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Re: NOTATIONS and DATA TYPES (was: Update on namespaces)

From: W. Eliot Kimber <eliot@isogen.com>
Date: Wed, 11 Jun 1997 11:31:09 +0100
Message-ID: <339E7E6D.72D4@isogen.com>
To: w3c-sgml-wg@w3.org
Andrew Layman wrote:
 
> My understanding on notations is that the ExternalId used refers to some
> processor capable of interpreting the data (e.g. a program that can read
> and display GIF bitmaps if the external binary resource is in fact a GIF
> bitmap).  Do I read this correctly?

As defined by SGML, notations first and foremost associate a local
notation name with the specification (documentation) for that notation. 
In particular, the public identifier for a notation identifies the
specification.  System IDs for notations are often (but not necessarily)
used to point to processors for the notation (they could also point to
the documentation).

But note that system IDs, being system-specific, are not reliable. 
Therefore, the declaration of a notation with a system ID should never
be taken as a permanent or persistent association of the notation with a
particular processor.

Or said another way, systems may map notations to processors, but any
such mapping is processor-specific and not required by SGML (or,
presumably, by XML).

> If my understanding of NOTATION is correct (from section 4.6 in the
> working draft at
> http://www.textuality.com/sgml-erb/WD-xml-lang.html#sec4.6), then a
> NOTATION in effect says "I don't know how to deal with this external
> resource, but here is the identifier of some processor that does."  Data
> types would be saying "This is a number (date, time, etc.) I make no
> recommendations regarding processing."

Notations can actually do both jobs (just as mime types do).  The
notation says "here's something in a notation that is different from (or
more than) SGML (or XML)", here's a pointer to the documentation for the
data type, and, by the way, here's a program you might try using to
process it, if you feel like it.

In our use of notations to do formal definition in the HyTime standard,
we've tried to distinguish between the use of notations to define
semantics applied to data and lexical types to define the syntax for
data.  While a notation may imply or require a certain syntax (e.g., the
notation "SGML" defines both the semantics of SGML processing and the
lexical constraints on SGML documents), lexical types only specify the
lexical rules and do not define semantics directly. 

Cheers,

Eliot
Received on Wednesday, 11 June 1997 14:33:33 EDT

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