XQuery - Grammar? Implementations?

[Yoshi Okamoto]

Dear XQuery subscribers,

I am sometimes fighting to understand all the details of the XQuery grammar. If you have some time, I would be delighted if you could have a look at the following example: I have defined a function with a single string argument:

  [0]  declare function local:f($a as xs:string) { $a };

If I pass on an element as follows:

  [1]  let $a := <X>Y</X> return local:f($a)

it is accepted by most XQuery implementations which I tried (I am testing with Saxon, BaseX, MonetDB, Zorba, Qizx, Kawa and MXQuery). However, when I cast the element before passing it on:

  [2]  let $a as xs:string := <X>Y</X> return local:f($a)

some implementations (also Saxon, which I regard as reference implementation) return [XPTY0004] as error. A third version with an explicit cast:

  [3]  let $a := xs:string(<X>Y</X>) return local:f($a)

is finally accepted by all implementations.

If possible, can someone tell me what the difference are between the versions, especially [1] and [2]? I tried to find the answer in the specification, but I was not successful.

Thank you very much for your attention and help!

Best Regards,
Yoshi Okamoto