Daffodil is the first DFDL implementation to provide the advanced features of
- dfdl:inputValueCalc (IVC for short)
- dfdl:outputValueCalc (OVC for short)
- dfdl:hiddenGroupRef
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These features are essential for highly complex binary formats like:
- PCAP
- Asterix
- MIL-STD-2045 and related MIL-STD and STANAGs
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SDE for element in hidden group that has no default, nor dfdl:outputValueCalc
Within a hidden group, if an element has no fixed/default value, and no OVC, then because it is hidden and so does not appear in the infoset, there's no way for it to get a value at all when unparsing.
Daffodil issues an SDE in this situation.
TBD: this SDE may be too strong: If the element has IVC, then it doesn't need a value for unparsing unless an expression that is used at unparse time (i.e., not in an assert or discriminator) references the element.
IVC and OVC on the same element
In many situations we have found that we need both IVC and OVC on the same element. This occurs when the element is in a hidden group, and is essentially being used as a variable.
The dfdl:newVariableInstance doesn't eliminate this problem, as there are situations where one needs an "array variable", that is a variable associated with each index of an array, but referenced from expressions used to compute things outside of the scope of that array element.
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