A few months ago I had the pleasure to do "all XPath programming".
I developed the Binary tree data type and the Set data type
essentially completely in XPath 3.0.
These can be found in my blog, respectively at:
http://dnovatchev.wordpress.com/2012/01/09/the-binary-search-tree-data-structurehaving-fun-with-xpath-3-0/
and
http://dnovatchev.wordpress.com/2012/01/12/the-set-datatype-implemented-in-xpath-3-0/
At the time this happened accidentally. I don't like to work with
XQuery, but at that time Saxon's XSLT 3.0 processor didn't fully
implement higher order functions. Saxon's XQuery 3.0 did, so I
reluctantly used XQuery 3.0 in my work. Doing so, I planned to move my
whole code to XSLT 3.0 --whenever Saxon's XSLT 3.0 processor was ready
for this (and now, in version 9.4 it is). Therefore I essentially
coded everything as much in pure XPath 3.0 as possible.
This experience clearly showed to me that it is possible to use XPath
3.0 as a stand-alone programming language. This has many benefits, and
one of them is that XPath-only code is fully transferrable between
XSLT and XQuery.
At the end of my latest blog post "Recursion with anonymous (inline)
functions in XPath 3.0", I list a number of nice features we still
need in XPath as a programming language, that could be added in a
future, post-3.0 version:
1. Stand-alone XPath processors.
2. Import/include directives for XPath-only files (expression-files).
3. Separate packaging/compilation of XPath-only programs.
4. New data structures such as tuples.
5. Generics – parametric data types.
Cheers,
Dimitre
On Wed, Oct 24, 2012 at 4:55 AM, Costello, Roger L.
<costello(_at_)mitre(_dot_)org> wrote:
Thanks a lot Michael for the explanation. Fantastic!
Below is a summary of my question plus Michael's answer (slightly expanded).
The XPath 3.0 specification says this:
XPath 3.0 is a composable language
What does that mean?
It means that every operator and language construct allows any XPath
expression to appear as its operand (subject only to operator precedence and
data typing constraints).
For example, take this expression:
3 + ____
The plus (+) operator has a left-operand, 3. What can the right-operand be?
Answer: any XPath expression! Let's use the max() function as the
right-operand:
3 + max(___)
Now, what can the argument to the max() function be? Answer: any XPath
expression! Let's use a for-loop as its argument:
3 + max(for $i in 1 to 10 return ___)
Now, what can the return value of the for-loop be? Answer: any XPath
expression! Let's use an if-statement:
3 + max(for $i in 1 to 10 return (if ($i gt 5) then $i*3 else $i*2)))
The fact that XPath is a composable language is very cool and very powerful.
Contrast this with XSLT, which is not fully composable: XPath expressions can
be used as operands to XSLT instructions, but not the other way around.
Similarly Java has constructs called statements that
cannot be nested inside expressions.
/Roger
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Cheers,
Dimitre Novatchev
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