W3C home > Mailing lists > Public > w3c-sgml-wg@w3.org > June 1997

Re: IDs - make them case sensitive

From: W. Eliot Kimber <eliot@isogen.com>
Date: Fri, 27 Jun 1997 21:30:40 -0900
Message-Id: <3.0.32.19970627212123.00df7da0@mail.swbell.net>
To: w3c-sgml-wg@w3.org
At 01:42 PM 6/27/97 -0700, Tim Bray wrote:
>At 05:14 AM 28/06/97 +1000, Rick Jelliffe wrote:
>>Everyone send in strings they have *actually* wanted to use as unique
>>IDs. ...If you ever changed your
>>SGML declaration to add a symbol to namechar, that would be acceptable
>>too.
>
>http://www.infoseek.com/Titles?qt=epistemology;col=WW;sv=IS;lk=noframes;nh=10

Surely nobody wants to use the above as an *ID*?  I mean, why would I do:

<mydoc>
 <para
id="http://www.infoseek.com/Titles?qt=epistemology;col=WW;sv=IS;lk=noframes;
nh=10">a;dsjfad</para>
 ...
<para><link
href="http://www.infoseek.com/Titles?qt=epistemology;col=WW;sv=IS;lk=noframe
s;nh=10">see this para</link>
</para>
</mydoc>

If what's wanted is for the latter (the href) to be an SGML "IDREF", it
doesn't make any sense, because IDs are, *by definition*, identifiers for
elements *within a single document*.  Using the full URL syntax for them
doesn't buy you anything.

However, if the real requirement is for simple cross-document addressing,
that's very different.  At the WG8 meeting in Munich last year, the SGML RG
accepted in principle a design change proposal submitted by Martin Bryan
that allows IDREFs to include entity/ID pairs, e.g.:

<!DOCTYPE Mydoc [
  <!ENTITY foo SYSTEM "foo.sgml" NDATA SGML >
]>
<mydoc>
<link idref="foo::bar">
</mydoc>

Where "bar" is an ID used in the document "foo.sgml".  We didn't decide on
the exact syntax and James pointed out some problems with the original
proposal (I don't remember his objections).  If you used URLs as the system
IDs of entities, you'd get very close to direct URL references without
having to bind the complete URL to every reference:

<!DOCTYPE MyDoc [
 <!ENTITY Clause6.2 SYSTEM "http://www.drmacro.com/hythtml/clause6.2.html"
   CDATA SGML >
]>
<mydoc>
<para>See <link refid="Clause6.2::clause-6.2.1">this clause</link> for more
about object representation in HyTime.
</para>
</mydoc>

[The refid above looks a bit redundant because my IDs for individual
clauses include the whole clause number, as do the filenames of the chunked
files.  Your mileage may vary.]

It might not be too late to get this into the WebSGML TC--I think it would
be a good thing, as it probably satisfies 70 or 80% of the cross-document
addressing requirements of SGML documents.

If you want to use URLs to do addressing, use URLs and don't try to pretend
they're IDREFs.  Certainly the rules for URLs are well defined (if not
always well understood).  HyTime defines a general mechanism by which the
use of URLs can be defined to a HyTime engine (and I recommend that all
HyTime implementations include support for URLs for direct addressing, in
addition to supporting them as system IDs for entities).

As to why IDs are NAMEs and not CDATA, probably to get the case folding
rules of NAME.  Also probably because that's the way IDs worked in GML.  My
initial reaction is that there's no compelling requirement to restrict
their syntax, although there might be a subtle one that I haven't seen yet
(you get a lot of that in SGML--the depencies among the parts are just
monsterous).

Note that in the grove new world, you can, in theory, have as many name
spaces as you want because the mechanism for defining and using namespaces
is completely generalized.  The SGML property set only defines the
document-wide names spaces defined by SGML (elements, entities, notations,
element types), but, for example, XML could define an application-specific
property set derived from the SGML property set that added either
additional name spaces (e.g., "xml-ID") or provided a general way to define
new name spaces within documents.
XML processors would implement to this property set [NOTE: this *doesn't*
mean they'd have to implement groves, just be consistent with what the
property set defines in terms of what you can address and what you get when
you do it.]

I think the latter would be a good idea for SGML (and I think it has been
discussed a bit, but I'm not sure).  For example, you might have a
declaration like this:

<!DOCTYPE MyDoc [
 <!NAMESPACE db-names -- Declare new name space called "db-names" --
     scope    document  -- Scope options might "element", "element type", 
                           "nested", etc. --
     lexmodel CDATA     -- Lexical rules for the names --
     namelen  32        -- Maximum name length --
 >
 <!ATTLIST PartNumber
     control-number ID db-names #REQUIRED
     -- Control number is a name-space ("ID") attribute but in 
        the db-names name space, not the default elements name space. --
 >
 <!ATTLIST PartNumRef 
     control-number IDREF db-names #REQUIRED
     -- Reference to name in db-names name space --
 >
]>

For XML, maybe it would be sufficent to define a processing instruction
that does more or less the same thing:

<?XML NAMESPACE db-name scope="document" lexmodel="CDATA" namelen="32"?>

And then use the name-space name as the attribute name or something [you
could use XML-specific attributes map name spaces to attributes if you had
to].

Note that in HyTime (and DSSSL), name-space addressing is generalized such
that you can do indirect addressing of names in any name space in any
grove. To address db-name names indirectly in HyTime I would use the
"name-space location address"
(www.drmacro.com/hythtml/clause-7.3.html#clause-7.9.3) element form like so:

<nmsploc id=part-102345-ABC namespace="db-names">102345-ABC</>

This allows me to redirect any normal ID reference to any name in any other
name space for any kind of data (given a processor that knows how to
resolve the name for that data type).  If the above is a reference to a
part in a database, all I need to do is declare the database as an entity
and use it as the location source for my name-space location address:

<!DOCTYPE MyDoc [ 
 <!NOTATION my-database PUBLIC "-//ME//NOTATION My Database//EN" >
 <!ENTITY parts-db SYSTEM "parts-db.sql" NDATA my-database>
]>
<MyDoc>
<nmsploc id=part-102345-ABC 
         namespace="db-names"
         locsrc="parts-db">102345-ABC</>
<para><partref refid="part-012345-ABC">framitz</part>

The grove-aware notation processor API for the "my-database" notation
(e.g., a thin layer of code over the real database) gets as input the
namespace name and the content of the nmsploc element (the name or list of
names to be addressed) and returns the "nodes" addressed.  In other words,
the grove-access API for the database gives back a program object
consistent with the grove API of the requesting program (for example, the
grove API in JADE or in GroveMinder).  The requesting program (a HyTime
browser, DSSSL engine, etc.) gets what it thinks is a grove node and goes
happily about its business.

Cheers,

E.
--
<Address HyTime=bibloc>
W. Eliot Kimber, Senior Consulting SGML Engineer
Highland Consulting, a division of ISOGEN International Corp.
2200 N. Lamar St., Suite 230, Dallas, TX 95202.  214.953.0004
www.isogen.com
</Address>
Received on Friday, 27 June 1997 23:34:39 EDT

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