Re: [xml-dev] XPath 2.0 Best Practice Issue: Graceful Degradation

[David Carlisle <davidc@nag.co.uk>]

> Doesn't that presuppose a certain environment: the XPath is running in
> an environment where the data has already been checked?


well I don't have to presuppose that. I can just state (that as a
general principle rather than a hard and fast rule) it's better to get
your data checking done up front rather than making every expression 10
times longer than it would otherwise be as it makes the code very hard
to understand and debug (and as written relies a lot on the optimiser to
re-write repeated exopressions. //airplane is a potentially expensive
operation, implying a full search of the whole input document, and your
rewrite changed that from being called once to being called 7 times.
If the system only does it once anyway, perhaps that's alright but
if it doesn't performance might be so bad that you never notice how
graceful the degredation is....

Even if there isn't a schema validation step and you want to
check the data validity using xpath I think it helps to keep that
separate from your program logic, so have schematron (or
schematron-like) templates that are just checking the input format is
OK, and keep those templates separate from your program logic.

David

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