> If you have a function f(x) and and expression
> 
> f(x) + f(x)
> 
> it may be optimised to 2 * f(x) which means you only have to 
> calculate the function once.
> 
> However you can't do that if there is a possibility of the 
> "function" doing something strange like incrementing teh 
> value of some second variable y as in the first case y goes 
> up by 2 and in the second it goes up by 1.

Unfortunately when it's

   f(x) is f(x)

then you do have to evaluate the function twice; expressions in XPath
2.0 are allowed to have the limited side-effect of creating new,
distinct nodes. 

The formal semanticists seem happy that this is under control, but I
can't say I've worked out all the implications for my optimizer yet!

Michael Kay


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