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Discriminators and potential points of uncertainty (Steve)
28/1: Steve to write up a proposal to prevent a discriminator from behaving in a non-obvious manner when used with a potential point of uncertainty that turns out not to be an actual point of uncertainty.
5/2: With Steve

I started on this by reading section 9.3.3 on points of uncertainty, which lists the potential PoUs. Here's the list to save getting the spec out.

1.        An xs:choice branch

2.        All xs:elements in an unordered xs:sequence (dfdl:sequenceKind is 'unordered')

3.        An optional xs:element

4.        An array xs:element

5.        All xs:elements in an xs:sequence containing one or more floating xs:elements.

The section then looks at each in turn and gives the circumstances when it is an actual PoU or not. As currently written, it is only 3 and 4 where a potential PoU might not be an actual PoU. For 1, 2 and 5 it says they are always actual PoUs.

But I'm not sure that's correct. A deeper analysis of what is actually going on with 1, 2 and 5 says to me that there are times when there might not be an actual PoU.

1. Given that there is no concept in DFDL of optional choice branches, then if the last branch is reached then there is no longer a PoU. It must be that branch else it is a processing error.

2. There can come a point in an unordered sequence when all that can be encountered is one element, and if that is (1,1) then there is no longer a PoU.

5. If all floating elements are (1,1) and all are encountered, then from that point on there are no longer any PoUs due to floating elements.

I'd like us to get straight on this before I proceed with the action proper.

Regards
 
Steve Hanson
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848
----- Forwarded by Steve Hanson/UK/IBM on 05/02/2014 10:12 -----

From:        Steve Hanson/UK/IBM
To:        dfdl-wg@ogf.org,
Date:        27/01/2014 17:39
Subject:        Fw: Thoughts on a discriminator scenario



Been thinking some more on the discriminator scenario below that I mailed out before xmas, and discussing it with the IBM DFDL team.

The 'confusing' aspect of the behaviour is that a discriminator within a potential PoU will act on a higher level PoU if the potential PoU is not an actual PoU. In the example, the array element 'Type1' is not an actual PoU for occurrence 1, only for occurrences 2+. So when the discriminator fires for occurrence 1 it will resolve a higher level unresolved PoU if one exists.  

Perhaps the spec should say that a discriminator can't 'leak' beyond the potential PoU that encloses it ? If so, then for occurrence 1 the discriminator has no effect, and only has an effect for occurrences 2+.  This makes for more predictable and robust schemas.

We'd need to go through spec section 9.3.3 carefully to see if this does not break any of the potential PoUs that are listed.

Regards

Steve Hanson
Architect, IBM Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848
----- Forwarded by Steve Hanson/UK/IBM on 16/01/2014 09:55 -----

From:        Steve Hanson/UK/IBM
To:        dfdl-wg@ogf.org,
Date:        20/12/2013 13:20
Subject:        Thoughts on a discriminator scenario


Take the following schema (simplified) for element Type1 (1,10) being a loop for elements A,B,C.  Type 1 does not have an initiator so I need to use a discriminator to establish the existence of an occurrence of Type1 so that incorrect backtracking does not occur after an error. Because occursCountKind is 'implicit', the 1st occurrence is not a point of uncertainty so the discriminator acts instead on any enclosing point of uncertainty, but for 2nd and subsequent occurrences it acts on Type1.  That is all working as designed, but I think users will the 1st occurrence behaviour a bit confusing. There are workarounds to avoid the problem, eg, use occursCountKind 'parsed' or split Type1 into two as (1,1) and (0,9). I think this is worth documenting in a tutorial as this is quite subtle stuff.

        <xs:element name="Type1" maxOccurs="10" dfdl:occursCountKind="implicit">
                        <dfdl:discriminator test="{fn:exists(A)}" />
                <xs:complexType>
                        <xs:sequence>
                                <xs:element name="A" dfdl:initiator="A:" ... />
                               <xs:element name="B" dfdl:initiator="B:" ... />
                               <xs:element name="C" dfdl:initiator="C:"... />
                       </xs:sequence>
                </xs:complexType>


Regards

Steve Hanson
Architect, IBM Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848
Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU

Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU

Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU