[DFDL-WG] Discriminator example, alternative presentation

[Simon Parker]

Although I don't yet grasp all the nuances of 'assert' and
'discriminator', your point is valid.
This potential capability is too useful to lose, and I feel we should
make the specification change you suggest.
 
Thanks for the thoughtful feedback,
 Simon
 


________________________________

	From: Steve Hanson [mailto:smh at uk.ibm.com] 
	Sent: 15 February 2008 11:47
	To: Simon Parker
	Cc: dfdl-wg at ogf.org; dfdl-wg-bounces at ogf.org
	Subject: Re: [DFDL-WG] Discriminator example, alternative
presentation
	
	

	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 

To
<dfdl-wg at 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
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