[xquery-talk] Global variables in XQuery

Michael Kay mhk at mhk.me.uk
Wed Oct 4 17:48:47 PDT 2006

XQuery is a functional language, and like other pure functional languages,
it has no assignment statement. Your attempt to write an assignment
statement is simply creating a local variable with the same name as the
global variable (something that some of us tried to ban because we knew
people would make this mistake).

It's a common fallacy among programmers to think that variables are so
called because they are mutable. In fact the term comes from mathematics
where an expression such as X = X+1 would be considered rather strange (or
at any rate, false). XQuery is an expression language, and the term
"variable" simply means an input to the expression that can have different
values on different occasions, just as "width" and "depth" are variables in
the formula area=width*depth.

I'm not sure what your example is trying to do. It's quite hard to
reverse-engineer a declarative algorithm from a procedural one, especially
an incorrect procedural one. It's usually easier to start again from the
statement of requirements.

Michael Kay


> -----Original Message-----
> From: talk-bounces at x-query.com 
> [mailto:talk-bounces at x-query.com] On Behalf Of Massimo Franceschet
> Sent: 04 October 2006 16:25
> To: talk at x-query.com
> Subject: [xquery-talk] Global variables in XQuery
> Hi,
> I am confused about the behaviour of (what are called) global 
> variables in XQuery. In paricular, I am not able to modify 
> the value of a global variable inside a user-defined 
> function. Here is an example:
> declare namespace my = 'my:stuff';
> declare variable $my:global as xs:integer := 1;
> declare function my:fact($n as xs:integer) as xs:integer {
> 	let $my:global := $my:global + 1
>         return if ($n < 2)
> 	then 1
> 	else $n * my:fact($n - 1)
> };
> let $x := my:fact(10)
> return $x div $my:global
> When I run this query the global variable $my:global is not 
> modified by the function my:fact and retains its original 
> value (1). The result of the query is always 10! (instead of 10!/11).
> Massimo
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