Is this XPath technique reliable in all situations?

Question

I am developing an application that accepts user-defined XPath expressions and employs them as part of its runtime operation.

However, I would like to be able to infer some additional data by programmatically manipulating the expression, and I am curious to know whether there are any situations in which this approach might fail.

Given any user-defined XPath expression that returns a node set, is it safe to wrap it in the XPath count() function to determine the number of nodes in the set:

count(user_defined_expression)

Similarly, is it safe to append an array index to the expression to extract one of the nodes in the set:

user_defined_expression[1]

Answer 1

Well an XPath expression (in XPath 1.0) can yield a node-set or a string or a number or a boolean and doing count(expression) only makes sense on any expression yielding a node-set.

As for adding a positional predicate, I think you might want to use parentheses around your expression i.e. to change /root/foo/bar into (/root/foo/bar)[1] as that way you select the first bar element in the node-set selected by /root/foo/bar while without them you would get /root/foo/bar[1] which would select the first bar child element of any foo child element of the root element.

Answer 2

Are you checking that such user-defined expressions always evaluate to node-set?

If yes, first Expr is ok. Datatype will be correct for fn:count

Second one is a lot trickier, with a lot of situations there predicate will overweight axis, for example. Check this answer for a simple analysis. It will be difficult to say, what a user really meant.

Answer 3

A more robust approach would be to convert the XPath expression to XQueryX, which is an XML representation of the abstract syntax tree; you can then do XQuery or XSLT transformations on this XML representation, and then convert back to a modified XPath (or XQuery) for evaluation.

However, this will still only give you the syntactic structure of the expression; if you want semantic information, such as the inferred static type of the result, you will probably have to poke inside an XPath process that exposes this information.