[Prev][Next][Index][Thread]

Re: PIs

At 1:13 PM 9/11/96, Tim Bray wrote:
>>7.6 (4th bullet)
>>PIs don't make sense on a network. Their use should be restricted to the
>>prolog, where they could be useful in setting up the parser (e.g. to
>>identify a relevant SGML declaration). The thought of allowing a processing
>>instruction within a document instance that is going to be displayable by
>>many different plug-ins scares the hell out of me.
>
>Hmm, I think I'm missing something.  In SGML, a PI can be freely
>ignored by any application... what harm is done in supporting PI's as a place
>to put, well, Processing Instructions?  In the full knowledge, of course, that
>nothing downstream can be guaranteed to understand or act on them, without
>prior arrangement.  But such prior arrangements are common.  Could you
>elaborate, Martin?

This started me thinking about something I've pondered before:

There are (at least) two *sorta-different* kinds of comment-like metadata
that we want to be able to embed in a document.  The question here is
whether these different things should be blessed with a different syntax.

What can you do with PIs that you can't do with comments starting with a
special well-known codeword that means "PI"?

Syntactically, I think the answer is "well...nothing" (but welcome corrections).

The standard answer:  comments get ditched by SGML parsers, so some
processing "engines" (that need to see the PIs) might never see them.  PIs
are good because they are not tossed by parsers.

But we're not talking about SGML parsers here, we're talking about XML
parsers, which (since they don't really exist yet) could be taught not to
throw away comments out of hand, but keep them around so that "engines"
could glance at them before throwing them away (or not).  If the
functionality of PIs is done in XML with the above-described special form
of comment, this makes the syntax of XML slightly ximpler (hehe, that was a
typo, but i decided to leave it) at the expense of moving the decision to
ditch them a little further down the line.

mag

--
.---o  Tom Magliery, Research Programmer                         .---o
`-O-.  NCSA, 605 E. Springfield                  (217) 333-3198  `-O-.
o---'  Champaign, IL 61820          O-        mag@ncsa.uiuc.edu  o---'