If a processing error occurs for an optional element in a sequence, the speculative behaviour of the DFDL parser says that the optional element is assumed not to be present, and the next alternative in the sequence is tried. That is fine when there are no separators involved, but we need to clear on what happens when there are separators.

1) Positional separators (separatorSuppressionPolicy is 'never', 'trailingEmpty' and 'trailingEmptyStrict').
The key point about positional separators is that they are expected in the data, so if an error occurs while parsing the optional element, it does not make sense to backtrack to the start offset the element and try to match the next element. Yes there's a point of uncertainty in the sense that the element is either there or it has empty representation, but if an error occurs I think it must be treated as a hard error, and not cause backtracking.

2) Non-positional separators (separatorSuppressionPolicy is 'anyEmpty').
This behaves like the non-separator case and the next alternative in the sequence is tried from the start offset. However, because 'anyEmpty' behavior is lax, it is possible that the next thing in the data is a separator, so the parser must cater for that when the element is found to have empty representation. But if an error occurs establishing representation, I think the parser should just backtrack and try to match the next element.

Does that sound correct?

Regards

Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848
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