This is really a question about how IBM's DFDL works, but I have to introduce the topic:

XPath 2.0 says that implementations have freedom about whether these expressions cause errors or not:

true() or error() = true() Let's call this the sequential semantics

true() or error() = error Let's call this the parallel semantics

error() or true() = true Let's call this the bizarre semantics

There are analogous cases for 'and' with false()

Saxon, which dates from XPath 1.0, implements the sequential semantics, which is required by XPath 1.0, and so that's what the various TDML tests and such that we have in the Daffodil project have come to depend on. Quite often we have things like

dfdl:occursIndex() = 1
or

../r[dfdl:occursIndex() - 1]/flag

where that 2nd operand is effectively an error if the first operand is true.

Strictly by the new XPath 2.0 rules, the only portable way to write this expression is with an if-then-else. But I am reluctant to change all these tests we have.

Part of me says the most conservative thing is the parallel semantics - because it prevents you from writing an OR statement like the one I have above that depends on the sequentiality.

It is probably more important that the initial implementations are consistent so that schemas are more likely to interoperate.

What does IBM DFDL do for the semantics of the OR expressions?

Thanks

Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | www.tresys.com

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