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Re: XML Schema quiz on default values

From: Daniel Dui <daniel.dui@gmail.com>
Date: Fri, 24 Aug 2012 13:33:11 +0100
Message-ID: <CAACJ0wZapU2GYd3SXNuSrFf_MqNNWrf+LEdN5+358-huseriLQ@mail.gmail.com>
To: "Costello, Roger L." <costello@mitre.org>
Cc: "xmlschema-dev@w3.org" <xmlschema-dev@w3.org>
Roger et al., thanks for the clarification.

I have used XML Schema for a long time and this is definitely a dark corner
of the specs.

I always warn people about using default values and often recommend not
using them. This non-obvious defaulting behavior looks to me like
another reason why many schemas should avoid using default values.

-daniel

On 24 August 2012 12:48, Costello, Roger L. <costello@mitre.org> wrote:

> Hello Daniel,
>
> Suppose Title is declared like this:
>
>     <xs:element name="Title" type="xs:string" default="Hello World" />
>
> Note that a default value ("Hello World") is specified.
>
> Now suppose we have this empty Title element in an instance document:
>
>     <Title></Title>
>
> What is the value of Title? Answer: it is the default value ("Hello World")
>
> Conversely, suppose Title is declared like this:
>
>     <xs:element name="Title" type="xs:string" />
>
> Note that no default value is specified.
>
> Now suppose we have this empty Title element in an instance document:
>
>     <Title></Title>
>
> What is the value of Title? Answer: it is a string of length zero (the
> empty string).
>
> /Roger
>
>
> So what should an instance document look like to have the default value
> "Hello World"? Does this mean that the default value is of no practical use
> for an xsd:string type?
>
> -d
>
>
> On 24 August 2012 10:05, Costello, Roger L. <costello@mitre.org> wrote:
>
> > Thank you Michael and Pete for correcting my misunderstanding.
> >
> > Allow me to correct the answer to my quiz:
> >
> > I wrote this as the answer to my quiz:
> >
> >     >  Answer: the value of Title is the empty string, not the default
> > value.
> >     >  The reason is that the empty string is a valid value of the string
> > data type.
> >
> > However, the correct answer is:
> >
> >  Answer: the value of Title is the default value ("Hello World"). The XML
> > Schema specification explains why:
> >
> >     An element with a non-empty default value whose
> >     simple type definition includes the empty string in
> >     its lexical space will nonetheless never receive that
> >     empty string value, because the default value will
> >     override it.
> >
> > /Roger
>
>


-- 
____________________________________________________________
   Daniel Dui - daniel.dui@gmail.com - skype: danieldui
Received on Friday, 24 August 2012 12:34:33 GMT

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