XQuery - Copy Constructor? - xquery

Question

Given the following query
let $a := xs:dateTime("2012-01-01T00:00:00.000+00:00")
let $b := xs:dateTime($a)
let $c := xs:dateTime($a cast as xs:string)
(: cannot - don't know how to - execute the function without assignment :)
let $d := adjust-dateTime-to-timezone($a, xs:dayTimeDuration("PT1H"))
return (<a>{$a}</a>,<b>{$b}</b>,<c>{$c}</c>)
the output is as follows
<a>2012-01-01T01:00:00+01:00</a>
<b>2012-01-01T01:00:00+01:00</b>
<c>2012-01-01T00:00:00Z</c>
Based on XQuery's documentation on constructor functions (the constructor function for a given type is used to convert instances of other atomic types into the given type) this is the expected behaviour. Calling xs:dateTime($a) simply returns $a as there is no need to cast, but xs:dateTime($a cast as xs:string) creates a new xs:string from $a first. However this requires an extra conversion.
Is there any other way to tackle this problem? Or conversions are cheap and I shouldn't care?
(If it makes any difference my XQuery processor is BaseX 7.2.)
It seems it does a make a difference that I'm using BaseX. I've really thought that this is the way the xs:dateTime constructor function and the adjust-dateTime-to-timezone function should be working, this is why I misinterpreted the XQuery documentation.
Given the input I've been given by Dimitre and Ranon it seems the problem described is gone.
By the why my use case is, or more like it was, that I wanted to make a date-time interval based query against my XML data set's date-time element. Because the input parameters and the source date-time values used different time-zones I had to make time-zone corrections with the above function, which modified its input parameter (the original source date-time in my case), however I wanted to preserve the original value. Given the function's name adjust-dateTime I thought that it's okay that it modifies its argument, so I automatically thought that I had to copy my original value using a constructor function to be able to keep the original date-time value.

Answer 1

Looks like you ran into some really weird bug.
Your line 5 shouldn't change $a-c at all as XQuery is a functional programming language with immutable variables (adjust-dateTime-to-timezone should not change your variables) and without side effects. Thats why you were forced to assign $d, otherwise your calculated results directly would have been thrown away.
I just submitted some bug request. Zorba is doing your query right, you can use it for understanding the problem.
BaseX as you preferred XQuery processor will do within few days, too. I or some other BaseX team member will trigger you here as soon as it's fixed.
I guess your problem arised from missunderstanding and wrong behaviour of BaseX and should be solved. Feel free to ask again if anything stayed unclear with your query.

Answer 2

The output that is reported is incorrect.
The correct output (produced running Saxon under oXygen) is:
<a>2012-01-01T00:00:00Z</a>
<b>2012-01-01T00:00:00Z</b>
<c>2012-01-01T00:00:00Z</c>
The adjust-dateTime-to-timezone() function, as any other function cannot modify its arguments -- its effect is only contained in the variable $d -- which you don't use in the return clause.