Question

I’m a little bit confused concerning variable updates in XQuery:
On [1] it says:

Variables can’t be updated. This means you can’t write something like let $x := $x+1. This rule might seem very strange if you’re expecting XQuery to behave in the same way as procedural languages such as JavaScript. But XQuery isn’t that kind of language, it’s a declarative language and works at a higher level. There are no rules about the order in which different expressions are executed (which means that the little yellow triangle that shows the current execution point in the Stylus Studio XQuery debugger and XSLT debugger can sometimes behave in surprising ways), and this means that constructs whose result would depend on order of execution (like variable assignment) are banned.

I’m wondering if there is really no way to reliable update a variable? Maybe I’m just to used to those things in other languages, but I can’t really imagine / believe it 😉

[1] http://www.stylusstudio.com/xquery_flwor.html, second paragraph below the screenshots of the chapter “L is for LET”

UPDATE:
I have to add a question to this: Shouldn’t it be possible to update an existing variable in an if statement because in this case the order of execution is clear? I guess you just aren’t allowed to use something like $x = $x+1 in a loop?

Answer 1

You are describing immutability, a feature of functional languages. It’s true; once a variable is set to a value, it cannot be set to something else.

Immutability has many benefits. In particular, concurrent programming is made much easier.

In the case of loops, what happens is that a new variable is created each time through the loop, replacing the original one. So immutability still holds. This is explained in detail in the article you linked:

Isn’t there a variable being updated
when you write something like the
following?

for $v in //video
let $x := xs:int($v/runtime) * xdt:dayTimeDuration("PT1M")
return concat($v/title, ": ", 
      hours-from-duration($x), " hour(s) ",
      minutes-from-duration($x), " minutes")

(This query shows the running time of
each video. It first converts the
stored value from a string to an
integer, then multiplies it by one
minute (PT1M) to get the running time
as a duration, so that it can extract
the hours and minutes components of
the duration. Try it.)

Here the variable $x has a different
value each time around the XQuery for
loop. This feels a bit like an update.
Technically though, each time round
the for loop you’re creating a new
variable with a new value, rather than
assigning a new value to the old
variable.