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Re: What are the characteristics of a good type system forXML?

• To: xml-dev@l...
• Subject: Re: What are the characteristics of a good type system forXML?
• From: "Thomas B. Passin" <tpassin@c...>
• Date: Tue, 13 May 2003 23:19:48 -0400
• References: <183.1adfd9d7.2bf204fc@a...><20030513214401.50a8f65d.amyzing@t...>

[Amelia A.Lewis

> It seems to me that the core XML 1.0 spec provides a definition of
> base-xml-string validation, in its well-formedness constraints for text
> and attribute nodes.  That is, base type validation is equivalent to
> well-formedness for text and attribute nodes.
>
> My sense is that when the spec authors begin talking about "value
> space", then the discussion may already strayed out of XML's yard and
> onto a busy street.  Is it important to be able to manipulate types?
> Sure.  Is it something that XML can do?  No.  But as Bob Foster points
> out elsewhere in this thread, it is something that transformation and
> query languages can do.
>

At first glance, this suggests that it is not __xml__ that needs a type
system, but the infoset (maybe), schema, transformation, and query
languages.  But there is a confounding factor, and probably that is what has
lead to so much confusion and complexity.

It is not enough to say "let this thing be an amy:duration". We also need to
know "this thing in such and such a location, but not in that other
location, must be an amy:duration".  So the value types easily get mixed in
with the element types, and bingo, the type system has become mixed up again
with the xml, which we hoped to avoid.

I think it will take some cleverness to see keep the type system from
getting mixed up with the structure, but I think it will be worth it.

Maybe a step towards keeping the type system out of the xml is to keep it
out of the infoset too.  Then the types get applied only after infoset
creation, not during it.  We have heard (I remember reading a post to this
effect but I forget who sent it) that this would be inefficient, but maybe
it would turn out to be not so bad after all.

In turn this notion implies the possibility of a validity state of "well
formed and correctly structured but with the wrong simple types somewhere".
That would be interesting because you could still picture doing useful
processing, and perhaps even casting the wrong types to the ones you were
interested in.

Cheers,

Tom P

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  • Re: What are the characteristics of a good type systemfor XML?
    • From: Amelia A.Lewis <amyzing@t...>

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