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
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Subject
| [DFDL-WG] Discriminator example, alternative
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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
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