[sc34wg3] TMQL: Predicate invocation and scope

Robert Barta rho at devc.at
Wed Jul 9 05:38:40 EDT 2008

On Tue, Jul 08, 2008 at 05:36:09PM +0200, Lars Heuer wrote:
> Hi Lars,
> [ $a = member-of(member: $p, group: $g)]
> > I know this doesn't work now. It's not even syntactically valid, as
> > far as I know. But we could define this in such a way that it *would*
> > work.
> I don't know if it is worth, because we'd change the meaning of
> predicate invocations seriously.

Predicate invocations generate association items, so in

  select $p, $q
     member-of(member: $p, group: $g)

the 'member-of(member: $p, group: $g)' gets converted (I'm simplifying)

    // member-of [ . -> member == $p ] [ . -> group == $g ] ..

> Not sure if it is a good or bad thing.

Not sure what you want to say here.

> What happens to the variables $p and $g if the predicate invocation
> returns associations?

I cannot see a problem. $p and $g will be instantiated with items, so
that the 'member-of(member: $p, group: $g)' gives some non-empty
result (in other words: "exists"). If you want all of it, then

  select $p, $q, $a
     $a == member-of(member: $p, group: $g)

will do the obvious.


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