[DFDL-WG] Discriminator example, alternative presentation

[Steve Hanson]

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 at uk.ibm.com
Phone (+44)/(0) 1962-815848



"Simon Parker" <simon.parker at polarlake.com> 
Sent by: dfdl-wg-bounces at ogf.org
13/02/2008 17:43

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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
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