IBM Research

© 2005 IBM Corporation

Adventures in Formal Methods at W3C:The XQuery 1.0 and XPath 2.0 Formal Semantics

Jerome Simeon, IBM

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A Formal Semantics is Controversial Famous computer scientist #1:

<< The XQuery Formal Semantics is unreadable, and you should just use english. >>

Famous computer scientist #2:

<< I couldn’t understand how XQuery works until I read its formal semantics >>

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A Formal Semantics is Useful XQuery 1.0 requirements: A language and an algebra

– Relational algebra was a success: simple core design, basis for optimization, countless research advances over 30 years of history

Support for Static Analysis over XQuery 1.0:– Static Typing Feature calculate type of expressions at compile time

– Soundness theorem: every XML data processed by an expression will have the type infered (or a subtype)

Deeper understanding of XQuery 1.0 semantics (translation effect):– Numerous bugs in english specification found (notably related to types)

– Exposes unnecessary complexity and opportunities for consolidation

Build the language on solid foundations to facilitate future evolutions

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XQuery 1.0 and XPath 2.0 Formal Semantics

Formalization using inference rules notations

Notations developed for specifying programming languages

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XQuery Processing Model

Formalized & normative:– Normalization of expressions into

a smaller “core” grammar (SQ5)

– Static typing (SQ6)

Formalized & non-normative:– Dynamic evalution (DQ1)

Formalized & informative:– Schema import (SI1)

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English vs. Formal Specification

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Formal Specification: One Example

Read:– To statically type a variable reference (e.g., $foo:x)

– Resolve the foo:x name to it expanded name (e.g., {http://example.com}:x)

– Lookup the type (Type) bound for that expanded name in the static environment (statEnv)

– if the rule cannot be applied (e.g., variable not in the environment), the inference fails and an error is raised.

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Formal Specification as an Implementation Guide

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Lessons Learned A formal specification is a large investment:

– 500+ pages for XQuery 1.0 and XPath 2.0

– Benefits must be clearly understood and resources are needed

Important to know what not to formalize:– In XQuery: Namespace resolution, validate expression (see XML Schema)…

Audience is not everyone:– Reader must invest in understanding the notations

– Useful for the working group, for developers, for academics…

– Not so useful for most end-users

Benefits are numerous, during the specification, and after:– E.g., fuels growing research activities around XQuery