[DFDL-WG] clarification wanted: fixed values

[Tim Kimber]

There are two situations where it would be be useful for DFDL to check XML 
schema constraints/facets when parsing, regardless of the validation 
setting:
a) The scenario below, where a fixed value is specified
b) The equivalent scenario where the simple type has a single enumeration 
facet . This scenario can be extended to cover all types of facets.

Both a) and b) are actually used in xsds for industry standards  where a 
choice is resolved by an identifier field. Sometimes the identifier is not 
the first element on the choice branch, so cannot be used as an initiator. 
Last time we discussed this, we agreed ( I think ) to add a new dfdl 
function that checks fixed values and facets and throws a processing error 
if they do not match the parsed value. Or maybe we agreed to make it a new 
type of dfdl:assert/discriminator test. 

I proposed an alternative scheme in which the DFDL parser unconditionally 
treats facet validation errors as processing errors. This was not accepted 
because
a) facet validation can be expensive in CPU, so it is useful to 
b) if facet validation was put under control of a parser feature then the 
info set produced by a parser would depend on information outside of the 
DFDL schema. 
I'm not sure about b), given that we support external variables that could 
affect backtracking behaviour and therefore affect the contents of the 
info set.

regards,

Tim Kimber, DFDL Team,
Hursley, UK
Internet:  kimbert at uk.ibm.com
Tel. 01962-816742 
Internal tel. 37246742




From:   Mike Beckerle <mbeckerle.dfdl at gmail.com>
To:     dfdl-wg at ogf.org
Date:   06/09/2012 03:09
Subject:        [DFDL-WG] clarification wanted: fixed values
Sent by:        dfdl-wg-bounces at ogf.org




The spec states that the fixed facet is only used for validation.

That is, it works like default in that it supplies a default that can be 
used when parsing and unparsing, but the comparison of an existing data 
value with the schema-specified fixed value is only done for validation. 
This means that a value not agreeing with the fixed value specified in the 
schema would never cause backtracking. 

Is this right? It seems awkward to me that I have to use a dfdl:assert 
annotation to achieve this. That said, however, I can achieve the 
behaviour I need. it's just more verbose than I wanted. 

Is this what the current IBM implementation does?

Thanks

-- 
Mike Beckerle | OGF DFDL WG Co-Chair 
Tel:  781-330-0412
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