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Paul Prescod wrote:
> Now what would we say to differentiate XML from SGML? How would I decide
> when my customers should use SGML instead of XML? What sort of problems
> are uniquely suited to non-XML SGML?
Documents that are valid, for starters. Well-formed documents are a double-edged
sword, while they may suit the data provider, they might not suit the recipient. In
XML, you're compelled to employ a business rule to ensure that what you get is what
you want. No such ambiguity exists for SGML - documents are either valid, or
rejected (leaving aside the semantics of the markup).
Generally, I think of SGML as being more suitable to certain datasets, not
necessarily to solve a different set of problems. I consider SGML data to be
static, and large, and fixed. I consider it to be further abstracted from delivery
than many XML systems. SGML is old, and staid, and predictable. (These are good
> But is anyone using SGML features that were left out of XML? CONCUR,
> LINK, DATATAG, OMITTAG, RANK, SHORTTAG? I still use some for my personal
> projects but I don't encourage my customers to. If the only difference
> between SGML and XML in common usage is the empty-tag and processing
> instruction syntax then SGML survives but not in a sense that I find
> very meaningful.
Some of them I have never used, but others I use on a daily basis, particularly
OMITTAG and SHORTTAG. Even for projects that require XML for the deliverable, we
almost always use SGML to get there. We are also doing a number of Defence projects
at the moment, so of course they are all SGML.
> Did SGML matter? Yes, of course, without it XML would not exist or might
> well be radically different (s-expressions?). Is SGML a thriving,
> growing technology separate from its XML incarnation? I personally do
> not think so. SGML has achieved its success under the name XML.
SGML wouldn't be a thriving growing technology even if XML had never come along.
It's not the sort of technology that was ever going to aquire mass acceptance. Due
to the narrow band of the data that it did well, it was kept pretty much "under the
hood", and I think that's where it belonged. XML is used in many more visible
contexts, that's probably why it was made simpler and the rules relaxed. As you
say, the differences between the two don't generally merit switching - the
differences to me originate more in philosophy than in syntax.
Marcus Carr email: email@example.com
Allette Systems (Australia) www: http://www.allette.com.au
"Everything should be made as simple as possible, but not simpler."