flowrrr - how to structure a function?

[Michael Kay]

> 
> You also need an execution model in which it is predictable 
> whether the function is actually called or not.
> 

Incidentally (responding to my own remark) that's a problem we had to tackle
in defining the semantics of xsl:result-document in XSLT 2.0. We ended up
with an approach in which the dynamic context for an expression contains a
boolean flag indicating "final output state" or "temporary output state",
and xsl:result-document can only be executed in "final output state". While
you're evaluating a function or a variable you're in temporary output state.
In final output state you get a guarantee that expressions are evaluated
exactly once, though with no guarantee of order of execution.

Because template calls preserve the current output state, while function
calls switch it to "temporary", templates and functions in XSLT behave
slightly differently, analogous to updating and non-updating functions in
XQuery Update: except that unlike an updating function, a template does
return a result.

Michael Kay
http://www.saxonica.com/