Flux: FunctionaL Updates for XML (extended report)

TL;DR

This paper introduces an improved, sound type system and path-error analysis for the Flux XML update language, addressing previous limitations related to recursion, error propagation, and dead code detection.

Contribution

It extends Flux with recursive types, formalizes a source language, and develops a path-error analysis to improve error detection and language robustness.

Findings

Type system now supports recursion.

Path-error analysis detects dead subexpressions.

Translation preserves and reflects typability.

Abstract

XML database query languages have been studied extensively, but XML database updates have received relatively little attention, and pose many challenges to language design. We are developing an XML update language called Flux, which stands for FunctionaL Updates for XML, drawing upon ideas from functional programming languages. In prior work, we have introduced a core language for Flux with a clear operational semantics and a sound, decidable static type system based on regular expression types. Our initial proposal had several limitations. First, it lacked support for recursive types or update procedures. Second, although a high-level source language can easily be translated to the core language, it is difficult to propagate meaningful type errors from the core language back to the source. Third, certain updates are well-formed yet contain path errors, or ``dead'' subexpressions which never do any useful work. It would be useful to detect path errors, since they often represent errors or optimization opportunities. In this paper, we address all three limitations. Specifically, we present an improved, sound type system that handles recursion. We also formalize a source update language and give a translation to the core language that preserves and reflects typability. We also develop a path-error analysis (a form of dead-code analysis) for updates.