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   Re: It's the syntax. . .

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  • From: "Eve L. Maler" <elm@east.sun.com>
  • To: XML Dev <xml-dev@xml.org>
  • Date: Fri, 10 Mar 2000 14:58:54 -0500

At 11:36 AM 3/9/00 -0500, W. E. Perry wrote:
>The ancillary standards promulgated or considered since XML 1.0
>are all (except SAX) attempts to canonize various
>semantically-intentioned structures. This is not a bad thing: well
>designed semantic shorthand can usefully simplify communication
>and drastically reduce the preliminaries required between
>well-acquainted correspondents. The original intent of XML itself,
>though, is different. SGML-on-the-Web was specifically intended
>for the autonomous nodes of an Internet topology who did not share
>the semantic infrastructure of a tight-knit markup community.
>Well-formedness was an explicit extension of the benefits of
>markup to those without the shared semantics of the DTD. Since
>those earliest days of XML, however, each new proposal has
>increased the requirement for semantics shared a priori, and
>thereby effectively subsetted the range of application for
>'conformant' XML. The monstrous complexity of XML Schemas and, at
>the apparent other end of the spectrum, the putative simplicity of
>SML are both pre-agreed semantic structures which brutally curtail
>what might otherwise be expressed, albeit in novel or unexpected
>ways, in the simple syntax of XML 1.0.

I'm sorry, but speaking as someone who was there, I disagree entirely with 
your perspective on the history of XML.  It is solely about syntax 
precisely *because* we knew it was impossible to anticipate the semantic 
needs of the world in a single standard; we fully expected all these 
various semantic efforts to get going.  In the old days, the SGMLers mocked 
the ODA (Office Document Architecture) effort not because it bothered to 
figure out semantics where none were needed, but because they dared to 
think that one sweeping standard would meet everyone's needs.

Also, we didn't invent well-formedness in order to eliminate the need for 
shared semantics; the original reason was to reduce bandwidth where the DTD 
(but really, the semantics, since DTDs have very little meaning all by 
themselves) was agreed on beforehand.  Being in possession of a single 
well-formed XML document, with no other information at hand to say how to 
interpret it, is extremely uninteresting.

>At this moment we should take a look at
>our roots, from which I (alone?) conclude that:
>     --XML was originally motivated by an Internet topology of
>autonomous, largely anonymous nodes. Given that in many cases the
>humans behind many of those nodes do not share the history nor the
>assumptions of the established players (whether 'established'
>means as standards-setters or, from the ecommerce perspective, as
>dominant forces in their vertical markets, or both) there is no
>easy--maybe no workable--way to cast the needs and assumptions of
>these newly-arrived participants into semantic structures which
>they had no hand in defining. One effect will surely be to
>exclude, at least from the more standard fora and markets, those
>for whom, presumably, XML went to the Internet in the first place.
>The irony is that XML 1.0, as syntax, continues to offer them the
>tools with which they might create a truly worldwide forum from
>which established players would be (self-)excluded by their own a
>priori semantic expectations.

Even "autonomous, largely anonymous nodes" need a shared context in which 
to exchange information.  If the point is to share information, then I 
don't see how standardized semantics can be considered a bottleneck.  I 
just called a fabric-cleaning person on the phone today, someone I've never 
met before.  I wouldn't have gotten anywhere if I'd simply said 
"subject-verb-object" to him, but because I could safely assume he spoke 
English, and knew something about fabric cleaning, I could say to him "My 
sofa has a stain" -- and we got something accomplished.

>     --The fundamental functions of an XML processor are syntactic.
>If the
>source of the XML were obliged to share, and therefore to know,
>sufficient semantics to be able perfectly to describe its intent
>to a separate processing node, why not simply do the processing
>itself and avoid the specifically-remote possibilities for failure
>which the RPC people have been drawing attention to these past few
>days? In other words, why shouldn't the
>assumption/expectation/prerequisite of XML processing be simply
>XML syntax, precisely as described in XML 1.0?

Different parties may want to perform different processing (e.g., 
translation of text vs. formatting it), but share a common understanding of 
what the content means in order to perform their different tasks 
successfully.  Or they may want to use different subsets of the shared 
information.  Or use different augmented *supersets* of the shared 
information.  Or it may be more efficient for the recipient to do the 
processing than the sender.  XML's standardized syntax facilitates all of 
these situations where nothing else could before.

>I believe that this is the processor which we should be building
>as the general case, and for which standards grounded in semantic
>infrastructure are merely a sideshow. The past two years have
>allowed the XML community to plumb fairly thoroughly the
>possibilities of pre-agreed semantics and perhaps usefully defer
>the really hard work of building on the broader syntactic base.
>That time of doodling has now passed.

Believe me, we've barely started to scratch the surface. :-)

>We need to build an XML
>processor that works like yacc, or, to take a more nearby example,
>which allows, as RDF intended, saying anything about anything in
>an XML syntax.

I don't mean to trivialize your points by eliding them, but I have no idea 
what it means to have a processor of XML (higher-level than a true "XML 
processor") that *doesn't* depend on semantics.  You can't glean any hidden 
instructions for what to do with XML stuff by merely parsing it; you need 
something in addition that tells you either (a) what things "mean" (perhaps 
in prose or perhaps by reference to shared semantical nuggets) or (b) what 
to do with them (something executable).

Eve Maler          Sun Microsystems    XML Technology Center
                    elm @ east.sun.com        +1 781 442 3190

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