- From: Henry S. Thompson <ht@cogsci.ed.ac.uk>
- Date: 23 Nov 2001 10:13:38 +0000
- To: "Martin Bernauer" <bernauer@dke.uni-linz.ac.at>
- Cc: <xmlschema-dev@w3.org>
"Martin Bernauer" <bernauer@dke.uni-linz.ac.at> writes: > Mike Leditschle pointed out that there was a thread "Restriction of complex > types by changing minOccurs" dealing with somewhat the same sort of problem. > Thank you Henry and Mike, I now see that the XML Schema Spec forces the type > of a content particle of a complex type derived by restriction to have > a restricted type of its complex type base type. > > Nevertheless I don't see the rationale behind it, as in oo pardigm and many > programming languages it is perfectly valid to derive a type R from a base > type B and to redefine the type of a structural attribute b of the base type > in the subtype. > Actually there wouldn't be any problem, if XML Schema allowed me to redefine > the type of a particle in derivation by extension. However, it is only > possible when I derive by restriction, forcing me to restrict the particle's > type too. Could someone motivate this decision? We started with some very simple invariants: Restriction: instances of a restricted type is always an instances of the base type; Extension: instances of an extended type always contain an instance of the base type as a prefix. The latter is indeed modelled on sub-classing in OO languages. Could you clarify your counter-example? Certainly in C++ for instance basic derivation simply adds new instance variables/methods, it doesn't change those of the base, does it? That's why you can always treat a pointer to an instance of the derived as an instance of the base, because on the stack it is actually realised as base+extras. ht -- Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh W3C Fellow 1999--2001, part-time member of W3C Team 2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440 Fax: (44) 131 650-4587, e-mail: ht@cogsci.ed.ac.uk URL: http://www.ltg.ed.ac.uk/~ht/
Received on Friday, 23 November 2001 05:12:36 UTC