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

Adventures in Formal Methods at W3C: The XQuery 1.0 and XPath 2.0 Formal Semantics. Jerome Simeon, IBM. 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:

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

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  1. Adventures in Formal Methods at W3C:The XQuery 1.0 and XPath 2.0 Formal Semantics Jerome Simeon, IBM

  2. 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 >>

  3. 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

  4. XQuery 1.0 and XPath 2.0 Formal Semantics • Formalization using inference rules notations • Notations developed for specifying programming languages

  5. 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)

  6. English vs. Formal Specification

  7. 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.

  8. Formal Specification as an Implementation Guide

  9. 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

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