Re: [xml-dev] What does "optional" mean?

[Michael Kay <mike@saxonica.com>]

On 26/02/2012 14:11, Costello, Roger L. wrote:
> Hi Folks,
>
> Every schema language (DTD, XSD, RNG) has the notion of optional -- optional elements and optional attributes.
>
> But what does it mean to declare an element or attribute optional?
>
>
Go and read the relational database literature from the early 80s (?) 
when there was a religious war about the meaning of null, with Ted Codd 
and co writing papers advocating that there should be four or five 
different kinds of null with different meanings. Then when you've read 
them, chuck them in the bin.

Just as the attribute iubce='qwudob' means whatever the sender and 
recipient agree it should mean, so equally the absence of the iubce 
attribute means whatever the sender and recipient agree it to mean, 
which might be "I don't know", or "not applicable", or "I don't think 
you want to know this", or "you already know this so I'm not telling you 
again", or perhaps it means exactly the same as iubce='qwudob'.

Michael Kay
Saxonica