Type checking in XOBE
XML is the upcoming standard for internet data. Java is the most important programming language for internet applications. Nevertheless, in today's languages and tools there is no smooth integration of Java and XML. The XML OBjEcts project (XOBE) at the University of Lübeck addresses this mismatch by defining XML objects by XML schemas and by making them to first-class data values. In XOBE, the distinction between XML documents and XML objects no longer exists. Instead, a running XOBE program works only with XML objects. XML documents in text form with explicit tags exist only for communicating with the outside world. This approach allows to check the validity of all XML objects within a program statically at compile time. This is accomplished by XML constructors. Previously generated XML objects are inserted in these constructors such that the validity can be checked at compile time. This paper concentrates on the type checking algorithm in XOBE which is used, among others, for checking the correctness of assignment statements involving XML objects. The type checking algorithm assures that all XML objects that can occur dynamically on the right hand side of an assignment statement are objects that can be assigned to the variable on the left hand side. This type checking is done statically without running the program. The algorithm is based upon regular hedge grammars and regular hedge expressions.
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