[Bug 5297] Drop restriction of assertions to down-pointing XPaths?

http://www.w3.org/Bugs/Public/show_bug.cgi?id=5297





------- Comment #4 from cmsmcq@w3.org  2007-12-07 18:33 -------
Comments #1, #2, and #3 all argue that we should not make any change
here, first because there is no need to support every conceivable use
case and second because there are good reasons for validation to be
context independent.

These arguments are not wholly satisfying.  

The first presents a generally accepted principle as if it were an
argument: we cannot support all conceivable use cases.  This is not in
dispute, in part because some of us are well aware that it is not
possible to do so even in theory -- but it does not address the
salient question, which is: should we seek to support *this* use case?

This particular example is not a discovery made after long search: the
context awareness of HTML form and input elements has been a topic of
conversation, in the context of schema languages and their expressive
power, for about as long as HTML has had DTDs.  The last I heard, HTML
was not an obscure language of no particular interest for the world at
large; the ability to specify features built into widely used markup
languages is, I think, something the designer of any mechanism for
specifying markup vocabularies should be thinking hard about, if the
mechanism being designed is intended to be of general use.

The case for taking this use case seriously is simple: HTML is the
flagship markup language of W3C, and the feature makes it possible to
define a well known part of HTML precisely and concisely.  Our earlier
decision not to support it was based on a false premise.  The case
against considering this use case seriously appears to be: the W3C's
schema language doesn't really have any need to support the features
needed to define W3C's markup languages, or any other obscure and
little-used languages.  These arguments are most charitably described
as laughable; perhaps there are others.

The second argument offered is that there are good reasons for
validation to be context-free.  If those reasons exist, deciding on
this issue will involve weighing them carefully against the reasons
for making it possible to write an XSDL schema that expresses an
important constraint in (X)HTML.  Such a careful weighing of one point
against another will be easier if the reasons for context-free
validation are identified.  Comment #3 does not identify any
arguments, architectural or otherwise; it only says they exist.  

Michael Kay does identify a specific reason: bottom-up construction.
I think he is correct that the locality of validity may make it easier
to guarantee the validity of larger constructs made from smaller ones,
and I expect that it will be useful to distinguish different kinds of
validity which depend on different parts of the document, just as we
do now in distinguishing local validity from 'deep' validity.

It's not entirely clear to me how to weigh against each other the
two situations 

  (A) Validity is simple to calculate but does not include some 
      simple mechanically checkable constraints imposed by the 
      definition of the vocabulary. So "validity" is less useful 
      as a concept than it might be.

  (B) "Validity" included more of the constraints imposed by the
      definition of the vocabulary, so it's a more useful concept
      than in (A).  But it's more complex to calculate.

But it does seem clear to me that dismissing the requirements of HTML
out of hand, as if they were a corner case, is not the right way to
weigh them.  And assertions that there are good architectural reasons
for something do not carry, in a careful Working Group, the same
weight as the architectural reasons themselves.

Received on Friday, 7 December 2007 18:43:48 UTC