Re: XPath "none" /type/ not available to the user

On Wednesday 24 May 2006 11:26, Florent Georges wrote:
>   Hi
>
>   In F&O, '3 The Error Function says:
>
>     fn:error() as none
>
>     [...]
>
>     Note that "none" is a special type defined in [XQuery
>     1.0 and XPath 2.0 Formal Semantics] and is not available
>     to the user.  It indicates that the function never
>     returns and ensures that it has the correct static type.
>
>   Why is it not available to the user?

Probably because no one thought a reasonable usage scenario exists for
it(although I don't know for sure).

>   As the result type
> of a function that never returns.  For example, what can I
> put in the @as of the following XSLT definition of an XPath
> function?
>
>     <xsl:function name="my:error" as="none()">
>       <xsl:param name="msg" as="xs:string"/>
>       <xsl:sequence select="
>           error($my:error-qname, concat('No luck: ', $msg))"/>
>     </xsl:function>
>
>   IMHO, it can be interesting for static type checking.
> Does this "type" introduces some complexity in the static
> type system?

I think implementors happily form a little choir singing "Yes it do! Tra la la
la!"

I've myself been thinking a bit about this, while writing XQuery test cases.
Try declaring a function with the return type empty-sequence():

     <xsl:function name="my:error" as="empty-sequence()">
       <xsl:param name="msg" as="xs:string"/>
       <xsl:sequence select="
           error($my:error-qname, concat('No luck: ', $msg))"/>
     </xsl:function>

I haven't yet confirmed in Formal Semantics that this shouldn't be a type
error(e.g, that the type 'none' should match empty-sequence()), but it really
should due to the purpose of fn:error() and the none type.


Cheers,

		Frans

(personal response)