Derivation by Restriction

Henry S. Thompson ht at
Mon Jan 24 09:05:20 GMT 2000

"Fabio Arciniegas A." <l-arcini at> writes:

> Hi all.
> During the definition of a schema for instances product of Object
> serialization, I begin to wonder about the implications of derivation
> by restriction in XML: Schema when used with OO apps...
> Is derivation by restriction necessary/desirable? could anyone
> please explain the need behind it a bit? Is D by R a "threat" to
> the conformance[1] principle when using schema to define object
> hierarchies? 
> just oblique thoughts... :) comments?
> Fabio
> [1] loosely put, the traditional conformance principle in OO states: If B
> inherits from A, B can be used wherever A can be used.

That's precisely the intention of restriction.  Any element
information item (EII) which is valid per a type (R) derived by restriction
from a type (T) by construction contains all the required components
of T.  It follows that all such EIIs are valid per T.

  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 at

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