W3C home > Mailing lists > Public > xmlschema-dev@w3.org > February 2001

Re: What is ur-definition ?

From: <Noah_Mendelsohn@lotus.com>
Date: Mon, 19 Feb 2001 10:44:10 -0500
To: Michael Anderson <michael@research.canon.com.au>
Cc: xmlschema-dev@w3.org, xmlschemadev@hotmail.com
Message-ID: <OF9CFAE61C.A0090413-ON852569F8.005679D4@lotus.com>
Michael Anderson writes:

>> Is this right?  I thought anyType can 
>> function as a complex type _only_,
>> not a simpleType.

There are a lot of subtleties in this area, but the simple answer is: " 
The anyType (the urType) functions as both simple and complex."  One way 
to think about this is to notice that anyType has mixed content, not 
element only.  So, if we have:

<element name="A" type="anyType"/>

that will validate, among other things,

 -or -
<A>Here is some <emph>interesting</emph> text </A>
<A>Here is some text</A>

You can see that the last example looks like simple content.  In the 
interest of having a single type hierarchy, that same declaration 48 also 
validates both of the following:

<A xsi:type="xs:integer">123</A>

The first one of these two is just the same as the last of the three 
above.  The second one is where we make the leap that makes it clear that 
even numeric types such as integer are viewed as restrictions of the 
urType (anyType).

And here is one more fancy example (presuming targetNamespace prefixed by 

<element name="A" type="anyType"/>
<element name="I" type="integer" substitutionGroup="myschema:A"/>
<element name="C">
                        <!-- the following allows substitution group 
members too -->
                        <element ref="myschema:A"/>

This will validate both of the following, among others:

<C><A>Here is some <emph>interesting</emph> text </A></C>

Note that the integer element <I> can be in the substitution group of <A> 
because its type integer is indeed a subtype of the urType.

Again, this is very subtle stuff, but it mostly just does what you want. 
Don't be misled: the urType is magic, just as "Object" is magic in Java. 
The specification shows you an approximation to the declaration of the 
urType, but it is just an approximation.  This type has characteristics 
that you cannot express my writing down an ordinary complex type 
declaration.  As noted in the original quote, this type has the unique 
characteristic that it functions as the root of the derivation tree for 
all types in the hierarchy, both simple and complex, including mixed.  We 
have given a name to its simple manifestation, anySimpleType, because that 
is what you need to declare things like attributes that do not allow 
element content under any circumstances.

I hope you find this explanation to be helpful.

Noah Mendelsohn                                    Voice: 1-617-693-4036
Lotus Development Corp.                            Fax: 1-617-693-8676
One Rogers Street
Cambridge, MA 02142
Received on Monday, 19 February 2001 10:56:56 UTC

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