Inheritance in XML (was Re: Problems parsing XML)

[Jeff Larson]

At 03:56 PM 4/17/98 -0400, Paul Prescod wrote:
>> Take for example an Excel spreadsheet.
>
>You can modify it reliably if the DTD/schema is complete. If not, you
>guess, just as with a partially documented API.

Excel was perhaps a bad example, but in general I disagree because the DTD
isn't enough to capture all of the various integrity constraints that may
exist between elements representing components of a data structure.  It
may be possible to represent some of these constraints through content
models, but certainly not all of them.  

If the schema was all you needed, then there wouldn't be much point
to the OO concept of the "setter" method.  You would just make all of
your data members public, and tell everyone not to break the rules, however
complicated those may be.  In practice, this doesn't work, and there is a
lot of value to be had in using methods (an API) to ensure that the
constraints are not violated.

>Not in my experience. There are dozens of tools that I can download that
>work with RTF, Frame MIF or PDF, and a small handful that talk to the
>Word, FrameMaker or Adobe Acrobat APIs. Furthermore, the formats
>described above have multiple independent implementations. The APIs do
>not.

Certainly there will be a few important and widely used data models around,
especially for the representation of documents, and XML is perfect for this.
The semantics of the data model will be well understood, enabling anyone
with enough time on their hands to write dozens of tools that operate reliably
upon the data.  To me though, this is still an API, my application isn't
parsing the file, its poking at it through the "tool" which is where the
semantics have been encapsulated.  If XML makes it easier for these
eager tool hackers to do their thing, then great, I'm all for it.

However, lets say I'm the vendor of some relatively esoteric thing, and I
need to design a file format to capture the state of my application.  Do
I use XML?  Sure why not, but do I really gain anything from this?  The
hackers with time on their hands are busy writing tools to edit RTF and
DOOM levels, they don't care about my nuclear power plant application.  
Even if someone did decide to write a nifty utility that operates on my
files, if they get it wrong, then Cleveland starts glowing, so I probably
don't want their help anyway.

I'm not against XML, I think its a great thing, and we should encourage
the vendors of major applications to support it, along with the copious 
DTD documentation that will be necessary to do anything useful with it.  
However, I think the notion that just storing my application data in 
XML will automatically make it more useful to the world is a bit presumptuous.

Jeff


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