From: Aaron Chang (anc327@yahoo.com)
Date: Sun Apr 18 2004 - 17:04:33 PDT
The Essence of XML
by Simeon and Wadler
Review Points
=============
1. The authors of this paper seek to study two major
XML grammar issues:
A. relation of named vs structural types
B. relation of matching vs validation of data
2. external formats for representing data require
self-description and round-tripping,
neither of which are in XML.
3. authors seek to clarify "the essence of XML" being
that of data typing and validation
4. XML has many typing systems. the authors suggest a
formalization of XML Schema typing system.
Schema uses XQuery and Xpath semantics.
5. XML does not match a value to a type, it validates
(it happens or it fails). authors present
a theorem that validation occurs IFF a value
matches a type and "erases" to an untyped value.
this simple statement appears to be an important
distillation of fact, learned by trial/error
6. Schema is written in XML notation. the authors
place specific restrictions on the notation
to make it more readible. the simplifications
offered sound reasonable.
7. some important aspects of their model on Schema:
A. simple vs complex types
B. global vs local declarations
C. atomic, list, and union types
D. "derivation" of simple and complex types (like
subclassing) -> called "restriction"
E. derivation by extension
conclusion: should restriction and extension be
essentially the same grammar?
8. specification of different types and untypes and
their values is well-thought out
9. "yields" and "resolves" are good, but not very
intuitive, perhaps "instanceOf" or "isType" like
in Java would've been better
10. not sure why "substitutes" is necessary, why not
just use "=" ?
11. what's the difference between "derives" and
"resolves"?
12. "matches" looks useful, but perhaps "isType"
could've also worked not only between types but also
between values and types
13. erasure and validation are important contributions
by the authors
14. the validation formal theory is very useful and
intuitively presented
15. sensibility in some cases seemed redundant, esp
subtyping
overall a very interesting read on how to place a
grammar on a largely unregulated semantics
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