Strong Typing in SGML and XML

[Peter Newcomb]

> Date: Mon, 05 May 1997 17:09:21 -0700
> From: Tim Bray <tbray at textuality.com>
> 
> Ever since about 15 minutes after SGML was born, database people have been
> discovering, to their surprise, that it contains no facilities for
> strong data typing.  You can have an element named <BIRTH-DATE>, and
> SGML will have no problem accepting 
> 
> <birth-date>purple bananas rule</birth-date>.

It seems to me that this sort of data typing can already be
accomplished:

<!NOTATION SQL-DATE PUBLIC "...//NOTATION SQL DATE//EN">
<!ELEMENT birth-date - - (#PCDATA)>
<!ATTLIST birth-date
   notation NOTATION (SQL-DATE) #FIXED SQL-DATE
>

Using data attributes, and HyTime's Data Attributes For Elements
(DAFE) facility (which acts as if the data content notation for an
element were an architectural form), one could implement a scheme like
the one you (Tim) propose:

<!NOTATION SQL-DATE PUBLIC "...//NOTATION SQL DATE//EN">
<!ATTLIST #NOTATION SQL-DATE
   sql-min CDATA #IMPLIED
   sql-max CDATA #IMPLIED
>
<!ELEMENT birth-date - - (#PCDATA)>
<!ATTLIST birth-date
   sql-type NOTATION (SQL-DATE) #FIXED SQL-DATE
   sql-min CDATA #FIXED "1900-01-01" -- some bogus restrictions --
   sql-max CDATA #FIXED "1999-12-31"
>

Or better yet:

<!NOTATION SQL-DATATYPE PUBLIC "...//NOTATION SQL DATATYPE//EN">
<!ATTLIST #NOTATION SQL-DATATYPE
   sql-type NAME #REQUIRED
   sql-size NUMBER 0
   sql-min CDATA #IMPLIED
   sql-max CDATA #IMPLIED
>
<!ELEMENT birth-date - - (#PCDATA)>
<!ATTLIST birth-date
   notation NOTATION (sql-datatype) #FIXED sql-datatype
   sql-type NAME #FIXED sql-date
   sql-min CDATA #FIXED "1900-01-01" -- some bogus restrictions --
   sql-max CDATA #FIXED "1999-12-31"
>

Of course, both of these require data attributes, which XML does not
(yet!) support.

-peter

--
Peter Newcomb                           TechnoTeacher, Inc.
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