[DFDL-WG] Spec question: Parsing sequence groups with separators

[Tim Kimber]

The rules outlined in section 14.2.2 'Parsing Sequence Groups with 
Separators' are not properly specified, and probably cannot be 
consistently implemented.

The last paragraph of Section 14.2.1 says this: "In the sections that 
follow, it is important to remember that the 
dfdl:separatorSuppressionPolicy property is carried on the sequence, while 
the XSDL minOccurs, XSDL maxOccurs and dfdl:occursCountKind properties are 
is carried on an element in that sequence."
This is true, and this 'local overriding' of separatorSuppressionPolicy ( 
by arrays within the group ) is the cause of most of the problems.

Problem #1: Complexity
Consider a sequence group that has SSP='never' and the separator is a 
comma. Its members ( A,B,C ) must always be represented as follows:
"a,b,c" or ",b,c" or ",,c"
but never "b,c" because that would imply that the separator for an empty A 
had been suppressed.

Now suppose that B is an array with minOccurs=0 and maxOccurs=3 and 
occursCountKind='implicit'. Acceptable representations are now:
"a,b1,b2,b3,c" or "a,b1,,,c" or even "a,,,c" 

But if occursCountKind is changed to 'parsed' then the acceptable 
representations suddenly alter, and empty occurrences of B can be 
completely omitted.
"a,b1,b2,b3,c" or "a,b1,c" or even "a,c" 
[ or should that be "a,,c" ]

This seems wrong. The logic that implements suppression policy is hard 
enough to implement already. Bringing in an extra layer of complexity 
around arrays will make it so hard that most implementations would contain 
defects, leading to interoperability issues.

Problem #2 Ambiguity
See the brackets in the preceding paragraph. 
[ or should that be "a,,c" ]

It is far from obvious whether the group should insist on having a 
delimiter for the array ( because its SSP is 'never' ) or whether the 
array should take liberty to suppress the separators for all of its 
members ( as I assumed when I wrote this email). The text of the 
specification is either silent or unclear on this point.

Possible resolution:
Rather than attempting to specify implied behaviours for the various 
occursCountKind settings, I believe the specification should 
a) prohibit the use of certain occursCountKinds within positional 
sequences
b) require array occurrences to use the same SSP as other sequence 
members.

After some discussion with the IBM team, I believe a) will not generate 
too many prohibited combinations, and the rationale for those prohibitions 
will be consistent with already-existing schema definition errors.
b) will simplify the implementation of separation suppression, thus 
addressing the complexity problem.

I expect we will need an action to be opened so that this can be discussed 
in the working group meetings.

regards,

Tim Kimber, 
IBM Integration Bus Development (Industry Packs)
Hursley, UK
Internet:  kimbert at uk.ibm.com
Tel. 01962-816742 
Internal tel. 37246742

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Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU
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