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Abstract Schemas, Schematron and Information Item declarations. Re: [xml


schematron cdata
This post deals with two related issues:
  A) Abstract Schemas
  B) Information Item declarations
and in relates two two non-W3C technologies: Schematron and Topologi's <informationItem>
schema. It then gives the suggestions which I think flow-on in 
  C) Practical Suggestions for DOM AS

A) Abstract Schemas
-------------------------
The DOM AS draft should not define an abstract schema. It defines a minimal grammar.

An abstract schema would have to, by plain language, abstract the common features of all 
schema languages and  paradigms in some way. So the name is quite misleading. I am
sure the Schema WG is aware of this, I hope they will look at this again.

An abstract schema language would have to provide all of
 1) a context traversal policy (e.g. traverse the document in document order)
 2) an abstract context selection mechanism (e.g. select each element, or select
    the element but use the form attribute value instead of the name if Architectural
    Forms are being used) 
 3) a context-sensitive validator state function (e.g. grammar based validators
     traverse through a content model so that x in one context has different
    followers than in another)
 4) a validation-rule traversal policy (e.g. validate attributes early, elements on exit)
 5) an abstract validation mechanism (e.g. children and attributes for grammars)
 6) error-handling policy
 7) create emergent properties for subsequent passes
 
We can use these three things to categorize various schema languages abstractly:
   
Schematron is multiple invocations of (for each active pattern)
   1) any traversal policy
   2) an XPath
   3) no state
   4) apply assertions in any order
   5) an XML expression
   6) implementation specific, but node-based invalidation or branch invalidation is OK
   7) N/A

DTDs are
   1) Document order
   2) Select current node
   3) grammar state (plus inclusion context in the case of SGML)
   4) not defined
   5) children content model, for attributes check tokenizing, ID uniqueness
   6) fail 
   7) extract IDs and IDREFs for IDREF checking

then we can say that the IDREF checking is a subsequent kind of schema.

XML Schemas is something like
  1) Document order
  2) Select current node
  3) grammar state, including local elements
  4) validate laxly etc
  5) complex and simple content, children and attributes, and uniqueness
  6) fail with particular reports 
  7) extract context for Key and Keyref checking

It seems that the DOMs AS mechanism abstracts away 1) and 2).
By not providing 3) an element can only be queried "are your contents valid?" 
but not "are you valid?"
 
B) Information Item Declarations
----------------------------------------

The AS mixes two things:
  1) declarations for document integrity
  2) constraints for validation.

I believe it would be better for these to be treated distrinctly.  In Topologi's
editor, we provide a file which provides basic declarations
for sets of information item declarations. This file can be sent in an XAR
application archive. Here is a reduced version.

<!-- A DTD for declaring sets of information item names.
       2002 (C) Topologi, Pty, Ltd
       Rick Jelliffe, ricko@t...
      The top-level element is information item.
-->
<!ELEMENT informationItems
    ( elementSets?, attributeSets?, entitySets?, processingSets?, commentSets?,
    notationSets) >

<!ELEMENT elementSets  (elementSet+)>
<!ELEMENT attributeSets (attributeSet+) >
<!ELEMENT entitySets (entitySet+) >
<!ELEMENT processingSets (processingSet+) >
<!ELEMENT commentSets (commentSet+) >
<!ELEMENT notationSets (notationSet+) >

<!ELEMENT elementSet  (element+)>
<!ELEMENT attributeSet (attribute+) >
<!ELEMENT entitySet (entity+) >
<!ELEMENT processingSet (pi+) >
<!ELEMENT commentSet (comment+) >
<!ELEMENT notationSet (notation+) >

<!ATTLIST elementSet 
    name NMTOKEN #REQUIRED
    prefix NMTOKEN #IMPLIED
    sysid  CDATA       #IMPLIED
    pubid  CDATA      #IMPLIED
    help CDATA #IMPLIED
>
<!ATTLIST attributeSet
    name NMTOKEN #REQUIRED
    prefix NMTOKEN #IMPLIED
    sysid  CDATA       #IMPLIED
    pubid  CDATA      #IMPLIED
    help CDATA #IMPLIED
>
<!ATTLIST entitySet
    name NMTOKEN #REQUIRED
    prefix NMTOKEN #IMPLIED
    sysid  CDATA       #IMPLIED
    pubid  CDATA      #IMPLIED
    help CDATA #IMPLIED
>
<!ATTLIST processingSet
    name NMTOKEN #REQUIRED
    prefix NMTOKEN #IMPLIED
    sysid  CDATA       #IMPLIED
    pubid  CDATA      #IMPLIED
    help CDATA #IMPLIED
>
<!ATTLIST commentSet
    name NMTOKEN #REQUIRED
    prefix NMTOKEN #IMPLIED
    sysid  CDATA       #IMPLIED
    pubid  CDATA      #IMPLIED
    help CDATA #IMPLIED
>
<!ATTLIST notationSet
    name NMTOKEN #REQUIRED
    prefix NMTOKEN #IMPLIED
    sysid  CDATA       #IMPLIED
    pubid  CDATA      #IMPLIED
    help CDATA #IMPLIED
>

<!ELEMENT element  ANY >
<!ELEMENT attribute  ANY >
<!ELEMENT entity  ANY >
<!ELEMENT pi   ANY >
<!ELEMENT comment  ANY >
<!ELEMENT notation ANY >

<!ATTLIST element 
    name NMTOKEN #REQUIRED
    status ( deprecate | unused | neutral | new ) "neutral" 
    content ( element | mixed | empty | pcdata | cdata | rcdata | default ) "default"
    help CDATA #IMPLIED
>
<!ATTLIST attribute
    name NMTOKEN #REQUIRED
    status ( deprecate | unused | neutral | new ) "neutral" 
    help CDATA #IMPLIED
>
<!ATTLIST entity
    name NMTOKEN #REQUIRED
    status ( deprecate | unused | neutral | new ) "neutral" 
    content ( xml | sgml | dtd | ndata | cdata)     -- cdata means "text", ndata means "binary" --
    sysid  CDATA       #IMPLIED
    pubid  CDATA      #IMPLIED
    help CDATA #IMPLIED
>
<!ATTLIST pi
    name NMTOKEN #REQUIRED
    status ( deprecate | unused | neutral | new ) "neutral" 
    help CDATA #IMPLIED
>
<!ATTLIST comment
    name NMTOKEN #REQUIRED
    status ( deprecate | unused | neutral | new ) "neutral" 
    help CDATA #IMPLIED
>
<!ATTLIST notation
   name NMTOKEN #REQUIRED
   status ( deprecate | unused | neutral | new ) "neutral" 
   help CDATA #IMPLIED
>

For example, an elementSet gives all the elements in a namespace.  
Note that there are no schematic rules here: which attributes belong to
which elements, or which data types anything can have. 

An example of a processingSet might be "the PIs that Arbortext
Publisher uses". An example of a commentSet might be "Editor
comments".  (In the Topologi editor, defining these sets allows
validation of PIs and comments, which then allows the documents
to be robust enough for friendlier automated tools.)

I think it is useful to consider this kind of declaration in the light
of, for example, James Clark's advocacy against DTDs. As the
HTML and MathML working group has discovered, it is not enough
merely to make a schema language, all the rest has to be considered
too.  

In the <informationItem> configuration files, we achieve several
goals for Topologi system integrators:
  1) We define a namespace (a list of names of elements or attributes)
  2) We bring comments and PIs to be first-class information items
  3) We relieve the schema language from having to worry about
   entity declarations 
  4) We expose notations which can be used for any datatypes
     that cannot be fitted into the schema language (or for xsi:type)
  5) By defining all namespace names, we make "open" schema
   languages even more useful: Schematron rules do not have to
   enumerate every possible element, but just concentrate on
   relationships. 
  6) The contents of the lowest-level elements contains (undisclosed)
   instantiation information for elements: default values etc.

If this <informationItems> system were used by DOM, the Sets
would be a hashtable and each Set would be a hashTable, and
each item would have a standard interface of its name, some
help text, and its status to a system.  

C) Practical Suggestions for DOM AS
---------------------------------------------

1) The DOM ASModel should be reworked into two separate interfaces:
       ASNamedInformationItems
       ASConstraintSets
 
2) The ASNamedInformationItems  interface should expose sets of sets of declarations.
These sets should allow various naming methods as appropriate. The declarations
should be minimal and be for elements, attributes, PIs, comments, entities, and notations.
The use case should be to expose all the information in a Topologi <informationItem>
configuration file, which we would contribute as part of the effort if desired.

3) The ASConstraintSet interface should expose a list of  ASConstraints objects.
Each ASConstraints object corresponds to a particular schema paradigm:
I think there are only three really:
   grammatical constraint, 
   datatype constraints, 
   path-based constraints
Each ASConstraints can have more than one ASConstraint object. 

The grammars in the current DOM AS draft are examples of these, but there
could be different ones, e.g. RELAX.  Perhaps in order to cop with RELAX NG,
the WG should at least provide a content model of "extension" which allows
any element in it in any order and occurrence: this would cope with interleave
and minimally validate many other things that might come along. 

Cheers
Rick Jelliffe
www.topologi.com

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