Bob Morris wrote:
Here's a proposed requirement for data exchange, brought home by one of Leigh's examples:
<RQT> It should be possible from the data itself, perhaps together with external resources to which it refers, for software to determine whether the order matters for any collection of tags in the data.</RQT>
I agree, especially that the software should be able to determine it. Regarding XML, tags, and DTDs, I believe, the assumption is that order does not matter, wherease an order can be imposed on attributes within a tag, correct?
One answer is "order always matters". That would be regrettable. For example, being able to detect that some list is unordered can make it easier to know whether two data streams represent the same object. (That's probably a deeper desideratum.)
Does order matters for our problem? Where would it matter, any examples from descriptive data?
In fact, thinking in my own relational information model, we have a problem here: Some parts of the structure will be relating to records, i.e. instances of an entity type. In my thinking, the order of items to be described, and the order of characters used to perform the description should NOT matter. It is a plausible user requirement to report any of them in any order.
That does not mean that characters or states do not have an order, but this order is secondary, and multiple orders are desirable. Being ordered should not be a requirement of the exchange format.
However, there are other data elements that refer to attributes of entities. Ordering them makes some parsing operations easier. However, what fundamental advantages would it have to impose an order on them?
Finally, some item are ordered, e.g. it will be desirable to impose an order on multiple states in a descriptions. Written in (loose) DELTA:
1,1/<rarely>2 -> Flowers blue or rarely white
may be different from
1,2/1<rarely> -> Flowers white or rarely blue
However, I believe that the DELTA method of using a text-sequence is not necessarily the best solution. Alternatively, a data element may be introduced to store the ordering information. For example, DeltaAccess handles this problem with the numeric SEQ attribute of the DESCR entity.
Gregor ---------------------------------------------------------- Gregor Hagedorn G.Hagedorn@bba.de Institute for Plant Virology, Microbiology, and Biosafety Federal Research Center for Agriculture and Forestry (BBA) Koenigin-Luise-Str. 19 Tel: +49-30-8304-2220 14195 Berlin, Germany Fax: +49-30-8304-2203
Often wrong but never in doubt!