[Bug 5810] [XQuery] reasoning about fn:error()


--- Comment #9 from Michael Dyck <jmdyck@ibiblio.org>  2008-06-30 20:13:51 ---
[Personal response]

(In reply to comment #8)
> Do you have an opinion on whether 
> count(fn:error())
> is 1,

Under a strict evaluation, the expression always raises a dynamic error, so the
question is more like "Is an implementation allowed to rewrite the expression
into one that always returns 1?". I believe the answer is yes, but really it's
allowed to rewrite it into *anything*. That is, when the original expression
raises an error, the XQuery rewrite rules don't constrain the value/error
resulting from the rewritten expression. (The classic example being an
implementation that returns 42 for the expression 1 div 0. It's allowed, but
frowned upon.)

If the question is whether it's a "reasonable" or "defensible" rewrite, I'm not
sure. Although FS 8.4 says
    quantifier(none) = 1
it's doing so in the context of defining a particular pseudo-function for use
in particular rules. Applying it elsewhere isn't guaranteed to give meaningful
results. (Note that FS 8.4 also says
    quantifier(empty) = ?
which could lead to some bad inferences if mis-applied.)

> and whether (more generally) it is permitted to count a function whose
> argument has quantifier 1 without evaluating it?

If an implementation can prove (via quantifiers or whatever) that a particular
call to fn:count() would, if evaluated, either raise an error or return 1, then
I believe it is permitted (among other things) to yield 1 for that expression
in all cases.

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Received on Monday, 30 June 2008 20:14:25 UTC