XML validation: looking backward - strongly typed and flexible XML processing are not incompatible

One major concept in web development using XML is validation: checking whether some document instance fulfills structural constraints described by some schema. Over the last few years, there has been a growing debate about XML validation, and two main schools of thought emerged about the way it should be done. On the one hand, some advocate the use of validation with respect to complete grammar-based descriptions such as DTDs and XML Schemas. On the other hand, motivated by a need for greater flexibility, others argue for no validation at all, or prefer the use of lightweight constraint languages such as Schematron with the aim of validating only required constraints, while making schema descriptions more compositional and more reusable. Owing to a logical compilation, we show that validators used in each of these approaches share the same theoretical foundations, meaning that the two approaches are far from being incompatible. Our findings include that the logic in [2] can be seen as a unifying formal ground for the construction of robust and efficient validators and static analyzers using any of these schema description techniques. This reconciles the two approaches from both a theoretical and a practical perspective, therefore facilitating any combination of them.