[DFDL-WG] Fw: Thoughts on a discriminator scenario

[Steve Hanson]

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 at 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 at 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 at 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
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