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I definitely plan to learn some Prolog, but I don't know when. For now I plan on learning Datalog because it is used (or a variation on it is used) in Datomic.

• jkbyc 14y ago
  Datalog is really a theoretical basis for declarative query languages. It amounts to a Horn-clause subset of predicate logic and in that it is similar to Prolog. One of the substantial differences is that Datalog doesn't allow functions in terms. That makes Datalog strictly less expressive but it also gives you a guarantee that a Datalog program will always terminate. In comparison, it is trivial to write even a simple Prolog program that does not terminate. Datalog is really almost just a special syntax for Horn-clauses; it is purely declarative. Prolog is much more than that in that it adds features like cuts, arithmetic, built-in functions, and everything else that's needed in a practical programming language. Also, a Prolog interpreter implements a sophisticated backward-chaining algorithm. So Prolog basically means backward-chaining. In comparison, I would say there is no default evaluation strategy for Datalog. You (resp. the interpreter) can use backward-chaining, forward-chaining, a combination of both (such as the magic-sets method) - the designer of the Datalog-based query language decides what's best.