Prolog doesn't have typed variables, but any Prolog term has a type, the set of Prolog terms that unify with itself. In practice, when you write Prolog, your ar

Prolog doesn't have typed variables, but any Prolog term has a type, the set of Prolog terms that unify with itself. In practice, when you write Prolog, your arguments to predicates are always implicitly typed, for example if you write a program that takes as argument a list, you'll write something like this: a_list_processor([H|T], ..., ):- process_list_head(H,Xs) ,process_list_tail(T,Ys) , ... Or at least you learn to do that quickly enough. And note that this implicit typing is used both to select clauses in a predicate, and also to index clauses in the Prolog program database by their first term (in which case the first term better be a constant though, not a list). So it's a bit of a bug-repellent, and a bit of an efficiency booster, both. Just like you get with static types.