RE: LC-93 disjoint datatypes

• From: C. M. Sperberg-McQueen <cmsmcq@acm.org>
• Date: Tue, 10 Oct 2000 17:22:51 -0600
• To: "Vun Kannon, David" <dvunkannon@kpmg.com>
• Cc: W3C XML Schema Comments list <www-xml-schema-comments@w3.org>
• Message-Id: <4.3.2.7.1.20001010114306.01fd9518@espanola.com>

At 2000-10-10 10:13, Vun Kannon, David wrote:
>Thank you. At first blush, the new proposal does satisfy my basic request.
>I'm unsure how the union of types across different schemas will look, but I
>have to admit to not having read the new spec carefully. I'm envisioning a
>situation in which one schema defines
>colorsA={red,yellow,green}
>, a second schema defines
>colorsB= first:colorsA union {pink,blue}
>eventually, an instance of a third schema containing
>attribute name="currentColor" type="schema2:colorsB"
>has
><trafficLight currentColor="schema2:pink"/>
Well, actually the value should be "pink" not "schema2:pink".
>but if the color was green what is the attribute value? schema1:green or
>schema2:green?
The attribute value is "green".  The attribute is identified as
having the union type "schema2:colorsB", and the value is
identified as being of member type "schema1:colorsA".
>Have I created a union with the type or the set of values the type
>aggregates? I hope that question makes sense.
Unfortunately, no, at least not to me when expressed in those terms.
So let me try it in my terms.
For a union type U, with member types V and W (in that order),
   - the lexical space of U is the the union of the lexical space of
V with the lexical space of W
   - the value space of U is the union of the value space of V with the
value space of W
If the lexical spaces of V and W overlap, then lexical forms in
the intersection will be interpreted as belonging to the first
member type (V); more generally, a value is interpreted as belonging
to the first member type in the union for which it is a legal value.
The corresponding values in the other types are effectively
shadowed and inaccessible.  (My earlier note said that xsi:type
could be used to overcome this shadowing; I was wrong.  This
was discussed speculatively in the WG, but is not in the spec.)
I hope this helps.
-C. M. Sperberg-McQueen

Received on Tuesday, 10 October 2000 19:24:49 UTC