words (RE: extensibility in XSchema?)
peter at ursus.demon.co.uk
Mon Jun 22 22:51:42 BST 1998
At 23:50 22/06/98 +1000, Rick Jelliffe wrote:
>This is why the particular word "semantics" is worthless. People
>use it so equivocally that sentences containing the word are rendered
>useless. I try to avoid the word now. I try to stick with SGML's
> text = data + markup
>and use "data" and "information" wherever possible.
I'll buy this - at least to the extent that I shall try to avoid the term
semantics. However I need *some* terminology to describe what I want to
do. Perhaps we can converge on some of that (given also that spec drafters
are also trying to firm up *some* of the terminological problems). I shall
use the word 'meaning' at present.
I'm a pragmatist and need to address the following problems:
- how do I add human 'meaning' to an object (e.g. an element or attribute)?
- how do I add machine 'meaning' to an object?
At present there are *no* defined mechanisms for either of these in XML. I
find this amazing and regrettable (I - and others such have Rick) have been
shouting for it). Consider:
That's it. All we can say is there is a foo element with a bar attribute
whose value is baz.
<!DOCTYPE foo SYSTEM "foo.dtd">
The DOCTYPE adds the meaning, doesn't it? Nope, it says the following:
- the root element is a foo element. But we know that already. It does NOT
define the document as of type 'foo'.
- there is a DTD whose sole purposes are:
- to expand minimisation (irrelevant for this document)
- to validate the document.
Yes, you can fill the DTD with comments, but they have no formal meaning
and there is no way even of knowing which comment belongs to which element
apart from guessing. It is even worse with attributes, unless each exists
in a single <!ATTLIST>
Now compare MIME:
If I get sent a file of type image/gif I can:
- determine that 'image/gif' is a MIME type
- see if it is in the central IETF registry. If so I can read the formal
- *and most important for me* I can associate the MIME type with a program
through the mailcap file, *even if the MIME type is not registered*.
This is all I am asking for, but there seems to be no way to do a similar
thing for an XML element or attribute.
This is what I used to call semantics. If someone can come up with a better
term, it would be useful. RickJ uses the word 'bind' - I like that.
Peter Murray-Rust, Director Virtual School of Molecular Sciences, domestic
VSMS http://www.nottingham.ac.uk/vsms, Virtual Hyperglossary
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