DTD question

James Tauber jtauber at jtauber.com
Fri Jul 2 05:13:50 BST 1999

>Greetings.  I'm new to this list and to XML.


>In a DTD, can I base the inclusion of an optional element on an

No, you can't.

>For example, I have:
><!ELEMENT NCSB-Message (Service-Version-Number, Service-Release-Number,
>Service-Completion-Code?, Service-Reason-Code?)>
>Is there a way to indicate in the DTD that if the attribute
>MESSAGE-TYPE=RESPONSE, then I want the elements Service-Completion-Code
>and Service-Reason-Code to be required instead of optional?

This kind of subclassing would either

(1) have to be checked by your application
(2) have to be achieved using different element types

(1) is not necessarily a bad thing, as you almost always end up doing some
kind of checking at the application stage anyway.

(2) would work something like:

<!ELEMENT Request (Service-Version-Number, Service-Release-Number,
Service-Completion-Code?, Service-Reason-Code?)>
<!ELEMENT Response (Service-Version-Number, Service-Release-Number,
Service-Completion-Code, Service-Reason-Code)>

The fact that a Request or a Response can occur wherever you previously
allowed an NCSB-Message could be modelled by wrapping the Requests and
Responses in an NCSB-Message. That way you capture the restrictions on where
NCSB-Messages can occur as well as differentiating between Requests and

<!ELEMENT NCSB-Message (Request|Response)>

Alternatively, you could just put "(Request|Response)" wherever you
previously had NCSB-Message in content specifications (possibly via a
parameter entity reference).

Hope this helps.

James Tauber

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