[DFDL-WG] what do we allow in DFDL expressions after "/"

[Steve Hanson]

Mike, 

More correctly, you mean Step[Index]/Step[Index]/Step[Index], but that's 
fine as that is handled by the definition of AxisStep as it includes 
Predicate.

However removing FilterExpr entirely removes PrimaryExpr and all it brings 
with it, like literals and function calls and the use of "." so your 
change on its own simplifies things too much. What I think you would need 
is:

StepExpr
   ::= 
FilterExpr | AxisStep

PathExpr
   ::= 
(("/" RelativePathExpr?)
| RelativePathExpr) | FilterExpr

I can see (at least) one problem with only allowing FilterExpr to appear 
on its own, namely that you lose the ability to use "." in conjunction 
with a path, so even the above is not sufficient.

Looking at what we have done in the original spec, all we did was edit the 
BNF to remove things we didn't support, making what resulted easily 
comparable to standard XPath 2.0. The resultant BNF implies several things 
that we do not support in DFDL, so we qualified the BNF with some notes. 
For example, DFDL only allows the use of Predicates to index arrays, so 
there is a note saying that a Predicate must result in an integer else 
it's an SDE. I think we should do the same for  errata 2.101 - leave the 
BNF alone and add a new note to the list.  And beef up the words at the 
start of section 23.4 to make it clear how the BNF should be read.

Also noticed that the BNF in the spec is not stand-alone as the constructs 
for StringLiteral etc are not reproduced, and rely on the reader reading 
XPath 2.0 spec.

Regards

Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848



From:   Mike Beckerle <mbeckerle.dfdl at gmail.com>
To:     dfdl-wg at ogf.org, 
Date:   05/02/2013 23:08
Subject:        [DFDL-WG] what do we allow in DFDL expressions after "/"
Sent by:        dfdl-wg-bounces at ogf.org




In reexamining the grammar for the DFDL expression language, I have the 
following question

In a path Step/Step/Step[Index]

What can Step be? I know it can be ".." or parent::QName or child::QName 
or just an NCName or QName, 

Those are all what the XPath grammar calls AxisStep. 

anything else? 

The grammar can be changed in a very  small way from the original XPath 
grammar if the above are the only possibilities. 

the clause

StepExpr  ::= FilterExpr | AxisStep

can just be changed to 

StepExpr ::= AxisStep





 


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Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | 
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