I agree with Mike's use of a nested choice, I have used this same pattern to model a catch-all branch on several occasions.
I would also add that working out which branch of a choice to take can be an expensive operation for a DFDL parser, and one where optimisations will be applied. The more alike that the branches are, the more likely it will be that the parser can optimise the selection.
We still need to answer James' question about schema definition error though. The spec says "When the dfdl:hiddenGroupRef property is specified, all other DFDL properties are ignored." which is saying other properties are not applicable. In other words, such a sequence is present in the schema purely as a device to hide the contents of the group that it refers to. So I think we have a couple of choices when the context in which the sequence appears means that it can not be silent about certain properties:
a) The use of dfdl:hiddenGroupRef in that context is a schema definition error.
b) Go to the model group content of the hidden group and see whether its properties satisfy the context.
Regards
Steve Hanson
Architect, IBM Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848
From: Mike Beckerle <mbeckerle.dfdl@gmail.com>
To: dfdl-wg@ogf.org,
Date: 31/05/2013 06:37
Subject: [DFDL-WG] Fwd: [dfdl] initiatedContent w/o initiators
Sent by: dfdl-wg-bounces@ogf.org
Forgot to copy the workgroup.
---------- Forwarded message ----------
From: Mike Beckerle <mbeckerle.dfdl@gmail.com>
Date: Fri, May 31, 2013 at 1:32 AM
Subject: Re: [DFDL-WG] [dfdl] initiatedContent w/o initiators
To: "Garriss Jr., James P." <jgarriss@mitre.org>
You need two nested choices. Only the inner one uses initiated content.
<xsd:choice>
<xsd:choice dfdl:initiatedContent="yes">
<xsd:element ref="Date"/><xsd:element ref="From"/>
<xsd:element ref="MessageId"/>
<xsd:element ref="Subject"/>
</xsd:choice>
<!-- all unknown and unwanted headers -->
<xsd:sequence dfdl:hiddenGroupRef="UnwantedHeadersGroup"/>
</xsd:choice>
On Thu, May 30, 2013 at 2:49 PM, Garriss Jr., James P. <jgarriss@mitre.org> wrote:
Spec says:
It is a schema definition error if any children have their dfdl:initiator property set to the empty string.
Does that include a sequence with a hiddenGroupRef?
For example, part of a schema for email headers is this:
<xsd:choice dfdl:initiatedContent="yes">
<xsd:element ref="Date"/>
<xsd:element ref="From"/>
<xsd:element ref="MessageId"/>
<xsd:element ref="Subject"/>
<!-- all unknown and unwanted headers -->
<xsd:sequence dfdl:hiddenGroupRef="UnwantedHeadersGroup"/>
</xsd:choice>
Date, From, and the other headers are all elements with initiators, but UnwantedHeadersGroup does not have an initiator (because I don’t know it a priori).
Should this throw an error in a DFDL implementation? I suspect it should, though I hope it shouldn’t.
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