 Prolog

Prolog expresses programs through facts and rules and answers queries by solving goals. A rule such as ancestor(X,Y) :- parent(X,Y). provides one way to establish a relationship. A recursive rule can search through intermediate parents. Unification matches a goal with a rule head and propagates bindings; backtracking explores alternative matches when a branch fails or another answer is requested.^[ref]

The logical core is associated with Horn clauses and SLD resolution, but clause order and goal order also determine a conventional Prolog program's search behavior. A logically valid recursive specification can loop before reaching an available answer. Predicates with input/output effects and control operations add further operational behavior. Arithmetic evaluation is also distinct from term matching: X = 1+2 builds a term, whereas X is 1+2 evaluates arithmetic in the usual Prolog interface.