Backward type inference for XML queries

Although XQuery is a statically typed, functional query language for XML data, some of its features such as upward and horizontal XPath axes are typed imprecisely. The main reason is that while the XQuery data model allows us to navigate upwards and between siblings from a given XML node, the type model, e.g., regular tree types, can describe only the subtree structure of the given node. To alleviate this limitation, precise forward type inference systems for XQuery were recently proposed using an extended regular type language that can describe not only a given XML node but also its context. In this paper, as a different approach, we propose a novel backward type inference system for XQuery, based on a type language extended with logical formulas. Our backward type inference system provides an exact typing result for XPath axes and a sound typing result for XQuery expressions.