• From: "Costello, Roger L." <costello@m...>
• To: "xml-dev@l..." <xml-dev@l...>
• Date: Wed, 24 Oct 2012 12:17:24 +0000

Hi Folks,

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.

-----------------------
Acknowledgement
-----------------------
    Thanks to Michael Kay for explaining to me 
    what it means for XPath 3.0 to be a composable 
    language.

/Roger