Types for path correctness of XML queries

If a subexpression in a query will never contribute data to the query answer, this should be regarded as an error. This principle has been recently accepted into mainstream XML query languages, but was still waiting for a complete treatment. We provide here a precise definition for this class of errors, and define a type system that is sound and complete, in its search for such errors, for a core language, under mild restrictions on the use of recursion in type definitions. In the process, we describe a dichotomy among existential and universal type systems, which is useful to understand some unusual features of our type system.This is the pre-print of the article: Dario Colazzo, Giorgio Ghelli, Paolo Manghi, and Carlo Sartiani. 2004. Types for path correctness of XML queries. In Proceedings of the ninth ACM SIGPLAN international conference on Functional programming (ICFP '04). Association for Computing Machinery, New York, NY, USA, 126–137. https://doi.org/10.1145/1016850.101686