Setting global variables

[Michael Kay]

Sounds like you're having difficulty getting procedural programming out of
your mindset... It's this concept of "later" that gives it away. In
functional programming, there is no time axis, no concept of "earlier" or
"later".

The answer is to initialize the global variable to the subset of the
document that's needed, from within its own initializer.

Incidentally, the problem of passing parameters through functions that don't
actually look at them directly sounds like a request for something like XSLT
2.0's tunnel parameters.

It looks as if XQuery is going to be extended to turn it into a procedural
language through "scripting extensions". As far as I'm concerned, that's a
great shame - being forced to learn to use functional programming has made
me a much better programmer.


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

 

> -----Original Message-----
> From: http://x-query.com/mailman/listinfo/talk 
> [mailto:http://x-query.com/mailman/listinfo/talk] On Behalf Of Ronald Bourret
> Sent: 31 October 2007 05:55
> To: xquery-talk
> Subject:  Setting global variables
> 
> Is there any way to set a global variable from inside a 
> function? (By "global variable", I mean a variable declared 
> in the prolog of a function library.)
> 
> Basically, I'd like to read a document and save a subset of 
> the document in a global variable, then refer to it later 
> when doing lookups. For example, I'd like to do something 
> like the following:
> 
>     declare variable $foo:listOfNames external;
> 
>     declare function foo:firstFunctionCalled(...)
>     {
>        let $foo:listOfNames := fn:doc("mydoc.xml")//Name
>        ...
>     };
> 
>     declare function foo:functionCalledMuchLater(...)
>     {
>        ...
>        if ($name = $foo:listOfNames)
>        ...
>     };
> 
> Currently, I pass such information from function to function 
> until it is finally used. While this works (and some would 
> argue is the correct style), it has the following drawbacks:
> 
> 1) It is difficult to follow the information through the 
> chain of functions.
> 
> 2) It can be confusing to read the intermediate functions, as 
> these never really use the information.
> 
> 3) It is brittle when refactoring the intermediate functions. 
> Because the information is not really used -- it's just 
> passed along -- it is easy to accidentally drop.
> 
> Thanks,
> 
> -- Ron
> 
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