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----- Original Message -----
From: "Jonathan Robie" <email@example.com>
To: "Dare Obasanjo" <firstname.lastname@example.org>; "Champion, Mike"
Sent: Thursday, January 03, 2002 4:26 PM
Subject: Re: [xml-dev] The use of XML syntax in XML Query
> 2. Typechecking without type inference can work fine as long as you do not
> have joins. If joins are involved, typechecking becomes undecidable.
So what happens with XQuery joins ( http://www.w3.org/TR/xquery/#id-joins )
then? Will there be a caveat in the recommendation that indicates that static
typechecking can't be done w.r.t joins?
> 3. Type inference is the most promising approach, but it does lead to some
> false negatives. He gives an example using a content model involving equal
> cardinality among three different elements in sequence. The schema for this
> can not be expressed in DTDs or in XML Schema, so it is not clear to me
> that this is a real limitation, but I just read this, and I need to do some
> thinking before I would want to draw a strong conclusion based on his
But his example is fairly simple and utilizes none of the really complex
abilities of XML schemas. For instance, how can static typechecking work for
schemas that use identity constraints like xs:unique? I fail to see how one
can guarantee that the following expression
f = <results>
FOR $x in /employee/age
$x * 13
will only return unique values for <employee-id> via type inference.
> 4. Dan concludes that type inference is still the most promising approach
> to static typechecking, but that further work needs to be done on its
> applicability and limitations.
> My own take on this is that static type checking using type inference is
> very promising, and seems to work well in theory for the kinds of queries I
> have looked at. An implementation using the current type system did catch
> interesting errors for me. I think our type system needs further work, and
> we need more practical experience using implementations that do static type
> checking. This is one of the highest priorities for me personally.
I'm interested in the type of queries you've looked at. I'm not convinced that
this is as straightforward a problem as you've implied but readily admit that
my theoretical CS skills are nowhere near excellent so you may be right and
all I need is a little convincing.
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