[DFDL-WG] Omitted array occurrences

[Alan Powell]

Tim

Comments below

Need more discussion on this

Alan Powell

 MP 211, IBM UK Labs, Hursley,  Winchester, SO21 2JN, England
 Notes Id: Alan Powell/UK/IBM     email: alan_powell at uk.ibm.com 
 Tel: +44 (0)1962 815073                  Fax: +44 (0)1962 816898




From:
Tim Kimber/UK/IBM at IBMGB
To:
dfdl-wg at ogf.org
Date:
19/11/2009 12:07
Subject:
[DFDL-WG] Omitted array occurrences




What should the DFDL unparser do when some or all of the elements of an 
array are missing? 

I have found the following statements in v0.36 which seem relevant: 

Section 5.2.1 
The minOccurs value is used: 
·        to determine if an element declaration or reference is scalar or 
array 
·        to determine the required minimum number of occurrences of an 
array both when parsing and unparsing 

Section 16.13 
( Note : this definition of 'required' is a repeat of the defintion in 
section 3 ) 
Definition: 'required' 
We define the term 'required' as follows: 
·        A scalar element is required. 
·        An element of a fixed-occurrence array is required. 
·        An element of a variable-occurrence array is required if its 
index is less than or equal to the value of minOccurs. 
All other elements are not required. 
... 
On unparsing, if an element is required, and is not part of the logical 
data and the element has a default value specified then it is used, 
otherwise it is a processing error. 

Section 17.3.1 : Sequence groups and separators 

re: the combination of separatorPolicy="suppressAtEnd" and 
sequenceKind="ordered": 
All separators must be found in the data except that when the sequence has 
trailing optional items, the separators are suppressed for any final 
missing items. Note suppressAtEnd can only be used when there is no clash 
with delimiters from the containing structure. 

My interpretation of the specification is: 
a) if separatorPolicy="require" then the unparser should output a 
separator for all missing required elements ( whether array members or not 
) 
Is this an additional definition of a 'required' element? In which case 
the default value should be output. (interestingly because default is a 
schema property rather than a dfdl property you cannot set a default 
default.)
b) if separatorPolicy="suppressAtEnd" then the unparser should output a 
separator for all non-trailing missing required elements 
Should set the default for any required element so it won't be missing.
"On unparsing, if an element is required, and is not part of the logical 
data and the element has a default value specified then it is used, 
otherwise it is a processing error. "

c) separators for missing elements must be output regardless of whether 
the element is required/optional, simple/complex, does/does not have a 
default value etc. I assume this because the term 'missing' is used rather 
than the very clearly-defined term 'required'. 
Missing just means not in the infoset and is orthogonal to 
optional/required. If you accept this is an additional definition of 
required then no. But it then forces you to set defaults for minOccurs=0 
elements which will only be used in this circumstance. I'm not sure what 
the default for complex elements would be: all the children must have a 
default? .

Reading between the lines, I also infer the following rules: 
d) if an array has maxOccurs="unbounded" and it is missing from the 
infoset then the unparser will not output any separators for the array 
If minOccurs > 0  then use default. If minOccurs= 0 then output nothing. I 
don't think maxOccurs has any effect.
e) if an array has maxOccurs!="unbounded" and it is missing from the 
infoset then  the unparser will output a separator for each missing 
occurrence ( so it will output maxOccurs separators ). 
See d)
f) if an element contains a child group, and none of the group members are 
present in the infoset, then the group is 'missing' and the unparser will 
output a separator for it. 
Not sure
Suggested changes to the specification: 
- As a minimum, I think it would be useful for the specification to 
include a definition of 'missing'.  'Not in the infoset'
- DFDL does not allow min/maxOccurs on groups, so they implicitly have 
cardinality 1:1. Specification should specify the behaviour of the 
unparser when none of a group's members are present in the infoset. Agree
- The wording in 17.3.1 could be more accurate. I don't think the word 
'optional' should be there ( if validation is off then the unparser will 
tolerate missing required elements -No. 'required' is not part of 
vaildation). I think the words 'trailing' and 'final' are intended to mean 
the same - we should standardize on 'trailing'. 

regards,

Tim Kimber, Common Transformation Team,
Hursley, UK
Internet:  kimbert at uk.ibm.com
Tel. 01962-816742 
Internal tel. 246742





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