logic languages

• Logic programming is based on Horn logic. A Horn clause is an if-then rule with one conclusion in the then part (head) and zero or more goals in the if part (body).
• The programmer specifies a problem by giving a set of Horn clauses describing facts and properties about the problem.
• The user can ask questions to the system by giving it goals to prove. The system automatically tries to prove the goals using the Horn clauses. If the system is unable to prove the goal from the Horn clauses it has available, it assumes that the goal is not true.
• A logic programming system that has the completeness-of-search property will always find a proof if one exists. Such systems typically use a search strategy based on backtracking.
• Horn clauses may use logical variables, denoting unspecified objects. A logical variable is initially unbound. It can be bound only once, unless its value is unbound again during the backtracking. Binding occurs as a side-effect of matching a goal with the head of a clause.
• Horn clauses express relations rather than functions. The arguments of a clause are not marked in advance as being input or output, but can often be used in both ways.
• Structures and lists are the most important data structures in logic languages.
• For efficiency, the programmer may control the order in which alternative clauses for a goal are tried as well as the order in which different subgoals in one clause are tried.
• Prolog is by far the most popular logic language.