Regular Expression Subtyping for XML Query and Update Languages

TL;DR

This paper explores regular expression subtyping in XML query languages like XQuery, proving the completeness of algorithmic typechecking and extending results to an XML update language, addressing issues of type precision and decidability.

Contribution

It establishes the completeness of algorithmic typechecking for a core XQuery-like language with subtyping and extends these results to an XML update language.

Findings

Proves completeness of algorithmic typechecking with subtyping

Extends results to an XML update language

Addresses type precision and decidability issues

Abstract

XML database query languages such as XQuery employ regular expression types with structural subtyping. Subtyping systems typically have two presentations, which should be equivalent: a declarative version in which the subsumption rule may be used anywhere, and an algorithmic version in which the use of subsumption is limited in order to make typechecking syntax-directed and decidable. However, the XQuery standard type system circumvents this issue by using imprecise typing rules for iteration constructs and defining only algorithmic typechecking, and another extant proposal provides more precise types for iteration constructs but ignores subtyping. In this paper, we consider a core XQuery-like language with a subsumption rule and prove the completeness of algorithmic typechecking; this is straightforward for XQuery proper but requires some care in the presence of more precise iteration typing disciplines. We extend this result to an XML update language we have introduced in earlier work.