The DSD Schema Language

Abstract

XML (Extensible Markup Language), a linear syntax for trees, has gathered a remarkable amount of interest in industry. The acceptance of XML opens new venues for the application of formal methods such as specification of abstract syntax tree sets and tree transformations. A user domain may be specified as a set of trees. For example, XHTML is a user domain corresponding to a set of XML documents that make sense as hypertext. A notation for defining such a set of XML trees is called a i>schema language. We believe that a useful schema notation must identify most of the syntactic requirements present in the user domains, and yet be sufficiently simple and easy to understand both by the schema authors and the users. Furthermore, it must allow efficient parsing and be modular and extensible to support reuse and evolution of descriptions. In the present paper, we give a tutorial introduction to the DSD (Document Structure Description) notation as our bid on how to meet these requirements. The DSD notation was inspired by industrial needs. We show how DSDs help manage aspects of complex XML software through a case study about interactive voice response systems, i.e., automated telephone answering systems, where input is through the telephone keypad or speech recognition. The expressiveness of DSDs goes beyond the DTD schema concept that is already part of XML. We advocate the use of nonterminals in a top-down manner, coupled with boolean logic and regular expressions to describe how constraints on tree nodes depend on their context. We also support a general, declarative mechanism for inserting default elements and attributes. Also, we include a simple technique for reusing and evolving DSDs through selective redefinitions. The expressiveness of DSD is comparable to that of the schema language XML Schema proposed by W3C, but their syntactic and semantic definition is significantly larger and more complex. Also, the DSD notation is self-describable: the syntax of legal DSD documents including all static semantic requirements can be expressed within the DSD language itself.