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Re: What is the meaning of the first child element foran elem

• From: "Liam R. E. Quin" <liam@fromoldbooks.org>
• To: "xml-dev@lists.xml.org" <xml-dev@lists.xml.org>
• Date: Tue, 13 Oct 2020 02:54:31 -0400

On Mon, 2020-10-12 at 16:43 +0000, Roger L Costello wrote:
> Hi Folks,
> 
> A specification says that a book has one or more titles. Here is an
> example of how the developers implemented the specification:
> 
> <Book>
>     <Title>Draft No. 4</Title>
>     <Title>On the Writing of Prose</Title>
> </Book>
> 
> The specification says that the first title is the primary title.

Then it should probably have a different name -
    <Title>Draft No. 4</Title>
    <SubTitle>On the Writing of Prose</Subtitle>
for example.

> 
> To obtain the primary title, the developers created an application
> that queries the <Book> element for the first child <Title> element.
> For the example, this <Title> element is fetched:
> 
>     <Title>Draft No. 4</Title>
> 
> Do you agree with the developer's approach to obtaining the primary
> title?

OK so far, given the premise.

> 
> The developers implemented the XML Schema this way:
> 
> <element name="Book">
>     <complexType>
>         <sequence>
>             <element name="Title" maxOccurs="unbounded" type="string"
> />
>         </sequence>
>     </complexType>
> </element>
> 
> That says Book contains one or more Title elements. It says nothing
> about ordering the <Title> elements. It does not say that the primary
> title must always be the first child <Title> element of <Book>.

I would want to see an explicit minOccurs="1", to emphaisze that it's
required.

> 
> It seems to me that there is a risk with creating applications which
> assume the first child <Title> element of <Book> is the primary
> author. Do you agree that there is a risk? 
The first Title is the primary title but not the primary author.

However, the information that one title element is different from the
others should be encoded, not least to help authors or programmers
generating the content, e.g. in case they accidentally omit the primary
title.


> 
> To avoid the risk, do you add additional information to explicitly
> indicate which <Title> element holds the primary title? E.g.,
> 
> <Book>
>     <Title primary="true">Draft No. 4</Title>
>     <Title>On the Writing of Prose</Title>
> </Book>

In general I'd say use a different element name, since it's different,
but an attribute is fine too, although possibly harder for authors.


> 
> Or is that being excessively cautious?
No, it's sensible.

Liam



-- 
Liam Quin, https://www.delightfulcomputing.com/
Available for XML/Document/Information Architecture/XSLT/
XSL/XQuery/Web/Text Processing/A11Y training, work & consulting.
Barefoot Web-slave, antique illustrations:  http://www.fromoldbooks.org

• References:
  • What is the meaning of the first child element for an element that isrepeatable?
    • From: Roger L Costello <costello@mitre.org>