[DFDL-WG] DFDL - debate around array specific properties

[Steve Hanson]

On Friday's call Mike & Suman were tasked with creating a proposal for the 
number of array specific properties that we expose in 1.0. 

Spec v028 lists a core set:

occursKind
occurs
occursSeparator
occursStopvalue
occursStopValueKind

The debate was whether array properties are needed for handling 
prefix/suffix region, the motivating example being leading skip count.

Here's a format I came across recently, JSON, meaning 
JavaScriptObjectModel (http://json.org/), used in conjunction with REST 
style web services. 

For a model that logically looks like...

String name;
String[] address;
String zip;

...an example JSON instance could look like...

{"name":"Steve Hanson",
 "address":["1 Woodlands","Awbridge","Romsey"],
 "zip":"SO51 0GP" 
} 

... and a DFDL model (ignoring white space) would look like. 

<xs:sequence dfdl:separator="," dfdl:initiator="{" dfdl:terminator="}" 
dfdl:lengthKind="implicit"> 
        <xs:element name="name" type="xs:string" 
dfdl:initiator="&quot;name&quot;:" dfdl:terminator="" 
dfdl:lengthKind="delimited"/>
        <xs:sequence dfdl:separator="" 
dfdl:initiator="&quot;address&quot;:[" dfdl:terminator="]" 
dfdl:lengthKind="implicit">
                <xs:element name="address" type="xs:string" 
dfdl:initiator="" dfdl:terminator="" dfdl:lengthKind="delimited" 
                        maxOccurs="unbounded" dfdl:occursKind="implicit" 
dfdl:occursSeparator=","/>
        </xs:sequence>
        <xs:element name="zip" type="xs:string" 
dfdl:initiator="&quot;name&quot;:" dfdl:terminator=""/>
</xs:sequence>

Note the need to wrap the address element in a sequence to consume the 
array markup **. I could have used a complex element, but that would have 
added an extra infoset level and I wanted to conform as closely as 
possible to the logical view of the data above.

To do this with explicit array properties the DFDL model would look like:

        <xs:element name="address" type="xs:string" dfdl:initiator="" 
dfdl:terminator="" dfdl:lengthKind="delimited" 
                  maxOccurs="unbounded" dfdl:occursKind="implicit" 
dfdl:occursSeparator="," 
                  dfdl:occursInitiator="&quot;address&quot;:[" 
dfdl:occursTerminator="]" />

Note the need for dfdl:occursInitiator, dfdl:occursTerminator. 

Personally, I don't think the extra properties increase the useability by 
much.


** The one thing that feels a bit odd is having dfdl:lengthKind="implicit" 
on the wrapping xs:sequence. I feel like I want to use 
dfdl:lengthKind="delimited", because the sequence is followed by the 
separator of the outer sequence.  And in this instance I could, as there 
are no 'final unused' bytes in the data. I'm still getting used to length 
on sequences, I guess.


Regards, Steve

Steve Hanson
WebSphere Message Brokers
Hursley, UK
Internet: smh at uk.ibm.com
Phone (+44)/(0) 1962-815848





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