Mike,
The
think the primary motivation for the useNullValueForDefault is to allow
out-of-band values to be output as default values. A default value has
to be in the value space of the missing element but a Null value can be
an out-of-band value. One scenario where useNullForDefaultValue is useful
is in the case of COBOL where a user wants to output HIGH-VALUES or LOW-VALUES
for missing elements.
Regards,
Geoff Judd
WebSphere Message Brokers Development
IBM United Kingdom Ltd.
Hursley
Telephone: +44-1962-818461
E-mail: JUDDG@uk.ibm.com
Mike Beckerle <beckerle@us.ibm.com>
Sent by: dfdl-wg-bounces@ogf.org
30/11/2007 20:34
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Subject
| [DFDL-WG] OGF DFDL - useNullValueForDefault |
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I'm trying to simplify something in the nulls/defaults space.
There's this property proposed: useNullValueForDefault.
This is a boolean.
I'm trying to understand what this is for.
On input, when the element is required but no content is found, and you
therefore want to create a default value, you would look, see that the
element is nillable, see that useNullValueForDefault is true, and set the
logical value of that element to null.
On output when no logical value is provided but the element is required,
you therefore want to create a default value, so you look and see that
the element is nillable and see that useNullValueForDefault is true and
so you provide the logical value null as the value. Then you output as
if the logical value was null in the first place, so you would output the
first of the dfdl:nullValues list as the representation.
These two are not symmetric.
I understand wanting empty content to cause null as the logical value of
a nillable element. Symmetry argues that one wants in this case for the
output side to output empty content as the representation of null logical
value.
The way I would express this is to provide a means for the list of dfdl:nullValues
to be able to contain the empty string as its first member of the list.
E.g., some syntax like:
dfdl:nullValues=" '' 'null' 'NULL' '\ ' "
Once you have this, there is no need for useNullValueForDefault. It's functionality
is subsumed by allowing nullValues to contain empty-string as a null representation.
This leaves the case where an element is both nullable and has a non-null
default value.
Is this case important or can we just say that an element is either nullable
or has default values, but not both. We already have some constraints
on this stuff in that we don't allow nullable complex types, only simple.
I believe allowing all combinations of nullability with default values
is probably overkill.
Comments?
I understand wanting a missing
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@us.ibm.com
508-599-7046
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