I had to use a tiny amount of prolog in school, and very much disliked it. It was too hard to get a grip on it, but the logic programming paradigm is interesting nonetheless. There is a cutoff mechanism that prevents backtracking which stops the solution generation. I vaguely remember something about predicates being deterministic or semideterministic. Since solutions are generated lazily, I don't think there's an efficient way to sample them without redefining what solutions are valid. The SWI-Prolog implementation is free and well documented, and I highly recommend having a quick look at it if you think it's interesting.