Prolog is useful everywhere proofs are useful. Now, a proof isn't what you may think of as a proof in the general sense. One example of a proof that Prolog is

Prolog is useful everywhere proofs are useful. Now, a proof isn't what you may think of as a proof in the general sense. One example of a proof that Prolog is good at is solving games, such as solving a particular instance of tic tac toe, or 4-in-a-row. Beyond these toy examples, you can see a good fit in any application which needs to follow a procession of rules. These rules may be in a feeding/bleeding relationship — this is essentially constraint logic programming. The problem goes: given these constraints, does this statement fulfil the constraints? How can I make the statement fit the constraints? Which is the set of statements that fit these constraints? I know of an industry application in German luxury cars, where the customer can order a car with a lot (a lot a lot) of different features, but some are mutually exclusive. If you have this particular leather seat, you can't have that particular heating unit or whatever. Now tell me if the customer selected a valid configuration, and if not, tell me what the valid configurations "next to" the desired one are. Another application are DCGs (Definite Clause Grammar), which have the expressive power of regular expressions (EDIT: see below for a correction, it appears they're at least context-sensitive.) If you've done Monadic parsing, DCGs feel kinda similar. They're super-close to Phrase Structure Grammar rules, and used to be part of my linguistics curriculum (I hear it's all ML nowadays. Pity.) Yes, Prolog fits niche domains. But if you find a domain, such as regular languages or a problem that can be solved by CLP, the elegance of logic programming can hardly be beat. It's really a joy to work with in the right domain. It's absolutely awful as a general purpose programming language, though — mostly because the debugging is so complex, it almost feels nondeterministic (it isn't, but there were many moments at 2 in the morning where I could've sworn it is.) In principle, Prolog is great for searching for solutions of a formally posed problem. Think of it as a logic search engine. If your problem can be stated in logic, and the logic problem can be evaluated in a finite time, Prolog's your friend. If not, you'll blow the stack.