seen as a simplification and abstraction of
XML schema languages. The type inference method, which is the core of this
work, may be seen as abstract interpretation. A non standard
way of assuring termination of fixed point computations is proposed,
as standard approaches are too inefficient. The method is proved correct
wrt. the formal semantics of Xcerpt.

We also present a method for type checking of programs. A success of type
checking implies that the program is correct wrt. its type specification.
This means that the program produces results of the specified type
whenever it is applied to data of the given type. On the other hand, a
failure of type checking suggests that the program may be incorrect. Under
certain conditions (on the program and on the type specification), the
program is actually incorrect whenever the proof attempt fails.

A prototype implementation of the type system has been developed and
usefulness of the approach is illustrated on example programs.

In addition, the thesis outlines possibility of employing semantic types
(ontologies) in Xcerpt. Introducing ontology classes into Type Definitions
makes possible discovering some errors related to the semantics of data
queried by Xcerpt. We also extend Xcerpt with a mechanism of combining XML
queries with ontology queries. The approach employs an existing Xcerpt
engine and an ontology reasoner; no modifications are required.

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Last modified on January 2008 by Anne Moe