can the length of two xml lists be defined as being required to be equal?

Question

Assuming you have two lists defined in an xml schema, call them A and B, is there a way to say A must be one or more items and B must be the same length as A?

Answer 1

Co-occurrence constraints are not supported in XSD 1.0. In XSD 1.1 which is still in a draft stage, they are. So, sadly, no.

Answer 2

As an alternative to storing

A = (a, a, a)
B = (b, b, b)

and enforcing the same length, you could try storing:

AB = ([a,b], [a,b], [a,b])

This way the constraint you need to enforce would be that each element from AB is of length two and consists of the types A and B - this should be possible in a normal schema.