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Re: [xml-dev] Use-case for a complex type that has no valid content?

The main use of a void type is that it helps you construct a system with closure properties, so you can have operators on types (e.g. the intersection of two types) where the result of the operation is always a legitimate type.

Use-cases are the bane of orthogonality. Any system with good orthogonality can do things for which it is very difficult to find a use case. What’s the use case for adding two negative numbers? Design focused excessively on use-cases leads to a lack of orthogonality.

XSD 1.1 introduced xs:error which is a void simple type. It’s defined similarly to <xs:choice/> as a union with no member types. The specific use case for introducing it was for conditional type assignment, it allows you to have the last alternative in a list of alternatives say “if none of the above conditions is true, then the type is xs:error, meaning no content is accepted”.

Michael Kay

On 2 Jun 2015, at 18:44, Costello, Roger L. <costello@mitre.org> wrote:

Hi Folks,


Thanks for the outstanding responses.


I have learned that an empty choice:


    <xs:complexType name="ValidationKiller">


has no valid content, i.e., the set of valid content is the empty set, {}.


When would one use such a complex type? Can you think of a use-case for it?



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