[DFDL-WG] How do I preserve order in an unordered list?

[Steve Hanson]

James - answers in-line below.

Regards

Steve Hanson
Architect, IBM Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848



From:   "Garriss Jr., James P." <jgarriss at mitre.org>
To:     "dfdl-wg at ogf.org" <dfdl-wg at ogf.org>, 
Date:   08/10/2013 15:21
Subject:        [DFDL-WG] How do I preserve order in an unordered list?
Sent by:        dfdl-wg-bounces at ogf.org



(Sent on behalf of Roger Costello)
 
Hi Folks,
 
I am trying out the sequenceKind="unordered" property.
 
I created this simple test:
 
    <xs:element name="Test">
        <xs:complexType>
            <xs:sequence dfdl:sequenceKind="unordered" 
dfdl:separator="%SP;">
                <xs:element name="A" type="xs:string" dfdl:initiator="A:" 
/>
                <xs:element name="B" type="xs:string" dfdl:initiator="B:" 
/>
            </xs:sequence>
        </xs:complexType>
    </xs:element>
 
That schema says the input data must consist of A:___ and B:___, in any 
order.
 
Here is sample input:
 
   B:Cat A:Dog
 
I processed the input using the schema and here is the result that I got 
from Daffodil: 
 
    <Test>
        <A>Dog</A>
        <B>Cat</B>
    </Test>
 
Notice that the order of the input data changed in the result XML. This 
was quite surprising to me. 
 
Upon consulting the DFDL specification, it appears that the exhibited 
behavior is expected:
 
            ... a DFDL processor must sort the members of an 
            unordered group into schema order when parsing.
 
QUESTIONS:
 
1. Is this really the desired behavior? I would not expect parsing to 
alter the order of any data. From my XML Schema experience, I would be 
shocked if an XML Schema validator altered the order of markup in XML 
instances simply because the XML Schema specified <all> (unordered 
sequence).

SMH: Yes it is the intended behaviour. It's because this is not xs:all, 
but xs:sequence with a DFDL annotation. As far as XSDL is concerned, it is 
a xs:sequence so the elements must appear in the sequence order (otherwise 
validation will fail). The original intent was to use xs:all but it 
imposes the restriction that maxOccurs = 1 on the elements in the group, 
precluding repeats, which was felt too restrictive, so it was not included 
in the DFDL subset of XSDL. 

2. If we grant that this really is the desired behavior, then how do I 
create an unordered sequence in which DFDL parsing preserves the order of 
the data? In the above example, if the input data lists B:Cat first and 
A:Dog second, then how do I get that order preserved in the result XML?

SMH: In DFDL 1.0 you use xs:choice within a repeating element. I agree 
this is not ideal but it does preserve the order, at the expense of 
introducing a parent element into the infoset.  The latest revision of the 
DFDL spec is in Public Comment, you are very welcome to create a comment 
on the OGF site if you feel that xs:all should be supported - the link is 
http://redmine.ogf.org/projects/editor-pubcom/boards/15.

 
/Roger--
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