At 2011-06-17 17:58 +0000, Will Thompson wrote:
>Can someone explain why passing this function () returns (), instead
>of an element or throwing an exception?
>
>declare function local:do-stuff(
> $bool as xs:boolean
>) as element() {
> if ($bool)
> then <x/>
> else <y/>
>};
I would expect passing the function an argument value of () would
return a runtime error because there is no optionality allowed on the
datatype of the argument. An argument of xs:boolean* would allow the
empty set to be passed as an argument and <y/> would be returned.
>I see that xs:boolean(())=(),
Correct if you are expressing a comparison operator there. Both
sides evaluate to false(), but for slightly different reasons ... or
rather, the same reason but different mechanisms.
The left-hand side is explicitly computing the
effective-boolean-value of the empty set and the right-hand side
implicitly (because it is an operand of the operator) expresses the
effective-boolean-value of an empty set.
The two false() values equate as true().
If you are using "=" in your statement as a notation for "returns",
then the statement is not correct ... xs:boolean(()) returns false().
>but it seems like this should either eval to false(), or the
>function should throw an exception since () isn't a boolean (or is it?).
xs:boolean does a cast of the argument. The first bullet of the
documentation cites what is returned when the argument is an empty set:
http://www.w3.org/TR/2007/REC-xpath-functions-20070123/#func-boolean
The implicit cast for the equality operator is defined here:
http://www.w3.org/TR/2007/REC-xpath-functions-20070123/#func-boolean-equal
I hope this helps.
. . . . . . . . . . . Ken
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