[DFDL-WG] question/clarification - asserts using test patterns

[Tim Kimber]

Good questions. 

The intent of this DFDL feature is as follows:
- a point of uncertainty (POI) cannot be resolved using an initiator ( the 
simplest option ). The data format just doesn't work that way.
- it could be resolved using an assert or a discriminator, but that would 
be too heavyweight.
- a simple inspection of the data format reveals that the discrimination 
can be done by testing the first few characters of each branch of the POI.

Example: SWIFT 50K ( multi-line address field):

:32A:060929EUR25,36&hex;0D&hex;&hex;0A&hex;
:33B:EUR56,78&hex;0D&hex;&hex;0A&hex;
:50K:/IT60X0542811101000000123456
ABC Corporation Times Square 7 NY 1
LINE 2
LINE 3
:52A:/<etc>

Note that field 50K contains lines of address data, but the actual number 
of lines is not known. So how will the DFDL parser know when the 50K field 
has completed? Answer:  it encounters a line that starts with a colon.

Now, the most natural way to model SWIFT field 50K is as a series of 
lines. The SWIFT XML format defines it this way. 
If you work through the possibilities, it turns out that the only way to 
achieve this using discriminators is:
- cause the parser to parse each line and put it into the info set
- add a discriminator to the repeating 'addressLine' element. The DFDL 
expression would be something like this: 
  { if ( fn:exists(./NameAddress_Line) ) then 
(fn:not(fn:starts-with(./NameAddress_Line, ':'))) else xs:boolean("true") 
}

That's a very expensive way to achieve the intended goal, which is 'treat 
the data as another addressLine if the next character is a colon'.
So that was the motivation for the feature. 

To answer the questions:
- not intended to be limited to xs:string only
- not intended to be limited to elements with text representation ( 
because dfdl:represention only applies to simple elements, and the POI 
might be a group or an element.)
- is intended to be matched against text or binary data, starting at the 
POI's byte offset. If the element's representation is binary then the 
'encoding' property will be required. 

Sounds as if the spec needs to be clarified in this area.

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:   15/10/2012 19:26
Subject:        [DFDL-WG] question/clarification - asserts using test 
patterns
Sent by:        dfdl-wg-bounces at ogf.org




Question: Is an assertion using a regular expression pattern allowed on 

(a) xs:string type elements
(b) any data with text representation
(c) any data with text or binary representation

and, does the regular expression apply to the representation or the 
logical data value?

(a) is the only case that is not ambiguous, because the representation and 
the logical value are the same thing. 

For everything else, there’s the question of whether the test is on the 
representation or the logical value. If it's the logical value, then how 
is a regex made meaningful on a logical value that is, for example, a 
number, without defining a canonical representation to which the logical 
value is converted?

If it's to apply to the representation, then exactly what data? (Eg., what 
grammar region) is subject to the regex?

...mikeb

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