[DFDL-WG] DFDL variables as path steps and with predicates

[Tim Kimber]

Interesting...I naturally interpreted $x/a/b/$y/../$z as a series of 
macro-expansions of the stringified values of the variables $x, $y and $z. 
You clearly assumed something different - that $x is a complex-valued 
object with a child called 'a'. 

I cannot see much value for DFDL in either interpretation, to be honest. 
If a path reference can contain variables then it will not always be 
possible to work out which parts of the schema will participate in 
expression evaluation. The example usage that you gave {  fn:exists($x[. 
eq 3] } could be replaced by { $x eq 3] }. Are there any other usages that 
could only be done using a variable in a path ref?

My proposal would be to allow var refs within the predicate expression of 
a StepExpr but not anywhere else in a StepExpr. Integer variables could 
then be used to calculate an array position in a predicate. This would 
require only a small change to the grammar, and I think it was probably 
the intention when this was originally put into the specification. 

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:   11/09/2012 17:10
Subject:        [DFDL-WG] DFDL variables as path steps and with predicates
Sent by:        dfdl-wg-bounces at ogf.org




Currently the DFDL spec's grammar productions are quite liberal about 
where a VarRef can appear:

PathExpr ::= ("/" RelativePathExpr?)
            | RelativePathExpr
RelativePathExpr ::= StepExpr (("/") StepExpr)*
StepExpr ::= FilterExpr | AxisStep
AxisStep ::= (ReverseStep | ForwardStep)
            Predicate
FilterExpr ::= PrimaryExpr Predicate
Predicate ::= "[" Expr "]"
PrimaryExpr ::= Literal | VarRef |
               ParenthesizedExp


in terms of XPath 2.0 syntax, you could write $x/a/b/$y/../$z. 

However, the spec also says the type of a variable can only be one of the 
simple types allowed by DFDL only.  So no path steps in the sense of 
children are meaningful after a DFDL variable. Furthermore, variables are 
all declared at top level. There is no notion of parent nodes for variable 
values; hence, a/$x is meaningless (or means the same as $x by itself), 
and $x/.. is similarly meaningless.

But, a variable reference can be followed by a predicate. The resulting 
node set, would either be one node, containing the value of the variable, 
or zero nodes. 

For example is {  fn:exists($x[. eq 3] }  is presumably a boolean valued 
expression true if variable x's value is 3.

Are there any issues here with predicates??

Should we update the expression language productions to enable only 
sensible use of DFDL variables in expressions or leave it to match XPath 
2.0's more general syntax.

If we update the productions should we disallow predicates after variable 
references also? This loses no expressive power, you can still write { if 
($x eq 3) then true else false }, which is to say I think there is no 
inherent capability lost if we require variable references to be atomic 
expressions that produce exactly a single node value. 

...mikeb





-- 
Mike Beckerle | OGF DFDL WG Co-Chair 
Tel:  781-330-0412
--
  dfdl-wg mailing list
  dfdl-wg at ogf.org
  https://www.ogf.org/mailman/listinfo/dfdl-wg

Unless stated otherwise above:
IBM United Kingdom Limited - Registered in England and Wales with number 
741598. 
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.ogf.org/pipermail/dfdl-wg/attachments/20120911/9edbfb7b/attachment.html>