Re: Compatibility issues and principle #3

On Mon, 16 Sep 1996 17:50:11 -0400, "Eve L. Maler" <elm@arbortext.com> wrote:

>Our design principle #3 currently reads, "XML shall be compatible with
>SGML."  I'm hoping we're ready to get more specific about what this
>means.  (I apologize for starting a thread and then bolting -- I'll be 
>largely out of email-touch from tomorrow through Sep 26.)
>  If we provide for only XML->SGML, then I think we're setting up a
>  situation where XML is like "SGML with macros," which gets expanded
>  as soon as it gets into a "real" system.  If we provide only for 
>  SGML->XML, then XML may just fill the role of "HTML Heavy" -- good 
>  only for static delivery, not for serious work.  But round trips could 
>  be problematic, if (e.g.) we remove the ability to have EMPTY elements
>  and users expect the ESIS of each form to be equivalent.
>  I think a round trip should be weighted as highly desirable, with
>  transforms relatively undesirable.

ESIS takes pains to not distinguish empty elements from others. It signals the
start and end of every element whether or not it is empty. And if it is empty,
"ESIS does not indicate why it was empty; that is, whether it was declared to be
empty, or whether an explicit content reference occurred, or whether it just
happened to contain no data characters."

That is why we can safely eliminate declared content, including declared EMPTY
element types. In all cases, there is an equivalent valid SGML DTD for which
there is a document instance that yields the same ESIS. This is as close to a
round trip as you can hope to get when going from a rich representation to a
poorer one. You'll need a trivial transformation when generating the XML and
when getting it back, but there is no loss of ESIS information.

Best regards,

Charles F. Goldfarb * Information Management Consulting * +1(408)867-5553
           13075 Paramount Drive * Saratoga CA 95070 * USA
  International Standards Editor * ISO 8879 SGML * ISO/IEC 10744 HyTime
 Prentice-Hall Series Editor * CFG Series on Open Information Management