Synonyms
Definition
To constrain the structure of allowed XML documents, for instance with respect to a specific application, a target schema can be defined in some schema language. A schema consists of a sequence of type definitions specifying a (possibly infinite) class of XML documents. A type can be assigned to every element in a document valid w.r.t. a schema. As the same holds for the root element, the document itself can also be viewed to be of a specific type. The schema languages DTDs, XML Schema, and Relax NG, are, on an abstract level, different instantiations of the abstract model of unranked regular tree languages.
Historical Background
Brüggemann-Klein et al. [3] were the first to revive the theory of regular unranked tree automata [14] for the modelling of XML schema languages. Murata et al. [10] provided the formal taxonomy as presented here. Martens et al. [8] characterized the expressiveness of the different models and provided type-free abstractions.
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Recommended Reading
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Neven, F. (2009). XML Types. In: LIU, L., ÖZSU, M.T. (eds) Encyclopedia of Database Systems. Springer, Boston, MA. https://doi.org/10.1007/978-0-387-39940-9_1563
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