I agree with all of Steve's description, and all of Mike's response. And I still think that in an ideal world we would include in the specification a set of grammars that describe the various 'styles' of group, including groups with no separator, positional separators and non-positional separators.

regards,

Tim Kimber, DFDL Team,
Hursley, UK
Internet:  kimbert@uk.ibm.com
Tel. 01962-816742  
Internal tel. 37246742




From:        Mike Beckerle <mbeckerle.dfdl@gmail.com>
To:        Steve Hanson/UK/IBM@IBMGB,
Cc:        dfdl-wg@ogf.org
Date:        11/02/2013 18:16
Subject:        Re: [DFDL-WG] Backtracing behavior for optional elements
Sent by:        dfdl-wg-bounces@ogf.org

---

This sounds right.

Let me run an array scenario past you. Tell me if you think I am interpreting consistently with your rules.

What you've said here is that we distinguish positional and non-positional separators. They are very different.

Positional separators are greedy and drive the parser decision. Once matched, they no longer tolerate failure to parse. So, if I have an array with occursCountKind='parsed', then finding a positional separator means I am NOT at the end of the physical array. I will have syntax for one more element to be parsed successfully, though I may suppress its value being added to the infoset if it is optional and I get the appropriate empty representation after the separator. Failure means the array is broken. Success means I will look for yet another element (because this is ock parsed).

The above makes sense to me. This is what 'separators' means to me for the most part, that they are a driving part of the syntax/format.

The non-positional separators case is 100% different.

In that case, the decision that a separator was found is revisited on failure. An ock='parsed' array/optional will be ended. The thing after it in the sequence will be attempted next.

This makes sense, I almost wish we didn't have to call it 'separator', but I think it is a useful behavior certainly, and the right interpretation of the properties we have in the spec and 140 stuff today.







On Mon, Feb 11, 2013 at 9:56 AM, Steve Hanson <smh@uk.ibm.com> wrote:
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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