Steve - here are some of my thoughts and rationale for forward reference in this particular case..

For the example I put together, if I push the discriminator from AddressRecord to Add-Rec and NameRecord to Name-Rec which are inside their respective complex type, the model will look somewhat awkward. It will be difficult for the user to understand how the choices are being resolved keeping in view that the complex type containing the Addr-Rec and Name-Rec could potentially be in different xsd file. From implementation perspective, I am specifically restricting the forward reference to very first token; may be we can make it more explicit.

Note this forward reference is required only when the choice branch element is of complex type, it adds an entry in the XSD model but actually the data in bitstream would correspond to the very first element of simple type within the complex element. From parsing perspective, the cursor is positioned pretty much at the data in the bitstream for which we are evaluating the branch condition.

One can argue -> what if the first element of complex type is itself a complex element ( e.g. USAddress in the example below), then the forward reference becomes multiple tokens. Even in this case the cursor would still be positioned correctly in the bitstream, it only changes the logical model.

05 AddressRecord redefines common-rec. reference becomes AddressRecord\USAddress\Addr-rec
07 USAddress.
10 Addr-rec pic x(4).
10 Street pic x(10).
10 Pin pic x(6).
10 filler pic x(70).

From pure technical standpoint and having strict restriction that forward references are forbidden/invalid, we could move the discriminator annotations to the respective elements whose value needs to be checked but at the cost of making the model look somewhat unnatural. May be we want to go with the strict approach first and relax the restriction later on a case by case basis..

-------------------------------------------------------------------------------------------------------------------------------------------------------------
This is how XSD will look if discriminator annotations are pushed down .. Not sure if it makes the parsing any easier

<?xml version="1.0" encoding="UTF-8"?>

<xsd:schema targetNamespace="http://www.example.org/choiceDiscriminators"
xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:dfdl="http://dataformat.org/"
xmlns="http://www.example.org/choiceDiscriminators">







<xsd:element name="msg_MultiRecord" type="MultiRecord" />

<xsd:complexType name="MultiRecord">
<xsd:group ref="RedefinedElement_multirecord_common__rec_addressrecord_namerecord">
</xsd:group>
</xsd:complexType>

<xsd:group name="RedefinedElement_multirecord_common__rec_addressrecord_namerecord">
<xsd:choice>
<xsd:element name="common_rec">
<xsd:annotation>
<xsd:appinfo source="http://dataformat.org/">

<dfdl:discriminator test=".='oth'" />




</xsd:appinfo>
</xsd:annotation>
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:maxLength value="100" />
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="AddressRecord" type="multirecord_addressrecord"></xsd:element>
<xsd:element name="NameRecord" type="multirecord_namerecord"></xsd:element>
</xsd:choice>
</xsd:group>

<xsd:complexType name="multirecord_namerecord">
<xsd:group ref="multirecord_namerecord" />
</xsd:complexType>

<xsd:complexType name="multirecord_addressrecord">
<xsd:group ref="multirecord_addressrecord" />
</xsd:complexType>

<xsd:group name="multirecord_addressrecord">
<xsd:sequence>
<xsd:element name="Addr_rec">
<xsd:annotation>
<xsd:appinfo source="http://dataformat.org/">

<dfdl:discriminator test=".='addr'" />


</xsd:appinfo>
</xsd:annotation>
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:maxLength value="4" />
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="Street">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:maxLength value="10" />
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="Pin">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:maxLength value="6" />
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="fill_0">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:maxLength value="70" />
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
</xsd:sequence>
</xsd:group>

<xsd:group name="multirecord_namerecord">
<xsd:sequence>
<xsd:element name="Name_rec">
<xsd:annotation>
<xsd:appinfo source="http://dataformat.org/">

<dfdl:discriminator test=".='name'" />


</xsd:appinfo>
</xsd:annotation>
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:maxLength value="4" />
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="first_name">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:maxLength value="10" />
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="last_name">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:maxLength value="10" />
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="fill_1">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:maxLength value="76" />
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
</xsd:sequence>
</xsd:group>

</xsd:schema>

Suman Kalia
IBM Toronto Lab
WebSphere Business Integration Application Connectivity Tools
Tel : 905-413-3923 T/L 969-3923
Fax : 905-413-4850 T/L 969-4850
Internet ID : kalia@ca.ibm.com

Steve Hanson <smh@uk.ibm.com>

02/14/2008 03:22 PM

To	Suman Kalia/Toronto/IBM@IBMCA
cc	dfdl-wg@ogf.org
Subject	Re: [DFDL-WG] Fw: DFDL: Minutes from OGF WG call, 13 Feb 2008

Hi Suman,

The DFDL expression language does not permit forward references when parsing. See Alan's document which has been rolled into draft 031. You'll have to push the discriminator down inside the group.

Regards, Steve

Steve Hanson
WebSphere Message Brokers
Hursley, UK
Internet: smh@uk.ibm.com
Phone (+44)/(0) 1962-815848

Suman Kalia <kalia@ca.ibm.com>
Sent by: dfdl-wg-bounces@ogf.org

14/02/2008 20:04

To	dfdl-wg@ogf.org
cc
Subject	[DFDL-WG] Fw: DFDL: Minutes from OGF WG call, 13 Feb 2008

I have reworked the example where the choice branch condition was specified outside the choice contents. We don't have to specifically mention switch, case statement programming paradigm in DFDL; thinking more about this, I believe switch/case statement paradigm is not relevant here. Attached is the reworked example .

I also created another example by importing COBOL copy book (containing redefine clause) using IBM message broker toolkit. Here the branch condition is described inside the choice . In this case the discriminator contain forward reference to check the condition value; implementations may want to restrict the forward reference to the first token within the contents of choice branch.

Note : Discriminators are useful when user is dealing with binary or text data which is not tagged..

Your comments/ suggestions are most welcome and appreciated..

Suman Kalia
IBM Toronto Lab
WebSphere Business Integration Application Connectivity Tools
Tel : 905-413-3923 T/L 969-3923
Fax : 905-413-4850 T/L 969-4850
Internet ID : kalia@ca.ibm.com
----- Forwarded by Suman Kalia/Toronto/IBM on 02/14/2008 02:47 PM -----

Ian W Parkinson <PARKIW@uk.ibm.com>
Sent by: dfdl-wg-bounces@ogf.org

02/14/2008 01:34 PM

To	dfdl-wg@ogf.org
cc
Subject	[DFDL-WG] DFDL: Minutes from OGF WG call, 13 Feb 2008

Open Grid Forum: Data Format Description Language Working Group

Weekly Working Group Conference Call
17:00 GMT, 13 Feb 2008

Attendees
Mike Beckerle (Oco)
Steve Hanson (IBM)
Suman Kalia (IBM)
Simon Parker (PolarLake)
Ian Parkinson (IBM)

Apologies
Alan Powell (IBM)

1. Specification Draft Status

Alan has distributed draft 31 of the DFDL specification. The meeting reviewed the plan for the next few versions of the drafts and updated it to reflect progress and revised target dates - a copy of the updated plan is attached to these minutes.

Simon's UML diagrams describing the schema components are now planned for inclusion in draft 32 but require further discussion. This discussion will be added to the agenda for the DFDL-WG call on 27th February, but Simon would appreciate comments via e-mail before then. These diagrams are intended to set a conceptual model for DFDL and to show where annotations may be attached, but will not be used more formally, e.g. to automatiically generate APIs.
The work on nulls/defaults/optionals is complete, except for some small details, which will be included in draft 32.
The 'valueCalc' work has been progressed but is not complete, and is also now targeted for draft 32.
Other items originally planned for draft 31 are complete and have been included in the draft.

2. Assertions, Discriminators and Choice

Suman has distributed an example showing the use of discriminators in choice constructs in DFDL schemas.
The meeting discussed the distinction between discriminators and assertions - Mike described an assertion as simply a predicate which, if encountered within a choice, can cause backtracking. In contrast a succesful discriminator expression would lock the choice into a particular branch. If no discriminator matches, then the parse would fail - unless, as Simon pointed out, the choice itself was optional. Simon also suggested that the last branch of a choice could be left without a discriminator to act a a catch-all, but felt that the purpose of a discriminator should be more to help disambiguate between the possible branches rather than form such a "swtich" construct.

Steve asked about the timing attribute on assertions, and in particular whether we'd need a similar attribute for discriminators. Mike suggested that the timing attribute might have been included simply to make implementation easier; as without it an implementation would need to perform a significant amount of static analysis. The motivation for discriminators was to allow a choice to be resolved by data encountered before the choice. However as Steve and Suman thought there were use cases where a discriminator might need to refer to elements inside the choice, and so a timing option would be useful. Suman will prepare such an example, and Mike will schedule a further discussion of this topic.

Simon distributed an alternative example, showing the use of fixed fields instead of discriminators or assertions., which he felt might form a useful starting point for a full description of choice disambiguation.

(Steve left the meeting)

3. Presentation for next OGF conference

Simon suggested that Mike highlight the recent discussion topics of the working group, and items which have recently been added to the specification, and asked whether it would be useful to include the UML diagrams. Mike would like to display the diagrams and see whether they trigger a discussion amongst the delegates. He would also like to present work on variable markup and valueCalc.

Meeting closed, 18:00 GMT

Attachment: revised plan for specification drafts

Draft 31:

Improve (finish?) nulls/defaults/optionals (Mike, with input from Steve) - Done, apart from minor edit task
Expression language (Alan) - Done
Property precedence for parsing (Steve) - Done
Entities, including basic white space (Alan) - Done

Draft 32 ("vX+2"):

valueCalc (Mike) - Feb 27, 2008
Remaining aspects of null/default/optionals (Alan) - Mar 5, 2008
2-level description of schema components, including UML (Simon) - Feb 27, 2008
Property precedence for writing (Steve) - Feb 15, 2008
Variable markup (Steve) - Feb 29, 2008
Regular expressions for lengths (Alan)
Bring supplements up-to-date (Steve) - Mar 7, 2008
Assertions, discriminators and choice, including discussion of timing option (Suman) - Feb 19, 2008
How speculative parsing works (combining choice and variable-occurence - currently these are separate) (TX person)
Reordering the properties discussion: move representation earlier, improve flow of topics (Alan)

Draft 33: ("vX+3"):

Escape schemes (Ian P) - Mar 21, 2008
String XML type (Ian P) - Mar 21, 2008
Variables (Mike)
Selectors (Suman) - Mar 3, 2008
Improvements on property descriptions (All - split TBD)
Envelopes and Payloads (Steve) - Mar 5, 2008

Extraneous to spec:

Develop Schema for DFDL xsd (Suman) - Mar 15, 2008
Develop Schema for Schema DFDL Subset xsd (Suman) - Mar 30, 2008 (might not be needed)

Ian Parkinson
WebSphere ESB Development
Mail Point 211, Hursley Park, Hursley, Winchester, SO21 2JN, UK

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