Based on our discussions on the call today, I was thinking about the definition of output “unparsing” and how outputValueCalc is dealt with.

I believe the following language is sufficient to explain how such expressions are evaluated.

When unparsing, an element declaration in the schema must have a corresponding value in the infoset.

If one exists then that value is serialized based on its properties. If there is no corresponding value in the infoset then a value is computed as follows:

a) If the element declaration is required, and has a default value specified, then an element item having the default value is created in the infoset

b) If the element declaration has an outputValueCalc property then the expression which is the property value is evaluated and the resulting value becomes the value of the element item in the infoset. References to other infoset elements from within the outputValueCalc expression must obtain their values from the infoset directly (when the value is already present) or by recursively using these methods (a) and (b) as needed.

c) If any infoset element’s value is requested and neither (a) nor (b) applies, then it is a processing error.

Seems ok to me. This is the ordinary stuff of language specification.

The function dfdl:length() needs some additional discussion.

I think we can restrict dfdl:length() to accept only paths to element info items. I.e., the first argument must be an explicit path.

(Alternatively, we can make the dfdl:length() be a member of an info item, as in ../x.dfdl:length(‘bytes’) - of however we want to notate obtaining the length from a path, instead of dfdl:length(../x, ‘bytes’). Either notation style is ok with me.)

The path for the dfdl:length must be to an element which has representation. That is, it cannot have the inputValueCalc property. This insures that it is meaningful to ask for the dfdl:length, that is the representation length of the item measured in the requested units.

There is already an Xpath function count() which returns the number of occurrences of an item. Both count() and dfdl:length() potentially imply buffering in the unparser implementation.

Comments?