[DFDL-WG] Action 033 : Assertions and discriminators
Tim Kimber
KIMBERT at uk.ibm.com
Wed Apr 29 07:41:21 CDT 2009
Hi all,
I have this action:
033
04/03: Assert/Discriminator semantics. AP to document. TK to check uses of
discriminator besides choice.
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 at 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
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