I just released version 0.2.0.2 of my datalog library to hackage. Datalog is a declarative query language similar in spirit to Prolog, though less expressive. If you have a problem that can be expressed as dynamic transitive closure, Datalog is one way to express your problem. I believe Datalog is equivalent to SQL with recursive queries. My implementation is currently fairly basic; I hope to add some more sophistication in the near future. The primary feature of my implementation is that it is usable as a library: you can add facts to a database and issue queries through just a few simple library calls. The facts in the database can be any Haskell type that is hashable and comparable for equality -- basically anything you could reasonably want to query.
This latest release has one new feature and an important bug fix. The feature is a new REPL implementation using haskeline, a very nice implementation of readline. The REPL is not ideal as a high-performance interface to the solver, but it is useful for exploratory query design.
The bug fixed in this release caused the solver to return some incorrect answers. Specifically, it would return the empty set when there were really some answers. The root cause of the problem was a bug in how queries with negation were handled. In Datalog programs with no negations in any rules, a very simple evaluation strategy works well. When a Datalog program includes negated clauses, evaluation order begins to matter a great deal. The simplest way to deal with negation is to stratify the rules in the Datalog program. This stratifies, or partitions, the Datalog program into strongly-connected components (based on dependencies between relations) and evaluates the strata (partitions) bottom-up. Embarrassingly enough, I was evaluating the strata top-down before this bug fix release.
I misstated the exact nature of the bug fixed in the latest release. Strata were not being evaluated in the wrong order. Instead, stratum numbers were being assigned in the wrong order. Most of the time, the numbers were correct and thus rules were evaluated in the correct order. However, in some cases, rules in the highest stratum would actually be assigned to the lowest.