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Re: (draft response) Re: XML CG comments on XHTML Role Attribute Module last-call draft of 7 April 2008

From: Steven Pemberton <steven.pemberton@cwi.nl>
Date: Tue, 30 Sep 2008 11:46:25 +0200
To: "Shane McCarron" <shane@aptest.com>, "XHTML WG" <public-xhtml2@w3.org>
Message-ID: <op.uiaf3neesmjzpq@acer3010.lan>

An excellent reply!

One slight nit:

"The interpretation of a CURIE is always a URI."
"A CURIE's lexical space is [ [ prefix ] : ] reference.  Its value space  
is IRI."

I think the second one is better. Maybe you should just change URI to IRI  
in the first sentence, or add "(an IRI actually, which can always be  
transformed to a URI)".

Best wishes,


On Wed, 24 Sep 2008 19:22:40 +0200, Shane McCarron <shane@aptest.com>  

> (draft response for consideration by the XHTML 2 working group)
> Thanks for your comments.  We have discussed these as a working group.   
> Our comments are scattered below.
> C. M. Sperberg-McQueen wrote:
>> Dear colleagues:
>> Comments (4) through (9), on the other hand, relate to areas where the
>> Working Groups of the XML Activity have particular responsibility and
>> competence.  If upon consideration you find you disagree with us on
>> them, then we have a cross-domain coordination issue that requires
>> attention.
>> (1) First, we congratulate the XHTML Working Group for providing a
>> useful and clear namespace document for the namespace
>>   http://www.w3.org/1999/xhtml/vocab/
>> We wish more groups responsible for namespace did so well by the users
>> of their namespaces.
> Thank you.  However, it should be noted that this is not a namespace.   
> We know that the document mistakenly uses the string "namespace" in a  
> couple of places, and have fixed that.  This is a vocabulary definition  
> document.  It defines a collection of terms that are used in some XHTML  
> family attributes in conjunction with CURIEs and of course with RDF via  
> their expanded values (xhv:banner, for example, expands to  
> http://www.w3.org/1999/xhtml/vocab#banner).   The terms in this  
> vocabulary are never referenced as QNames.  More about this later.
>> (2) That said, we think the namespace document could be improved by
>> the addition of some more information.  A document date would be
>> helpful, and the identity of those responsible for the text of the
>> document, and for the namespace, could be stated more explicitly.
>> (From the fact that "The XHTML specifications are developed by the W3C
>> XHTML 2 Working Group as part of the W3C HTML Activity", it may be
>> thought to follow that it is the XHTML 2 Working Group which is
>> responsible both for the namespace document and for the namespace.
>> But at least this reader thought it might usefully be clearer; there
>> are cases where more than one group is involved.)  It might also be
>> desirable to provide hyperlinks from the namespace document into the
>> main documentation for the XHTML vocabulary (possibly in multiple
>> versions).
> Thanks for this.  We are updating the document to hopefully make this  
> clearer.  For avoidance of doubt, however, note that this document is  
> THE definition of these terms.  Also, as stated above, this is not a  
> namespace document.  It is an XHTML+RDFa document that provides prose  
> and machine readable definitions for terms defined and used in XHTML  
> family attribute values.  Think "ontology" or "taxonomy" - whichever  
> word resonates with you to mean "dictionary of terms and their mappings  
> to fundamental datatypes."
>> (3) The namespace document also needs a reference to a namespace
>> change policy.  At least, that is our reading of the following passage
>> from the document "URIs for W3C Namespaces" (13 July 1005, rev. 25
>> April 2006) at http://www.w3.org/2005/07/13-nsuri :
>>     The TAG finding titled The Disposition of Names in an XML
>>     Namespace explains how the use of a particular namespace may
>>     evolve over time. At the W3C, it is important for a group to state
>>     clearly its expectations for how use of the namespaces it controls
>>     will or will not change over time. Groups SHOULD document those
>>     expectations in [or clearly linked from] the Namespace Document.
> We agree that the change policy should be clear.  We will include the  
> policy in the document itself.  We believe that the policy is that the  
> term collection will never get smaller, but may expand as additional  
> basic terms are defined through the normal evolution of the associated  
> specifications.
>> (4) Our first concern is with the spec's reliance on CURIEs, which are
>> not as well defined and not as well integrated with other XML
>> technologies as one might wish.  That is perhaps a comment better
>> raised against CURIEs themselves than against this specification.  It
>> suffices here to notice that if the role attribute were defined as a
>> list of QNames, existing XSD-based technology would provide convenient
>> access to the namespace names of the individual tokens in the value of
>> the 'role' attribute; this is not the case with CURIES.
>> We note that as far as we can tell from the namespace document,
>> everything currently defined in the XHTML namespace is in fact an
>> NCName, so that QNames could be used in lieu of CURIES, without loss
>> of functionality as regards the items in the XHTML namespace -- and,
>> for software working with standard schema-aware infrastructure, some
>> substantial gain in functionality.
> The XHTML 2 Working Group is aware that some people in the community  
> have some concerns about the introduction of CURIEs as a way of  
> expressing compact URIs.  However, the XHTML 2 Working Group remains  
> convinced that compact URIs are the correct way to represent attribute  
> values that are to be interpreted as URIs.  QNames, while a fine  
> notation, have some restrictions that our constituents found  
> unacceptable (e.g., the requirement that the reference portion of the  
> QName be an NCName).  QNames are also only meaningful in the context of  
> XML languages - some of our constituents want to be able to use XHTML  
> Role in the context of HTML.  CURIEs can be readily supported in HTML.   
> Finally, and most importantly, QNames do not expand to URIs.  They map  
> into a tuple.  The interpretation of a CURIE is always a URI.  Since the  
> point of XHTML Role is to define the "role" of an element in a document,  
> and those roles are normally defined via RDF, and RDF relationships are  
> defined using URIs, this direct correspondence between a role value and  
> its URI is ideal.
> Note that the XHTML 2 Working Group, in conjunction with the Web  
> Accessibility Initiative and the Semantic Web Deployment Group, are  
> using CURIEs in several specifications for all the reasons stated  
> above.  Note also that the CURIE specification is a Rec-track document  
> that has already completed last call and will soon transition to  
> Candidate Recommendation.
>> (5) If it's desired to provide the better validation and easier access
>> to the namespace binding which would be provided by using the
>> xsd:QName type, but nevertheless not to rule out the use of CURIEs
>> which are not QNames, then we suggest the best way to define the role
>> attribute right now would be to define (1) a union of QName and CURIE
>> (in that order), and (2) a list of values from that union, and to make
>> the latter the type of the role attribute.  That would ensure that
>> XSD-aware software would provide access to the namespace names when
>> possible, and leave the task to the application only when necessary.
> We have an XML Schema definition for the datatype xh11d:CURIE that we  
> believe addresses this concern.  You can see that definition in the  
> CURIE specification at  
> http://www.w3.org/MarkUp/2008/ED-curie-20080617/#s_schema (or the latest  
> version of same).  We hope that these data type definitions address your  
> concern here.  Basically the Schema definition is an expansion of the  
> QName schema definition.
>> (6) A second concern is that we are unable to locate an XSD definition
>> of the datatype xh11d:CURIEs.  In general, the documentation for the
>> namespace <http://www.w3.org/1999/xhtml/datatypes/> falls, we regret
>> to say, somewhat short of the standard you set with your namespace
>> document for <http://www.w3.org/1999/xhtml/vocab/>.
>> Because we have not been able to find the XSD definition, we have not
>> been able to evaluate the XSD implementation of the role attribute.
>> What's present in appendix B.1 looks fine as far as it goes, but the
>> utility of the module really depends on the definition of the datatype
>> xh11d:CURIEs.
>> If you can point us to the XSD schema document which contains the
>> definition of that datatype, we will be happy to review it.
> Thanks for pointing out that the datatype document is not up to scratch  
> - we have neglected that for quite some time.  I (Shane) have taken an  
> action to update it.  In the interim, please look at the datatype  
> definitions in XHTML Modularization  
> (http://www.w3.org/TR/2008/PR-xhtml-modularization-20080611/schema_module_defs.html#a_module_XHTML_Datatypes)  
> - those are definitive.  Or of course at the definition for just the  
> CURIE datatypes using the reference already provided.
>> (7) We note that unprefixed names in the value of the 'role' attribute
>> effectively default to the XHTML namespace.  The first paragraph of
>> section 3 says in part:
>>     Any non-qualified value MUST be interpreted as being from the
>>     XHTML vocabulary at http://www.w3.org/1999/xhtml/vocab#.
>> We are of mixed mind about this; the longer we have thought about the
>> matter, the less certain we are that we have understood just what the
>> sentence is intended to say, and the more likely it seems that it
>> touches on important fundamental design issues for the use of
>> namespaces in XML.
>> On the one hand, this rule seems parallel to the rule in XPath 2.0 and
>> related specifications that there is a separate default namespace for
>> function names, which means that a call to count(), for example, need
>> not be qualified even if a default namespace in the context in which
>> it occurs.  Since the XML Query and XSL Working Groups provided a
>> specialized default namespace in this way, it would seem inconsistent
>> to object to your making a somewhat similar rule for the role
>> attribute.
>> On the other hand, the specialized rule for the default function
>> namespace was forced upon XPath 2.0 by the requirement for
>> compatibility with XPath 1.0, and might well have been avoided had
>> compatibility not made it necessary.  Do similar compatibility issues
>> arise for the role attribute?
>> The biggest problem is just that the existing xsd:QName datatype, and
>> datatypes constructed from it in the usual ways, already provide a
>> rule for deciding how to interpret unprefixed names in a context
>> where namespace prefixes (e.g. as part of QNames or CURIES) can
>> appear: they are assigned to the default namespace.  There is no
>> general-purpose mechanism for changing the default namespace just for
>> the value of a single attribute.
>> Either the idiom you seem to be proposing is a good one, and the
>> definer of an attribute or element or type should be able, as a
>> general principle, to specify that what namespace bindings should
>> apply, or at least what the default namespace should be, in values of
>> that attribute or element or type, or else the idiom is not a good
>> one, no general mechanism is needed, and you need to be persuaded that
>> the idiom you seem to be proposing is not a good idea.
>> For a mixture of technical and aesthetic reasons, we lean toward the
>> latter view.  The aesthetic reasons are simple: there are already too
>> many different rules for interpreting unprefixed names (in the
>> default namespace, for elements and QName values; in no namespace, for
>> attribute names; in the default function namespace, for function
>> calls), and adding new ones will not make the world a better place.
>> The technical reasons are also simple: we see no prospect of being
>> able to support this kind of mechanism in the generic XML tool stack;
>> it raises too many issues, and introduces too many incompatibilities
>> with the existing XML infrastructure.
> I think the fundamental disconnect here is that you are conflating  
> CURIEs and QNames.  And that is surely our fault - the XHTML Role  
> specification assumes a knowledge of CURIEs and what they are.  The  
> CURIE spec is referenced normatively, of course.  But that doesn't mean  
> most people will have read it nor understood it.  We will add some text  
> to help clarify what a CURIE is.
> However, to respond to your concern:  CURIEs are not QNames.  While  
> CURIEs permit the (re)use of xmlns declarations to define prefix  
> mappings, CURIEs do not in general take advantage of the "XML Namespace"  
> infrastructure.  In particular, CURIEs do not recognize the concept of  
> the "default" XML Namespace as being any sort of a CURIE mapping at  
> all.  Instead, the CURIE specification indicates that grammars are  
> permitted to define their rules for default prefix mapping.  In XHTML  
> Family documents, we have declared that unqualified (unprefixed) CURIEs  
> must be interpreted as being from the XHTML Vocabulary - a basic set of  
> terms.  So to respond to your first point above, the role values do not  
> default to the XHTML namespace.  They default to the XHTML Vocabulary  
> namespace.
> To get back to your point about the general idiom:  The working group  
> has considered a number of ways to deal with the problem of unprefixed  
> values.  Relying upon the default XML namespace was felt to be  
> inappropriate for a number of reasons - but mostly for reasons of  
> content portability (so-called cutting and pasting).  We also have a  
> strong requirement that it be possible to use basic terms without  
> specifying a prefix - this is associated with ease of use.  Our solution  
> to these two requirements was to resolve that unprefixed CURIEs *can* be  
> mapped to some pre-defined prefix.  In the case of the XHTML family, we  
> have declared that prefix to be the XHTML Vocabulary URI.
>> (8) On yet another hand, we note with a mixture of relief and anxiety
>> that we were obliged to speak above about "the idiom you seem to be
>> proposing" -- it's not entirely clear what you are proposing, and so
>> it's possible that we have misinterpreted it.
>> Since it is usual for XHTML documents to use
>> <http://www.w3.org/1999/xhtml> as the default namespace, it will
>> normally be the case that unprefixed names in the value of the role
>> attribute are asigned by the usual rules for interpreting namespace
>> prefixes to the XHTML namespace.  If the remark
>>     Any non-qualified value MUST be interpreted as being from the
>>     XHTML vocabulary at http://www.w3.org/1999/xhtml/vocab#.
>> refers only to this fact, then we suggest only that it be rephrased.
>> If, on the other hand, it is intended to mean that any token in the
>> value which has no namespace prefix and no colon is to be assigned to
>> the XHTML namespace, then we think that this is inconsistent with the
>> normal rules of namespaces (see point (7) above).
> Again - CURIEs are not QNames.  They must never be interpreted as such.   
> A CURIE's lexical space is [ [ prefix ] : ] reference.  Its value space  
> is IRI.  For purposes of processing, there is no "namespace".  If you  
> are doing lexical space processing, then the CURIE is a token.  It won't  
> mean much, but if you want to look at it as a token go ahead.  In order  
> to do real processing on a CURIE (comparison, dereferencing, whatever)  
> you need to look at the value space.  In the value space, there are no  
> prefixes.  There are IRIs.
>> (9) In point (8) we took the sentence
>>     Any non-qualified value MUST be interpreted as being from the
>>     XHTML vocabulary at http://www.w3.org/1999/xhtml/vocab#.
>> to be referring to tokens in the value which lack namespace prefix and
>> colon.
>> Strictly speaking, though, the usual terminology for such tokens calls
>> them "unprefixed", not "unqualified" -- unprefixed names are in fact
>> namespace-qualified if they are assigned to the default namespace.
>> So a third interpretation of the sentence is possible, namely that it
>> is intended to be read as speaking not about unprefixed tokens but
>> about unqualified tokens, and as saying about them, in effect, that if
>> the normal rules for resolving namespace prefixes leave a token
>> unqualified, then (since the namespace rules have NOT assigned the
>> token to a namespace) an application-specific rule associated with the
>> role attribute specifies that they should be interpreted as belonging
>> to the XHTML namespace.
>> This interpretation relies crucially on the subtle point that
>> unqualified names are not assigned by the namespace specification to a
>> magic or default or anonymous namespace, and similarly are NOT said by
>> the Namespaces Rec to be in no namespace at all (although this
>> paraphrase is frequently encountered); they are simply not assigned to
>> a namespace by the Namespace Rec.  There seems no reason that they
>> could not be assigned to a namespace by an application-level
>> convention.  But we note that this area is rife with confusion, and we
>> suggest that if you intend this interpretation, you explain it very
>> carefully.
>> No matter which interpretation of this passage you intend, it probably
>> should be recast to make the meaning clearer.  As indicated above, the
>> interpretation outlined in comment (7) would cause us grave
>> misgivings; that in (8) no misgivings at all; that in (9) would give
>> food for thought.
>> In any case, we believe that this point in your design requires
>> careful coordination between the XHTML Working Group and the Working
>> Groups in the XML Activity, and we invite you to a dialog about the
>> relevant issues.
> I don't think you need to read too much into the sentence.  I have  
> changed the wording in the current development draft so that we do not  
> use the term "qualified" but instead talk about "prefixed" and  
> "unprefixed".  At issue here is the fundamental theory of CURIE  
> operation.  All of your points above seem to want to treat CURIEs as  
> QNames.  CURIEs are not QNames.  There are no "normal rules" for  
> processing such items.  There are rules as defined in the CURIE  
> specification. The issue of the "default" prefix for CURIEs is a  
> difficult one, and the working group has debated this for many months.   
> In the context of RDFa, we cheated.  I suspect that, in the interest of  
> reducing confusion, we should cheat here as well. In RDFa Syntax, we  
> define a collection of "reserved values" for @rel and @rev. We declare  
> that there is no support for unprefixed CURIEs, and instead define the  
> datatype for @rel and @rev to be ( Reserved Word | CURIE )+ or something  
> like that.  We further say that when encountering reserved words, they  
> must be interpreted as being from the XHTML Vocabulary.  I do not think  
> that this addresses the fundamental problem though.  That seems to be a  
> confusion between CURIEs and QNames. Please review your comments in the  
> context of CURIEs instead of in the context of QNames and see if that  
> helps to allay your concerns.
Received on Tuesday, 30 September 2008 09:47:11 UTC

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