On Wed, Sep 19, 2012 at 11:11 AM, Stathis Sideris <side...@gmail.com> wrote:
> Hello,
>
> Is there a way to negate a goal in core.logic? I have written this goal
> below, and I would like to use it to query all the pairs of people who are
> *not* relatives according to my facts (for dating purposes of course :-D)
>
> (defn relative [p1 p2]
> (conde
> [(parent p1 p2)]
> [(parent p2 p1)]
> [(sibling p1 p2)]
> [(cousin p1 p2)]
> [(nephew p1 p2)]
> [(uncle p1 p2)]))
>
>
> Thanks,
>
> Stathis
core.logic currently only supports Prolog's negation as failure. You can
write a goal that looks like this:
(defn not [g]
(fn [a]
(if (nil? (g a))
a)))
But you must be careful if you use this, all logic vars passed to the goal
g must be ground. So it's not relational.
I have some leads based on recent Prolog research (CiaoProlog, and an
interesting dissertation on the abstract interpretation of Prolog) for
really handling negation in core.logic but I don't think I'll have time to
deal with it any time soon. Again, more than willing and excited to
collaborate with people who would like to see it done sooner.
David
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