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
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