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:Famous computer scientist #2:,3,A Formal Semantics is Useful,XQuery 1.0 requirements: A language and an algebra Relational algebra
2、was a success: simple core design, basis for optimization, countless research advances over 30 years of historySupport 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 t
3、ype 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 consolidationBuild the language on solid foundations to facilitate future evolutio
4、ns,4,XQuery 1.0 and XPath 2.0 Formal Semantics,Formalization using inference rules notationsNotations 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-
5、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:/:x) Lookup the type (Type) bound
6、 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
7、for XQuery 1.0 and XPath 2.0 Benefits must be clearly understood and resources are neededImportant 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-usersBenefits are numerous, during the specification, and after: E.g., fuels growing research activities around XQuery,
