[DFDL-WG] OGF DFDL WG minutes 2007-12-05 call

[Mike Beckerle]

I tend to trust your instincts about things Steve,

I would summarize it as this: regardless of how people think nulls 
*should* work, in XSD nillables are orthogonal to value and whether or not 
this matches people's past experience we should support it if we're going 
to overload nillable at all.

To me this reasoning is pretty compelling, so I withdraw my suggestion 
(the "either nillable or default value but not both" idea). 

...mikeb





Steve Hanson <smh at uk.ibm.com> 
12/06/2007 04:59 AM

To
Mike Beckerle/Worcester/IBM at IBMUS
cc
dfdl-wg at ogf.org, dfdl-wg-bounces at ogf.org
Subject
Re: [DFDL-WG] OGF DFDL WG minutes 2007-12-05 call







Unfortunately I have been roped into something else which will likely 
occupy me full time until middle of next week, so I can't look at the 
defaults/nulls issue in detail right now. But my first reaction to the 
proposal below is that elements should be allowed to have both null and 
default values. They are separate concepts in XML Schema, so why are we 
making the DFDL logical model different?  IMHO subtle differences like 
this cause more issues with customers than the odd extra DFDL property. 
The DFDL subset of XML Schema should be just that - a subset. For those 
features of XML Schema that we do support, the rules should be the same. 

Regards, Steve

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


Mike Beckerle <beckerle at us.ibm.com> 
Sent by: dfdl-wg-bounces at ogf.org 
05/12/2007 23:21 


To
dfdl-wg at ogf.org 
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Subject
[DFDL-WG] OGF DFDL WG minutes 2007-12-05 call









OGF DFDL WG minutes 2007-12-05 call 

Suman Kalia, Simon Parker, Alan Powell, Mike Beckerle 

(who else? - was someone else on also) 

We discussed 

Output issues in the DFDL expression language: 

E.g.., an outputValueCalc for a field in the header of a data stream may 
contain information that requires you to know the rep, or length of the 
rep, of the whole data item. 

We concluded that this kind of thing can't be ruled out. Some formats just 
require buffering and are not streamable; however, implementations can 
vary on just how large a data item they're able to cope with here. 

Expression language section will include a subsection highlighting this 
issue and that implementations can vary here. 

Alan will update his expression language proposal and include this. 

Also suggested was a path length-from-to function that takes 2 path 
expressions and gives you the size of the represntation between them. 
(start of first, to last bit before start of 2nd). 
(I don't think we discussed a clear use case motivating this, but there 
may be one. We did discuss applications trying to fit data into limited 
size boxes, but the use case is not clear. 
Also note that all representation lengths are subject to change due to 
different starting alignments.) 


Nillable and Default: 

We also discussed the interaction of nillable and having a default. 

The sense of the group on the call is that we can restrict these so that 
if something is nillable it cannot also have a default value, and that the 
behavior of DFDL on output for a required element that is nillable but not 
in the logical data, is to create a null value. Everyone agreed that there 
is no need for  a property useNullValueForDefault because this should 
always be the behavior. 

Mike will forward a proposal. 


...mikeb 

Mike Beckerle
STSM, Architect, Scalable Computing
IBM Software Group
Information Platform and Solutions
Westborough, MA 01581
direct: voice and FAX 508-599-7148
assistant: Pam Riordan 
                priordan at us.ibm.com 
                508-599-7046
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