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Re: [ontolog-forum] OWL and lack of identifiers

To: edbark@xxxxxxxx
Cc: Ontolog Forum <ontolog-forum@xxxxxxxxxxxxxxxx>
From: Waclaw Kusnierczyk <Waclaw.Marcin.Kusnierczyk@xxxxxxxxxxx>
Date: Tue, 17 Apr 2007 22:03:07 +0200
Message-id: <462527FB.1050507@xxxxxxxxxxx>

Ed Barkmeyer wrote:
> Wacek
> Now that we agree we are splitting hairs, ...
> you wrote:
>> A formal language is a set of strings over an alphabet -- a set of 
>> symbols.  You may define the language ostensibly, by listing all the 
>> strings, which would imply which symbols are in the alphabet.  You may 
>> define the language by means of a grammar, that is, specify the 
>> alphabet and rules for forming strings in the language.  You may do 
>> both, but the definitions must agree.
>> When you say that a URL may be a well-formed string and yet not in the 
>> language (i.e., not registered by IANA as an element of the language), 
>> then you state a contradiction.  Either the string is not well-formed, 
>> or it is in the language.
> Note that the XML Recommendation also makes a distinction between 
> "well-formed" and "valid".  "Well-formed" XML fits the grammar given in 
> the XML Recommendation; "validity" is defined with respect to some 
> "document definition", which is, in effect, a further grammar imposed on 
> the XML lexical tokens.  The original DTD document definitions were 
> clearly only syntactic specifications, whereas XML Schema document 
> definitions have greater pretensions (they are more than syntax and less 
> than content models).  But if we ignore the bastard aspects of XML 
> Schema, what we have is two levels of syntactic requirements.  A string 
> can therefore be well-formed, and an instance of the "XML language" as 
> defined in the Recommendation, and still fail to be a valid "document", 
> i.e. fail to be an instance of the "document language" defined by the 
> document definition.  But they are both "syntactic" grammars, and a 
> failure to meet the specifications of either may therefore be considered 
> a "syntax error".  XML prudently avoids use of the term.    (01)

I was actually curious whether you will refer to the XML example.
The use of 'well-formed' and 'valid' here are synonymous in that both 
are used to speak of a document's conformance with a grammar;  equally 
well it might be said that a document is valid as a XML document and 
well-formed as an RDF document.  This is just a terminological convention.    (02)

The issue with IANA is a bit different, in that a domain name may be 
registered and unregistered, i.e., its status as valid or not may 
change, though the rules of the language do not change, in general.  On 
the other hand, a document that is XML-well-formed and XXX-valid remains 
well-formed and valid, unless the grammars are redefined.  (You might 
say that a change in IANA registry is, analogously, a change in the 
languages rules, if you insist.)    (03)

> My position was only that there are also two levels at work in the 
> "syntactic" validity of URLs.  One is being "well-formed", conforming to 
> the lexical requirements for a URL per RFC 2396.  The other is that the 
> 'host' token is "valid", as defined by the IANA registry.  One could 
> equally well argue that the latter is a "semantic" concern.  But there 
> are clearly separable semantic concerns -- whether the URL refers to 
> something and whether that something is accessible.    (04)

The issue is that in the case of XML the use of 'well-formed' for the 
one and 'valid' for the other is well-defined and documented.  In the 
case of URLs, I am not sure whether there is an officially established 
convention of calling a URL 'well-formed' if it conforms to the RFC and 
calling it 'valid' if it is registered;  maybe there is, but maybe this 
is only wishful thinking.    (05)

vQ    (06)

> The problem seems to be that in these ad hoc languages with multiple 
> levels of parsing grammars, the traditional terminology of formal 
> languages -- lexical, syntactic, semantic -- is inaccurate and 
> insufficient.  By the same token (so to speak), the OMG Model-Driven 
> Architecture has run aground on the inability to define how many levels 
> of model there are and how to associate content with levels.
> What is at work here is that computer science has made a major advance 
> in the last 10+ years:  Where we used to have tools that defined the 
> capabilities, and languages that matched the tool capabilities, we now 
> have dynamically configurable tools that support the capabilities, and 
> languages that let the user define the capabilities.
> -Ed
> P.S. With respect to the last, I'm sure people will tell us that tool 
> suite XYZ had this capability 30 years ago.  The point is that this is 
> now mainstream behavior, not the uncharted island of technical 
> excellence in a sea of average competence.
>     (07)

Wacek Kusnierczyk    (08)

Department of Information and Computer Science (IDI)
Norwegian University of Science and Technology (NTNU)
Sem Saelandsv. 7-9
7027 Trondheim
Norway    (09)

tel.   0047 73591875
fax    0047 73594466
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