On 19 Apr 2014, at 05:26, Dimitre Novatchev <dnovatchev(_at_)gmail(_dot_)com>
wrote:
A simple search in the XQuery and XPath Data Model (XDM) 3.0 W3C
Recommendation for the string "xs:error", returns 0 results.
You are right, this area is not well covered by the XDM specification; you have
to treat xs:error as being included implicitly by virtue of the fact that all
XSD built-in types are included.
In the XDM 3.0 Recommendation there are three type diagrams:
The type xs:error is absent in any of these charts.
It does say that the diagrams are incomplete, in particular it only includes
types defined as part of a hierarchy.
This raises a number of questions:
4. Where in the type hierarchy is the type xs:error located --
which, if any is its immediate base type?
XDM states that the type system is not a hierarchy but a lattice. xs:error is
the "bottom" type in this lattice, it is a subtype of every other type.
5. Section "2.5.7 xs:error" of the XPath 3.0 Recommendation
describes the xs:error type. This is different from the formal
definition of xs:error in the XML Schema 1.1 Recommendation (Section
3.16.7.3 http://www.w3.org/TR/xmlschema11-1/#xsd-error). Which of the
two definitions of xs:error (in XPath 3.0 or in XSD 1.1) is the
normative one and which is the redundant one?
XPath doesn't contain anything resembling a definition of this type; it merely
refers to it by name. The definition is in XSD 1.1.
6. Section "2.5.7 xs:error" of the XPath 3.0 Recommendation states
that "A cast to xs:error raises an error or returns the empty
sequence." In what cases is an error raised and in what cases is the
empty sequence returned? What are examples of the two different
situations? In which document should we find the answer to this
question?
Casting is described normatively in F+O. xs:error is a union type (a union with
no member types) and the semantics are therefore defined by 18.3.5 Casting to
union types. It's a consequence of the general rules for casting to a union
type that casting to xs:error throws an error if the argument is a single item,
and returns an empty sequence if the argument is an empty sequence.
By way of background to this, it was realised at a fairly late stage that if
all built-in types from XSD 1.1 were to be supported in XPath 3.0, then this
included xs:error and t would be a good idea to say something about how
xs:error should behave; the only alternative was to specifically exclude it.
Everything the specs say about xs:error is a consequence of the fact that (a)
xs:error is a built-in type, and (b) it is defined as a union type with no
member types. There is no special treatment of xs:error, merely explanations of
the consequences of its existence and the way it is defined.
Michael Kay
Saxonica
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