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[xsl] [Summary] XPath 3.0 is a composable language ... what does that mean?

2012-10-24 06:56:06
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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