I believe the rules should be:

1. A point of uncertainty is any of
* an element which has minOccurs != maxOccurs
* a choice
* a sequence with sequenceKind="unordered"

2. Nested within the scope of a point of uncertainty, there might be other points of uncertainty.

3. A discriminator which evaluates to true resolves the nearest in-scope point of uncertainty.

4. An assertion which evaluates to false causes a processing error

5. Any processing error ( from an assertion failure or otherwise ) will cause the parser to backtrack to the nearest unresolved point of uncertainty and try the next available branch, if any. If there are no more branches available, the parser will backtrack to the next nearest unresolved point of uncertainty.

6. A processing error which reaches the root tag is reported to the host application.

7. Assertions and discriminators are allowed on any point of uncertainty ( not only on the branches of a choice )


Rationale:

If we only allow a discriminator on a choice branch, then it will be difficult to model this common style of message

Tagged header, minOccurs="1", maxOccurs="1"
Untagged body, maxOccurs="unbounded"
Tagged trailer, minOccurs="1", maxOccurs="1"

An example with 3 occurrences of the body would be:

HE,headerfield1,headerfield2,headerfield3
John Smith, 100, bodyfield3
John Brown, 200, bodyfield3
Elton John, 30Z, bodyfield3
TR,trailerfield1,trailerfield2,trailerfield3

And the DFDL schema would look something like this ( excuse the almost inevitable errors, this is just for completeness ):

...
<xs:element name="message">
  <xs:complexType dfdl:lengthKind="implicit">
     <xs:sequence dfdl:separator="\r\n">
        <xs:element name="header" initiator="HE,">
          <xs:complexType>
            <xs:sequence dfdl:separator=",">
              <xs:element name="header1" type="xs:string">
              <xs:element name="header2" type="xs:string">
              <xs:element name="header3" type="xs:string">
            </xs:sequence>
          </xs:complexType>
        </xs:element>

        <xs:element name="body" maxOccurs="unbounded">
          <xs:complexType>
            <xs:sequence dfdl:separator=",">
              <xs:element name="body1" type="xs:string">
              <xs:element name="body2" type="xs:int">
              <xs:element name="body3" type="xs:string">
            </xs:sequence>
          </xs:complexType>
        </xs:element>

        <xs:element name="trailer" initiator="TR,">
          <xs:complexType>
            <xs:sequence dfdl:separator=",">
              <xs:element name="trailer1" type="xs:string">
              <xs:element name="trailer2" type="xs:string">
              <xs:element name="trailer3" type="xs:string">
            </xs:sequence>
          </xs:complexType>
        </xs:element>
     </xs:sequence>
  </xs:complexType>
</xs:element>


The '30Z' value for the final occurrence of element Body2 is incorrect. It is not a valid integer, and will trigger a processing error.

Without a discriminator, this failure will cause the parser to backtrack to the optional field and try the next element ( the trailer element ). The initiator will not be found, and the reported error will be "Initiator 'TR,' not found for element 'trailer'". The user would almost certainly prefer "Invalid value '30Z' for element 'body2'. Value could not be converted to simple type 'xs:int'"

For this example, the discriminator would need to detect unambiguously that it really was dealing with a Body element and not a Trailer element. Due to the message style ( which is quite common ) the only way to do this is to detect that it is *not* a Trailer. I cannot think of an elegant way to do that using the facilities in v0.33 of the specification. I have raised this with Alan and Steve.

regards,

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

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