[xsl] XPath "none" /type/ not available to the user

Subject: [xsl] XPath "none" /type/ not available to the user
From: Florent Georges <darkman_spam@xxxxxxxx>
Date: Wed, 24 May 2006 13:26:04 +0200 (CEST)

  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?  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

    <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))"/>

  IMHO, it can be interesting for static type checking.
Does this "type" introduces some complexity in the static
type system?  It seems that it is well defined in Formal
Semantics (but I don't know well this document, I just had a
quick look at it).




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