[DFDL-WG] DFDL variables as path steps and with predicates
Suman Kalia
kalia at ca.ibm.com
Wed Sep 12 12:20:58 EDT 2012
How about the expression -> aaa/$var $var returning an atomic
value would be perfectly valid.. Are we going to allow this ?
Suman Kalia
IBM Canada Lab
WMB Toolkit Architect and Development Lead
Tel: 905-413-3923 T/L 313-3923
Email: kalia at ca.ibm.com
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From: Steve Hanson <smh at uk.ibm.com>
To: Mike Beckerle <mbeckerle.dfdl at gmail.com>,
Cc: Mark Frost <FROSTMAR at uk.ibm.com>, dfdl-wg at ogf.org
Date: 09/12/2012 11:47 AM
Subject: Re: [DFDL-WG] DFDL variables as path steps and with
predicates
Sent by: dfdl-wg-bounces at ogf.org
The reason that a DFDL variable appears to be allowed in a path is because
XPath 2.0 allows it. Specifically, XPath 2.0 allows:
$var/aaa/bbb
Here $var has to return a node. It can't return an atomic value. It's a
nice feature as it allows what I would describe as dynamic absolute paths.
In DFDL though all expressions must return an atomic value so we can't
exploit this.
XPath 2.0 does not allow $var/aaa/bb or aaa/$var/bbb where $var is an
atomic value. Hence DFDL can not allow this.
I think an errata is needed to the DFDL EBNF to remove the possibility of
a variable appearing in a path other than as a predicate.
Regards
Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848
From: Mike Beckerle <mbeckerle.dfdl at gmail.com>
To: Tim Kimber/UK/IBM at IBMGB
Cc: dfdl-wg at ogf.org
Date: 12/09/2012 14:48
Subject: Re: [DFDL-WG] DFDL variables as path steps and with
predicates
Sent by: dfdl-wg-bounces at ogf.org
That notion, that a variable could contain "a", and that could be used in
a path so that .../$x/... would mean to descend to child 'a' at that
point, looks to me that it goes beyond what XPath 2.0 allows. I'm not an
XPath expert, but XPath 2.0 is strong-typecheck-capable, and that requires
the path-steps in it, each of which can cause the type to change, to be
statically apparent in the expressions. A really dynamic path can only
have the vacuous type Any.
You could of course construct a string that is such a path first, and then
as XPath 2.0 to evaluate it, but that requires work outside the
expression.
But I totally agree with you that there's no value to DFDL, and no need
for anything other than where a variable like $x provides a single node
value of simple type, and can be used exactly and only where such a value
would be meaningful. So I think $x should appear alone as a
value-producing expression, including, as you pointed out things like
a[$x] should work because $x could be an integer value.
...mikeb
On Tue, Sep 11, 2012 at 6:54 PM, Tim Kimber <KIMBERT at uk.ibm.com> wrote:
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
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Mike Beckerle | OGF DFDL WG Co-Chair
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