SGML, XML and SML
Daniel.Brickley at bristol.ac.uk
Mon Nov 22 08:26:09 GMT 1999
On Sun, 21 Nov 1999, Paul Prescod wrote:
> Don Park wrote:
> > I don't understand what 'this' is. Are you suggesting that SML should
> > be abandoned in favor of S-expressions or are you saying that S-expressions
> > didn't fly so SML won't either?
> Here's what I'm saying:
> * XML has lots of stuff that most of these applications do NOT need.
> * XML *lacks* some important things that these applications DO need,
> the most important being a structured property syntax
> A language with neither of these flaws could be designed in a week and a
> parser for it could be written in half a day in any language. Writing
> such a parser would be not that much more work than strapping in an XML
> parser -- actually it would be less if there is no parser that meets
> your needs. Such a language could be DOM-compatible, XPath-compatible,
> XSLT compatible and so forth if it was designed to be.
> Therefore, why force unminimized angle brackets to do something that
> they are weak at? Just to prove the point that every data type in the
> universe can have a single syntax?
Hmm... reminds me... The original PICS-Next-Generation (aka PICS
2) proposal, the precursor to RDF, used an s-expression based
syntax. This was of course dropped in favour of an XML syntax.
Excerpt from http://www.w3.org/TR/NOTE-pics-ng-metadata
5.1. S-Expression Syntax
The syntax of PICS-NG is greatly simplified from that of PICS version
1.1. Basically PICS-NG syntax in a straightforward manner consists of
s-expressions where additional restrictions are placed on the types of
values of certain elements of s-expression structures. PICS-NG parsing
is a multi-step process. Parsing of a single label happens as follows:
1.A simple s-expression parser is used to parse (and verify) the
overall syntactic structure (given below in the form of a BNF
This is the only step necessary if one is not interested in any
semantic interpretation of the label (if the data is only passed
through, if the
parsing agent has no knowledge of the schemata used, etc.).
2.Information from each of the schemata of a label is used to verify
that attribute values have legal values.
3.Any other information from the schemata is used for semantic
interpretation of the label.
This syntax has been chosen because it is simple to parse, provides a
straightforward correspondence between the model and the syntactic form
of the data, is brief (good "over the wire" -characteristics), and (by
not being too verbose) is easy for humans to read and write. A BNF
of the overall syntactic structure is given below (despite that fact
that BNF rather poorly lends itself to describing s-expressions)
What goes around comes around! The RDF Interest Group  is currently
discussing alternate syntaxes for RDF in XML. Sounds like we're being
told we'd be better off returning to PICS-NGesque s-expressions. I doubt
that this would be well received... ;-)
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