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Re: XML Schemas patterns (was: Re: Defining recursive elements?)

From: Pete Cordell <petexmldev@tech-know-ware.com>
Date: Thu, 17 May 2007 23:06:56 +0100
Message-ID: <001401c798cf$b4bc73d0$4200a8c0@Codalogic>
To: <noah_mendelsohn@us.ibm.com>
Cc: <xmlschema-dev@w3.org>

I often see things like:

  <element name="thing">
        <element name="height" type="measurementType"/>
        <element name="width" type="measurementType"/>

<complexType name="measurementType">
        <extension base="float">
            <attribute name="units" type="string"/>

To me, this results in a lot of duplication if everything is to be made 
global.  I think it would also be harder to read as you would see the local 
ref to height and then have to find the global definition (which could be a 
long way away) to find out what it's type is.

Similarly, the following seems simpler (and thus preferable) to me:

  <element name="tvProgramme">
        <element name="start" type="datetime"/>
        <element name="stop" type="datetime"/>

instead of:

  <element name="tvProgramme">
        <element ref="start"/>
        <element ref="stop"/>


    <element name="start" type="datetime"/>
    <element name="stop" type="datetime"/>


Pete Cordell
Tech-Know-Ware Ltd
for XML Schema to C++ data binding visit

----- Original Message From: <noah_mendelsohn...>

> Michael Kay writes:
>> But it was written before anyone had any awareness of the impact on
>> schema-aware queries and stylesheets. This changes the rules,
>> for example it
>> becomes much more important to define global elements and types
>> so that you
>> can use their names in function signatures.
> Yes, and at the risk of being controversial, I'll go further:  I think
> local element declarations have been oversold in XML schema.  In part,
> this is because we made what is a mistake in my opinion, which is to make
> it convenient syntactically to define local elements, and somewhat
> clumsier to use global ones.  Because the form:
>  <element name="outer">
>    <sequence>
>        <element name="inner1" type="t1"/>
>        <element name="inner2" type="t2"/>
>    </sequence>
>  </element>
> is convenient, obvious, and isomorphic to the instances it validates,
> people use it.  Local elements also appear in early examples in the
> primer, so people think they're the obvious way to do things.   I believe
> that all this is to some extent a mistake.    Local element declarations
> are essential in the particular case where you need conflicting
> declarations for the same named element according to context.  When that's
> what you need, use locals.  Otherwise, I think globals are more robust,
> for the following reasons, among others:
> * It's a single uniform model: even when you want to use locals, you need
> a global to wrap them in.  That makes your schema asymmetric, with some
> globals and some locals.  Using all globals means all elements are defined
> using the same constructs.
> * You can begin validation from any global element, which means that you
> can validate incrementally when editing pieces of documents.
> * XML vocabularies are often designed to be reusable across documents. You
> can share references to globals, not locals.
> * The whole business of elementForm and elementFormDefault doesn't come
> up.
> * As Michael says, you have a first class construct for use in function
> signatures, etc.
> Global element declarations are the ones that correspond most closely  to
> what you get with DTDs, and thus are reasonably well understood in their
> implications.  Historically, locals were added to the schema language
> primarily to allow for mappings of existing programming language
> structures that might themselves provide local scoping (the same named
> property x as an int or a float according to the struct in which it
> appears.)  For those cases, fine, use them.
> I think the schema language looks a lot simpler, conceptually if not
> syntactically,  if you start by forgetting about locals.  Don't learn
> them, don't use them.  You'll find a language that's easier to teach and
> easier to learn.   You can always learn about locals in the rare cases you
> need them.  (Or, you can learn about locals so you can understand the 98%
> of schemas or whatever it is that use them unnecessarily.)  In retrospect,
> I would have preferred if the syntax above created globals for inner1 and
> inner2, with some override needed to cause local scoping.
> Noah
> --------------------------------------
> Noah Mendelsohn
> IBM Corporation
> One Rogers Street
> Cambridge, MA 02142
> 1-617-693-4036
> --------------------------------------
Received on Thursday, 17 May 2007 22:07:29 UTC

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