The negation as failure constraint nafc is supposed to succeed if and only if the goal provided by a relation and its args fails. The test cases just cover very simple cases like (nafc == q 'b) which is essentially equivalent to (!= q 'b) (at least to my understanding). But with a slightly more complex case, it doesn't seem to work anymore.
The constraint specifies that the number 2 must NOT be contained in the list q but still it is. I expected to get the single answer (1 3) here.
core.logic 0.8.10, clojure 1.7.0
It seems that this problem could be specific to clojure.core.logic.fd. At least this example works:
This is not a bug. It's expected behavior because not all arguments to the nafc goal are ground.
From `nafc` docstring:
EXPERIMENTAL: negation as failure constraint. All arguments to the goal c must be ground. If some argument is not ground the execution of this constraint will be delayed.
In the example using fd, q is not ground (because a and b are not ground), so it's almost the same as if the nafc wasn't there.
My interpretation of the docstring is that the check will be delayed until the point in time where the variables become ground. And eventually q is ground in my first example. I mean, otherwise nafc would be pretty useless.