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

[Mike Beckerle]

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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