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XPath as a stand-alone programming language (Was: Re: [xsl] [Summary]XPath 3.0 is a composable language ... what does that mean?)

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:




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.



On Wed, Oct 24, 2012 at 4:55 AM, Costello, Roger L. <costello@mitre.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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Dimitre Novatchev
Truly great madness cannot be achieved without significant intelligence.
To invent, you need a good imagination and a pile of junk
Never fight an inanimate object
To avoid situations in which you might make mistakes may be the
biggest mistake of all
Quality means doing it right when no one is looking.
You've achieved success in your field when you don't know whether what
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Facts do not cease to exist because they are ignored.
Typing monkeys will write all Shakespeare's works in 200yrs.Will they
write all patents, too? :)
I finally figured out the only reason to be alive is to enjoy it.

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