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Let me explain the kind of contradictions possible --
Let me first introduce tree types, and hedge types. Tree types produce
trees, and hedge types produce hedges (where hedge is an ordered list of
trees).
Example of tree type is
Book -> book (Title, Author*)
Example of hedge type is (Title, Author*)
In XML-Schema "effectively", subtyping is defined on the basis of hedge
types.
For example consider hedge type H1 -> (Title, Author*)
and hedge type H2 -> (Title, Author, Author)
Now consider a function which takes a parameter an object of type H1 and
returns nothing, which is defined as say:
F : H1 -> null
Now one contradiction is: Suppose we call the function F as
F (o:H2) -- that is we call F with an object o of type H2.. Is this
valid?? In other words does this statement type check correctly when we
compile??
There are two possible solutions, because any hedge produced by H2 will
also be of type H1, so it is fine... -- this is what is done when
subtyping for XML is what I would call "Implicit subtyping based on
inclusion" -- this is XDuce.
The other is no -- because H2 is not declared as a subtype of H1.. This is
how it is done in XML Schema.
The good thing is subtyping defined in XML Schema does not seem to have
"problems" in whatever sense, but the question is whether that is the way
subtyping should be defined in the first place... and what other options
are there..
cheers and regards - murali.
On Thu, 16 May 2002, Dare Obasanjo wrote:
> As for the question on using block for restriction which I assume means
> using the block attribute on a complexType definition, the answer is
> that you'd block restriction to prevent types derived by restriction
> from replacing the content model of base types in an instance document.
>
> This is also described in the XML Schema Primer in
> http://www.w3.org/TR/xmlschema-0/#ref28
>
> --
> PITHY WORDS OF WISDOM
> Experience is something you don't get until just after you need it.
>
> This posting is provided "AS IS" with no warranties, and confers no
> rights.
> You assume all risk for your use. (c) 2002 Microsoft Corporation. All
> rights reserved.
>
>
>
> > -----Original Message-----
> > From: Nada Reinprecht [mailto:Nada_Reinprecht@nemmco.com.au]
> > Sent: Thursday, May 16, 2002 4:04 PM
> > To: Murali Mani
> > Cc: xml-dev@lists.xml.org
> > Subject: Re: [xml-dev] Two quick questions regarding
> > restriction in XML Schema
> >
> >
> >
> > Muralli,
> >
> > a) You do not need to use xsi:type in the document if you
> > derive the type by restriction. In the schema you can simply
> > associate the derived type with an element.
> >
> > However, if you want to implement alternative contents for
> > the same element, than you use the xsi:type which allows you
> > to specify desirable content in each document.
> >
> > Cheers,
> >
> > </nada>
> >
> >
> >
> >
> >
> > Murali Mani
> >
> > <mani@CS.UCLA To:
> > <xml-dev@lists.xml.org>
> > .EDU> cc:
> >
> > Subject:
> > [xml-dev] Two quick questions regarding
> > 17/05/02 restriction in XML
> > Schema
> > 02:42
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > I think these are related.. Can someone tell me --
> >
> > a) If we derive types by restriction, do we have to use
> > xsi:type in the document? Is it true if I say that the
> > document will be valid irrespective of whether we use
> > xsi:type or not .. We use xsi:type to know the "precise" type??
> >
> > b) Is there any point in using block for restriction
> >
> > thanks and regards - murali.
> >
> >
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