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Re: PIs considered harmful Was: XML-SW, a thought experiment

From: Tim Berners-Lee <timbl@w3.org>
Date: Mon, 4 Mar 2002 16:14:32 -0500
Message-ID: <01ef01c1c3c1$947e8ab0$84001d12@w3.org>
To: "'www-tag'" <www-tag@w3.org>, "Elliotte Rusty Harold" <elharo@metalab.unc.edu>

----- Original Message -----
From: "Elliotte Rusty Harold" <elharo@metalab.unc.edu>
To: "'www-tag'" <www-tag@w3.org>
Sent: Saturday, February 23, 2002 7:27 PM
Subject: Re: PIs considered harmful Was: XML-SW, a thought experiment

> At 3:48 PM -0800 2/23/02, Piotr Kaminski wrote:
> >That's interesting, I didn't know.  However, I think my original point
> >still stands.  If you're using PIs to make DC annotations, how would you
> >"annotate the annotations" so to speak if nested PIs are not permitted?
> The annotation vocabulary could be explicitly designed to be
> recursive if this seems important.
> >But wouldn't that completely defeat the original point of the PIs, which
> >was that they're "outside" the scope of validation?  So what happens if I
> >now inherit an unmodifiable schema that specifies what PIs are allowed
> >where -- how do I add the custom new PIs necessary for my application?  I
> >think we've come full circle.  :-)
> >
> There can be more than one schema for a document. There can be more
> than one validation layer. There is more than one schema language.

Ah ... voila.  You have now come around full circle.  For if there
can be more than one validation layer, more than one schema
language... then you can use the same trick on the original document
to treat it with and without the elements you use for things you would
have put in PIs. And you can use the schema to say which elements
can be treated as PIs and ignored if one wants to.

Piotr Kaminski is right that you can't have your cake and eat it.
If you have to use PIs at a meta markup (mml) then you will need
a meta meta markup (mmml) to make statements about that,
and so on to infinity, or as Hofstadter might say,
a General Overlay Document.   I don't have a copy of "Goedel,
Escher, Bach" around to  quote from, but it doesn't work -- you
have to accept that the level 0 has to have the power to do
what you want as it.  if you introduce a way to make it
clear that an XML element can be removed without harm
(an XML function which elabrates to nothing) then you
can do everything you want.  The question is whether
one can go through the method of using PIs for that,
with their "ignore me" functionality all built in, or whether
one should add "ignore me" flag to elements.

> +-----------------------+------------------------+-------------------+
> | Elliotte Rusty Harold | elharo@metalab.unc.edu | Writer/Programmer |
> +-----------------------+------------------------+-------------------+
> |          The XML Bible, 2nd Edition (Hungry Minds, 2001)           |
> |              http://www.ibiblio.org/xml/books/bible2/              |
> |   http://www.amazon.com/exec/obidos/ISBN=0764547607/cafeaulaitA/   |
> +----------------------------------+---------------------------------+
> |  Read Cafe au Lait for Java News:  http://www.cafeaulait.org/      |
> |  Read Cafe con Leche for XML News: http://www.ibiblio.org/xml/     |
> +----------------------------------+---------------------------------+
Received on Monday, 4 March 2002 16:15:41 UTC

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