inp.fr

XQuery is a functional language dedicated to XML data querying and manipulation. As opposed to other W3C-standardized languages for XML (e.g. XSLT), it has been intended to feature strong static typing. Currently, however, some expressions of the language cannot be statically typed with any precision. We argue that this is due to a discrep-ancy between the semantics of the language and its type algebra: namely, the values of the language are (possibly inner) tree nodes, which may have siblings and ancestors in the data. The types on the other hand are regular tree types, as usual in the XML world: they describe sets of trees. The type associated to a node then corresponds to the subtree whose root is that node and contains no information about the rest of the data. This makes navigation expressions us-ing ‘backward axes, ’ which return e.g. the siblings of a node, impossible to type. We discuss how to handle this discrepancy and give first steps towards a solution. We describe a logic-based lan-guage of extended types able to represent inner tree nodes and show how it can dramatically increase the precision of typing for navigation expressions, yet is not adapted for typ-ing copy operations which selectively extract subtrees: reg-ular tree types are better in that case. As inclusion between the extended types and the regular tree types is checkable, in both directions, with a tool which is efficient in practice, we finally suggest that a hybrid system combining both type languages would be the way to go. 1