Hi Simon I see where you are coming from, but what you propose does not agree with the spec (section 5.2.8). This states that use of 'fixed' is the equivalent to dfdl:assert, not dfdl:discriminator. The difference between the two is that encountering a dfdl:discriminator that evaluates to 'true' in a choice branch switches off speculative parsing and a subsequent processing error in that branch of the choice will not cause backtracking. Using dfdl:assert does not provide that same semantic as a subsequent processing error in that branch of the choice will cause backtracking . I think all we need to do is change 5.2.8 to say that use of 'fixed' is equivalent to dfdl:discriminator instead. This semantic difference is the reason why we have both dfdl:assert and dfdl:discriminator. As I pointed out on the call, it's been lost from sections 7.3 and 7.4. Here's 5.2.8 fyi. The 'fixed' attribute is used: to constrain the logical value of a required element while parsing. to provide the logical value of a required element when unparsing when the DFDL information set does not have a value for the element. Note that a 'fixed' attribute can cause parsing to backtrack and try other alternatives (see footnote). When data is encountered and it does not match what is specified on the 'fixed' attribute, then it is a parse error. That is, the fixed value is used for parsing, not only for validation checking. Footnote: The semantics of fixed=?$$$? is as if it were translated into an assertion and a dfdl:outputValueCalc property: <element name=?x? type=?string? dfdl:outputValueCalc=?$$$? > <annotation><appinfo> <dfdl:assert>$. = ?$$$?</dfdl:assert> </appinfo></annotation> </element> Assertion failure causes a processing error. The outputValueCalc fills in the value when writing out data. Regards, Steve Steve Hanson WebSphere Message Brokers Hursley, UK Internet: smh@uk.ibm.com Phone (+44)/(0) 1962-815848 "Simon Parker" <simon.parker@polarlake.com> Sent by: dfdl-wg-bounces@ogf.org 13/02/2008 17:43 To <dfdl-wg@ogf.org> cc Subject [DFDL-WG] Discriminator example, alternative presentation Good evening. As discussed at the conference, here's a more succinct way to express this simple requirement. Perhaps discriminator's motivating example should be more complex. Comments and corrections welcome, Simon Simon Parker Software Consultant PolarLake 80 Harcourt Street Dublin 2, Ireland Phone +353 1 449-1075 Fax +353 1 449-1011 Web http://www.polarlake.com/ The information transmitted in this email is intended for the addressee only and may contain confidential and/or privileged material. Any review, retransmission, dissemination, reliance upon or other use of this information by persons or entities other than the addressee is prohibited. If you think, for any reason, that this message may have been addressed to you in error, we would ask you to notify the sender immediately by return email and delete the material. PolarLake Limited | Registered in Dublin, Ireland | Number 357324 | Registered office as above. -- dfdl-wg mailing list dfdl-wg@ogf.org http://www.ogf.org/mailman/listinfo/dfdl-wg 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